So, can DC pass through an Inductor or Capacitor?

DC can pass through an Inductor, but not through a Capacitor. In DC circuits, when fully charged a capacitor behaves like an open circuit not allowing current to pass, whereas an Inductor behaves like a short circuit allowing current to pass. 

What is DC?

All electronic circuits need current to operate. Current is like blood coursing through our body. Just like how it is essential that our bodies need blood to live, so too is it important for electrical and electronic circuits to have current to operate.

There are two types of current that can flow in electrical and electronic circuits; Direct Current (DC) and Alternating Current (AC). 

The main difference between Direct Current and Alternating current is the direction of flow. DC flows in one direction, while AC flows in both directions.In AC circuits the voltage also follows the current. 

DC is best represented with a steady straight line, and AC with a ‘alternating’ waveform like a sinusoidal wave (as seen in the images below).

What is a capacitor?

The electrical and electronic world is filled with a range of different components each having their own set of unique characteristics that help in different applications. One of the most widely used is the Capacitor. A capacitor is a two terminal electronic component that has the ability to store energy in the form of an electrical field when voltage is applied to it. 

The capacitor consists of two terminals connected to two metal plates. These metal plates are separated by an insulating material known as a Dielectric. The insulating material could be a range of materials such as glass, ceramic, mica, or even air. 

What happens when you connect a capacitor to a battery?

When a voltage is applied across the terminals of the capacitor, the metal plate connected to the negative terminal of the capacitor starts to attract electrons (making it negatively charged), while the plate that is connected to the positive terminal of the battery starts losing electrons (making it positively charged).

Once fully charged, the voltage across the capacitor will equal the voltage of the power supply it is connected to.

What is an Inductor?

An Inductor is another widely used component in electronic and electrical circuits. An Inductor is also a two terminal component which has the ability to store energy in the form of a magnetic field when current flows through it. It consists of an insulated wire which is wound into a coil. Many Inductors are accompanied by a magnetic core made of Iron to help increase the strength of the magnetic field.

The strength of the magnetic field can also be increased by several other factors which include the number of turns of the wire, diameter of the coil, coil length, and layers of winding in the coil. 

When an ‘changing’ current is applied to an Inductor, the magnetic field acts to oppose this change in current by producing a voltage known as an Electromotive Force (EMF) . This voltage has a polarity which opposes the change in current. In short, Inductors oppose change in current. 

Why does an Inductor allow DC to pass?

Now that we have learnt the basics of a Capacitor and Inductor, we can understand better why an Inductor allows DC to pass, but a Capacitor blocks it.

As we just saw above, an Inductor opposes changes in current. When current changes within an Inductor, it produces a voltage (EMF) of equal force to oppose this change in current. We know that DC is a steady current in one direction whose magnitude does not vary with time. Therefore, when DC is applied to an Inductor it does not offer any resistance (EMF), and acts like a short circuit allowing DC to pass. 

Why does a capacitor not allow DC to pass?

This comes down to what we learnt earlier about capacitors.

When voltage is first applied to the capacitor, a small current flows until both plates become saturated.This indicates that the voltage across the capacitor equals the supply voltage. At this point no more current can flow and is essentially an open circuit. This is why a Capacitor blocks DC. 

Increasing the voltage further will damage the capacitor. 

Why are capacitors used in DC circuits if they block it?

If capacitors block DC, why can you find them in so many DC circuits? Even though they block DC we can use that ability to good effect in DC applications. This comes down to the fact that it takes time to fully charge a capacitor. This unique ability can be used to great effect in DC circuits to reduce noise in power supplies and reduce high voltage transients. It is very common (and advised) to place a capacitor across an IC’s (Integrated Circuits) supply to source current which helps in avoiding drops in voltage. They are also found in rectifiers to smooth out voltage ripples from AC inputs.

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So, what exactly is the difference between an Inductor and Inductance? Inductance is the characteristic of a conductor to oppose changes in current by producing an electromotive force.  An Inductor is an electronic device whose main purpose is to oppose changes in current in a circuit by utilising inductance. The inductance of an inductor can be altered by changing its physical properties which include the core material used for the inductor, increasing the cross sectional area of the magnetic core, and increasing the number of turns of the coil. 

What is inductance?

Conductors have many different properties which include electric resistivity, magnetic permeability, conductivity, malleability, ductility, thermal conductivity, etc. Another important property of a conductor is its Inductance, which defines the ability of the conductor to oppose changes in current in the form of an electromotive force or voltage. The units of the inductance are given in Henrys (named after Joseph Henry who first discovered inductance) denoted by H. However, you might also associate inductance with the letter L. One Henry causes a Voltage of one volt, when current is changing at a rate of one ampere per second.  The inductance of an inductor is largely influenced by the cross-sectional area of the conductor, as well as the magnetic permeability of nearby materials. 

Below is the formula for inductance for a coil of wire;

What is an inductor?

An Inductor is an electronic component whose main purpose is to provide inductance in electrical and electronic circuits. Inductors are constructed using an inner core material (with high magnetic permeability), and a coil of wire wrapped around this core (as seen in the image below).

An inductor is designed to have a certain amount of inductance which is controlled by manipulating the different variables in the equation for inductance;

• Number of turns of wire of the coil (the more turns, the higher the inductance).
• Coil area 
• Coil length
• Core material permeability (the greater the magnetic permeability of the core material, the greater the inductance). 
• Core material cross-sectional area

The ability of an inductor to provide inductance in circuits has many different applications which include;

• Tuning circuits,
• Choking, 
• Blocking,
• Attenuating and
• Filtering/Smoothing high frequency

Difference between an inductor and inductance

The electrical and electronic world is filled with a plethora of components, devices, terms, units, etc. So things can get confusing fast. One major confusion is the difference between an Inductor and Inductance. As we have just taken a look at inductance and inductors individually, we can look at the differences between them. Inductance defines the property that all conductors have, which is to oppose changes in current. Inductors are components designed to have higher levels of  inductance (compared to a straight piece of wire) and provide inductance in circuits. So, inductance is a property, and inductors are components with a set value of inductance. The amount of inductance that an inductor has can be controlled by varying the number of turns of coil, coil area, coil length, core material cross-sectional area, and core material magnetic permeability.

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So, can a potentiometer be used as a fixed resistor? Yes, a potentiometer can be used as a fixed resistor. To do this you will need to set the potentiometer as a variable resistor and then set the desired value of resistance. Potentiometers come in a range of resistances, which you will need to choose accordingly. 

But, is using a potentiometer as a fixed resistor the more important question. This shall be discussed in detail in this article. 

What is the difference between a resistor and potentiometer

The main difference between a fixed resistor and a potentiometer is resistance. A fixed resistor is aptly named that because it has a ‘fixed’ value of resistance. Its resistance value cannot be altered. On the other hand a potentiometer is a component that can vary its resistance (when set up as a variable resistor) between a minimum value (usually 0), and a maximum value (which could be 1K, 2K, 5K, 10K, 22K, 47K, 50K, 100K, 220K, 470K, 500K, 1 M). 

The fixed resistor has two terminals, while potentiometers have three terminals. The potentiometer also has the ability to vary voltage (but we won’t worry too much about that in this article). 

Can a potentiometer be used as a fixed resistor?

Yes, a potentiometer can be used as a fixed resistor. The potentiometer has the capability to vary its resistance (between a minimum and maximum value). Its resistance can be any value between these two points. So you can set the resistance to your desired value. 

Note, the required resistance value will need to be within the range of the potentiometer. For example, if you require a resistance of 1K, and the potentiometer you have has a range of 0 -1K you are fine. You just set the pot to its maximum value. However, if you need larger resistance, say 2.2K, this pot will not be suitable. 

How to use a potentiometer as a fixed resistor?

A potentiometer can be used to vary voltage and resistance. But, if we require it to be used as a fixed resistor, we need to set it up to vary resistance. There are two ways to set up a potentiometer to vary resistance.

Option #1: 

The first option is to connect one of the outer terminals of the potentiometer to its middle terminal (as seen below). Then the pot becomes a two terminal device just like the fixed resistor. Then you only need to use the outer terminals. 

Option #2:

For the second option, we ignore one of the outer terminals and just use the middle and other outer terminals (as seen below). 

Is it effective to use a potentiometer as a fixed resistor?

While you can use a potentiometer as a fixed resistor, you gotta ask yourself is it really effective? And, unfortunately the answer is no. There are a couple of reasons to ponder and reconsider using the potentiometer as a fixed resistor.

The first reason is that the potentiometer is too big compared to a normal fixed resistor. If space isn’t an issue then you need not worry. However, if your circuit is confined to small boundaries, the potentiometer is going to take up a lot of space which isn’t ideal.

Next, the potentiometer does not have the same temperature stability as a fixed resistor. If it is going to be subjected to varying temperatures (high and low), the potentiometer is not going to provide a stable resistance.

Potentiometers have poor performance compared to a fixed resistance at higher frequencies. When potentiometers are subjected to high frequencies, their inductance and capacitance increase. This directly affects their overall performance.

Lastly, the potentiometer has less mechanical stability compared to a fixed resistor. To change the resistance of the pot you need to mechanically do so (using a knob, or slider). This means it has moving parts. Having moving parts means that there is a chance that the knob or slider could be disturbed thereby affecting the resistance as well.

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• Reducing voltage
• Temperature control
• Acting as a fuse
• Timing, Waveshaping, Filtering
• Pull-up and Pull-Down resistors
• Biassing active elements
• Terminating transmission lines
• Heating

This article shall take an in-depth look at each of these different functions of a resistor in a circuit. 

What is a resistor?

The electrical and electronic world is filled with many different components, and devices, each have a set of unique abilities. One of those components is the Resistor. The resistor is one of the most common components which you will find in almost every circuit. It is a passive electronic component whose main purpose is to provide resistance and limit the flow of current. The resistance of a resistor is measured in Ohms (Ω). The amount of current that is allowed to flow is determined by a resistor’s resistance level. A higher resistance means less current can flow, and a lower resistance results in more current flow. 

The resistance of the resistor is determined by Ohm’s law which states that the current through the resistor is proportional to the voltage across its terminals.

Common resistor functions in a circuit

Resistors are widely known for being able to limit the flow of current. But, there is more to this unassuming component than meets the eye. A resistor has many other functions in electrical and electronic circuits. 

Resistor function #1 in a circuit: Limit flow of current

The first function of the resistor is its main purpose; limiting the flow of current. So, why is limiting the flow of current in a circuit important? All components have Power ratings which tell us the maximum levels of current and voltage they can handle. Exceeding these levels for extended periods of time will damage the component. So, the power to components needs to be limited to avoid damaging them. But, a power supply is fixed in its value. To combat this issue you can place a resistor which will be able to limit the current to components and protect them from damage. 

Limiting the flow of current also has other applications. As we just saw that by controlling the current to a component (using a resistor) we also gain the ability to control the power it receives. This power control can be used to control the speed of a motor, adjust the brightness of a light, alter musical tone pitch, control the amplitude of an amplifier, as a few examples. 

Resistor function #2 in a circuit: Reduce voltage

The next common function that a resistor has in a circuit is Reduce voltage. A resistor is going to have a voltage drop across it. The magnitude of the voltage drop is determined by ohm’s law which is equal to the current through the resistor, multiplied by the resistance of the resistor. The greater the resistance the greater the voltage drop. 

Now when two resistors are placed in series and connected to a supply voltage (as seen below), the configuration is known as a Voltage Divider. 

This configuration gives us the ability to reduce the supply voltage to a desired value. This value is determined by the resistances of the two resistors and is calculated using the formula below.

To achieve a desired output voltage, we first set the resistance for R2. Then we rearrange the formula to calculate the resistance of R1 which will give us our desired output voltage.

For example, say we required a 4V at our output (the supply voltage is 5V). We first select the resistance of R2 (1K). We then rearrange the equation to make R1 the subject (as we just saw above), and plug in the values. This gives us a value of 250. So to get a 4V output, you will require a resistor with a resistor of 250Ω for R1. However, a note has to be made. The value of 250Ω might not be available, so you will just have to select the next best value according to the Standard Resistor Value E Series

This ability of a resistor to reduce voltage in the voltage divider configuration can be utilised as a voltage regulator to step down a voltage. reduce the input voltage to a microcontroller or bias active elements (as we shall see later). 

Resistor function #3 in a circuit: Temperature control

When current flows through a wire, electrical energy is converted to heat energy. This phenomenon was discovered by an English physicist named James Prescott. He formulated that the heat generated per second by a conductor carrying current is proportional to the conductor’s electrical resistance and the current through the conductor squared. This formula and phenomenon is known as Joule’s Law. 

Q = I² x R x T (Joule’s Law)

Q = Heat

I = Current

R = Resistance

T = Time

As you can see from the formula, heat has a directly proportional relationship with resistance. This means that by varying the resistance of a resistor we can control temperature. 

Resistor function #4 in a circuit: Fuse

As we saw earlier, all components and devices have maximum power ratings which should not be exceeded. By properly designing our circuits we can ensure that these current and voltage levels are within safe operating conditions. However, even with proper designing, circuits can still be subjected to high currents which are known as Overcurrents. Overcurrent is a situation where the current rises to levels above the normal current levels of a circuit. 

Possible causes for over currents include;

• Short circuits
• Excessive loads
• Incorrect design
• Arc fault or,
• Ground fault

Fuses are electrical safety devices that protect circuits and components from overcurrent. It consists of a thin wire that melts when excessive current (above the nominal levels) flows through it. 

A resistor has the ability to operate as a fuse. This type of resistor is known as a fusible resistor. It melts just like a fuse when excessive current flows through it, protecting the circuit and components from damage. But, the benefit of a fusible resistor is that it acts as a normal resistor under normal operating conditions.

Resistor function #5 in a circuit: Filtering, Wave-shaping, Timing

Resistors can do awesome things by themselves, but when they combine their abilities with other components they can achieve even more. The two components that are best friends with the resistor are the Capacitor and Inductor. The resistor can be combined with these components to form RC (resistor and capacitor) and RL (resistor and inductor) circuits.

Each type of circuit has its own set of unique applications which include Filtering, Waveshaping and Timing.

Resistor function #6 in a circuit: Pull-up and Pull-down resistors

Microcontrollers are very important devices in Embedded Systems. They act as the brains of these systems in charge of controlling operations, flow of information and input and outputs. Microcontrollers are digital devices with pins that can be in either two states or logic levels;  ON (HIGH) or OFF (LOW) which is represented by binary; 1 and 0 respectively. These pins can be configured either as inputs or outputs. Buttons and switches can be connected to these pins when configured as an input. However, if we just connect a button to a microcontroller, the input would be left floating (which essentially means the pin could float between the two logic states) which is an unwanted scenario. 

Resistors are used to help tie or secure an input pin to a logic level so it is not left floating. A resistor can be connected to ground (0 or LOW). This type of configuration is known as a Pull-Down Resistor and helps secure a LOW logic state. Or, it can be connected to the supply voltage (1 or HIGH). In this configuration it is known as a Pull-Up Resistor and helps secure a HIGH logic state. 

Resistor function #7 in a circuit: Bias active elements

Active elements are devices with the capability of generating electrical energy. They take an input signal, and produce a larger signal at its output. Transistors are common active elements and an important component in the world of electronics. They have two specific roles; amplify electric current or block it. 

Transistor biassing is a crucial part when working with transistors which involves setting a transistor’s operating voltage or current conditions to the right levels so as to ensure input signals are amplified correctly. Resistors are used to help bias transistors. It can be done in two ways either by using a single feedback resistor or a voltage divider configuration.

Resistor function #8 in a circuit: Terminate transmission lines

We live in the age of information. We are constantly sending and receiving information on a daily basis. One way we send information is via transmission lines. Resistors are used in transmission lines to dissipate energy in the form of heat. But, why? They are used to absorb energy to prevent reflections within the cable. In an ideal transmission line (one without a resistor), when a piece of data reaches its destination it will still have energy and reflect back and forth interfering with other data. As we know now, resistors dissipate energy in the form of heat which comes in useful in this exact application. The resistor can absorb this energy so that the data does not reflect back and forth. 

Resistor function #9 in a circuit: Heating

Last but not least for resistor functions is Heating. It has been mentioned numerous times already, but it won’t hurt mentioning it again one last time; when electrical current passes through a resistor, energy is lost in the form of heat. This heat comes from electrical energy being converted to thermal energy. Most of the time this is unwanted. The less hot a resistor gets the better. But, there are applications where this heat can be put to good use. Fan heaters, toasters, are some of the most common devices that make use of resistors for heating purposes. Other applications that make use of this include Aeronautics, Automotive, Military and Smart clothing industries.

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Just like many other components, resistors are prone to failure. Each type of resistor will have a limit to the amount of current and voltage it can handle (depending on factors such as material, size, resistance, etc). These are known as their ratings. Exceeding these ratings for extended periods of time can cause the resistor to fail. 

So, what exactly happens when a resistor fails? When a resistor fails, it can burn out and become permanently damaged causing an open circuit. The failure of a resistor can have a domino effect causing overload conditions (high voltage and current) on other components in the circuit, which could lead to their damage as well.

How does a resistor fail?

Before delving into what happens when a resistor fails, it will help to understand how exactly a resistor can fail. 

Power ratings are an important characteristic of all electrical and electronic components. Power ratings include voltage, and current. These ratings tell us the maximum amount (of current and voltage) that a component like a resistor is able to withstand while being able to perform its function efficiently and effectively. Exceeding these limits for extended periods of time can cause permanent damage. 

But, how does exceeding the ratings cause it to fail? Resistors come in a variety of shapes, sizes, materials, and resistances. These factors ultimately determine the ratings of the resistor. For example, a larger resistor will be able to allow more current (electrons) to pass through it compared to one with smaller physical dimensions. Now if we take those electrons flowing through the large resistor and force them through the smaller resistor, the electrons are going to start bumping into each other as there isn’t much room for them to move about freely. The collisions between these electrons is going to cause friction which is going to lead to the generation of heat. More collisions will increase the level of heat. If the levels of heat get too high, they will damage the resistor. 

The two images below show two scenarios, where few electrons can move freely with few collisions (Scenario A), versus when there are too many electrons flowing through the same area causing more collisions (Scenario B). 

So resistors of different sizes will have different ratings. A note has to be made though. Sometimes smaller resistors might be able to handle higher levels of power compared to bigger ones. This comes down to the material being used, as some materials allow the flow of electrons more easily.  

Things that can happen when a resistor fails

Exceeding a resistor’s power ratings is not ideal, and should be avoided at all costs. However, sometimes even with careful planning accidents can occur and a resistor might end up failing. So, what exactly happens when a resistor fails? When a resistor fails, it is going to get damaged physically. It will first start to get hot to the touch. Then if the heat persists for too long, it is going to start to melt (which you will notice visually, or be able to smell) and possibly smoke as well. This is going to permanently damage the internal structure of the resistor. Now, when a resistor does fail and gets permanently damaged, it is going to become an open circuit. 

A resistor failing and becoming an open circuit is going to cause issues in other parts of the circuit by possibly causing overload conditions such as placing a high voltage on other components (which could potentially damage them as well).

Things that can cause a resistor to fail

Sometimes the failure of a resistor might not have any dire consequences (it might be part of a simple circuit). Other times it could end up causing havoc on the other parts of the circuit. Overall, a resistor failing is best avoided. Knowing what causes a resistor to fail can help prevent exactly that. Below are a few things that could result in the failure of a resistor.

Things that can a cause a resistor to fail #1: Improper Circuit design

The first cause is improper circuit design. This can involve choosing a resistor with the wrong resistance, or power rating. This can occur from having calculated the wrong resistance initially in the design process, or by mistakenly placing the wrong value resistor in the circuit. As we saw above, if a resistor’s power ratings are exceeded, it will fail.

Things that can a cause a resistor to fail  #2: Overload Conditions

Just as the failure of a resistor can cause overload conditions in other parts of a circuit, so too can the failure of a component elsewhere in a circuit cause overload conditions on a resistor inevitably placing excessive power on the resistor.  

The possible cause for other components to fail include ageing of components (causing their characteristics to change and behave differently), short circuit, or choosing the wrong values (improper circuit design).

Things that can a cause a resistor to fail  #3: Short circuit

Just as a short circuit on another component (thereby damaging it), can cause overload conditions, a short circuit on a resistor can cause it to fail as well. A short circuit is described as the connection of two nodes in an electrical circuit which are at different voltages. This can cause an excessive current to flow through the resistor. Possible causes for short circuits can include water, loose connections, improper soldering connections, loose debris (that is conductive) etc.

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So, what is the main difference between a resistor, capacitor and inductor?

The main difference between a resistor, capacitor and inductor is what each does with energy. A resistor dissipates energy in the form of heat, a capacitor stores energy in the form of an electric field, and an inductor stores energy in the form of a magnetic field. Also, each of these components have different functions which play an essential role in electrical and electronic circuits. This article shall take a more in-depth look at each of these.

The resistor, capacitor and inductor

Before we take a look at the differences between these three components, let’s take a brief look at each component to see what they are all about.

What is a resistor?

Of the three components, the resistor is the most commonly used. The resistor plays a vital role in electrical and electronic circuits. Its main purpose is to limit the flow of current. It does this by providing a resistance to the flow of current. The greater the resistance, the less current can flow, and the lower the resistance, the more current can flow. Current is a form of electrical energy and when it flows through a resistor, this energy is converted into heat energy (which is dissipated into the surroundings). 

Other than just limiting current, a resistor can be used for many other purposes which include, voltage division, heat generation, matching and loading circuits, gain control and setting time constants. 

What is a capacitor?

Next up we have the capacitor. A capacitor is composed of two conducting plates that are separated by a dielectric (which is an insulating material). The main purpose of a capacitor is to store energy in the form of electrical energy. This stored energy can be released back into the circuit when required. The amount of electrical energy a capacitor is capable of storing is determined by its capacitance. The higher the capacitance, the more energy it can store, and vice versa. Capacitors allow Alternating Current (AC) to pass, but block Direct Current (DC).

Other than energy storage, capacitors are used for power conditioning, noise filtering, remote sensing, and signal coupling/decoupling. 

What is an inductor?

Last, but not least, is the Inductor. Inductors, also sometimes referred to as a coil or choke, are an electronic component that stores energy in a magnetic field when current flows through it. They are constructed using an insulated copper wire that is wound into a coil around a core (which is usually magnetic iron or ferrite). Inductors also have the ability to oppose changes in current which is determined by their Inductance. The higher the inductance the more effective an inductor is at opposing changes in current. Because of this fact, inductors block AC, but allow DC to pass. A wire with more coils (turns) is going to have a higher inductance. 

The applications of inductors include; choking, blocking, attenuating, filtering/smoothing high frequency noise, storing and transferring of energy. 

What is the difference between a resistor, capacitor and inductor?

The main difference between a resistor, capacitor and inductor, is what happens with current flowing through them. Energy is the common theme they share, however, what happens with energy in each of them varies. When current flows through a resistor, energy is dissipated in the form of heat. In a capacitor energy is stored in the form of an electric field when current flows through it. And when current flows through an inductor, energy is stored in a magnetic field. 

Other than that, the other major differences between these components include;

• Main functionality
• Construction

Main functionality

Other than what each does with energy, the other difference between a resistor, capacitor and inductor, is the main functionality and the applications they are used in. The main function of a resistor is to limit current, whereas the main function of a capacitor is to store charge for later use and the inductor’s primary purpose is to oppose any change in current. As there is a difference in their functionality, so too are the applications they will be used for.

Construction

How a resistor, capacitor and inductor is constructed is another difference. As we just saw, each has a different function. They get these unique functions due to the way that they are constructed (as well as the materials that they are constructed with). 

Difference between a capacitor and inductor

Of the three components, the capacitor and inductor are quite similar in that they both store energy. But, the way they store energy is their major differentiation. A capacitor stores energy in an electric field, while an inductor stores energy in a magnetic field. But, there is another difference between these two components. 

There are two types of current that can flow through an electrical/electronic circuit. The current can either be an Alternating Current (AC) or a Direct Current (DC). When it comes to a capacitor, it blocks Direct Current, but allows Alternating Current to pass. Inductors on the other hand allow Direct Current to pass, but block Alternating current.

Summary of the differences between a resistor, capacitor and inductor

Below is a table summarising the differences between a resistor, capacitor and inductor. 

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Since they are so vital to circuits, it is essential that they have longevity and do not damage easily. 

But, can a resistor go bad?

Yes, a resistor can go bad. A resistor will go bad under overload conditions which include excessive current flowing through the resistor (past its rated limits). This excessive current is going to cause the resistor to overheat and get damaged. When it gets damaged, it will not be able to perform its primary function which is  limiting current flow. There are other factors such as  extreme conditions (high heat and humidity),  which can potentially cause the resistor’s resistance to increase, as well as build up of dirt that can break down the insulation causing a short circuit. 

What do we mean by a bad resistor

Before we dive into what possible factors could cause a resistor to go bad, we first need to define what ‘go bad’ means. When we say go bad, we are saying that the resistor has failed (physically), and cannot perform its primary function (of limiting current) anymore. 

But, a resistor failing can appear in different forms which can include;

• Open Circuit 
• Short Circuit
• Decrease or Increase in resistance 

We shall cover how each of these failures occur in the next section. 

What can cause a resistor to go bad?

Below is a list of the possible factors that could cause a resistor to go bad;

• Overload conditions
• Mechanical stress 
• Accumulation of dust/dirt
• Extreme conditions

Overload conditions

Resistors, just like other components, have certain voltage and current ratings. These ratings tell us what values of voltage and current a specific resistor can operate at effectively. Venturing past these limits will cause the resistor to act differently. But, if these ratings are exceeded well above their limits, permanent damage ensues. This is the first way a resistor can go bad. Under overload conditions, when current or voltage values exceed the resistor’s ratings, the resistor is going to start to heat up. If the resistor gets too hot, it is going to melt or burn which will destroy the internal structure of the resistor.

Mechanical stress

With fixed resistors this is rare, but can still occur. Mechanical stress relates to excessive shock or vibration. Resistors are quite robust components, but under extreme mechanical stress they will fail. This will most likely end up in an open circuit. 

Accumulation of dust and dirt

Unfortunately, the world is filled with dirt and dust, and they seem to get everywhere, even in the smallest of electronic circuits. If dirt and dust accumulate on or in a resistor, it can break down the insulation coating of the resistor which can lead to a short circuit. 

Extreme conditions 

Like voltage and current, resistors also have temperature ratings which is a range of temperature that tells us the ideal temperatures that a resistor will operate effectively. Veering outside these limits is going to affect the characteristics of the resistor, mainly being its resistance. This is unwanted as a resistor is chosen for a specific resistance. A resistor’s resistance changing will cause a domino effect and affect other parts of a circuit. Under persistent temperature change outside its normal temperature range, a resistor’s resistance might permanently change. 

Also, if humidity and heat levels rise to extreme levels, , this could potentially melt the resistor’s insulation coating causing a short or open circuit condition. 

How do you know if a resistor has gone bad?

Now we know that a resistor can go bad, the next step is being able to identify when a resistor has been damaged. There are a couple ways that you will be able to do so which include physically and electrically. 

When a resistor has been damaged physically, you will be able spot this visually. The outer insulation of the resistor will be cracked, broken, burnt (appear black), etc, due to excessive heat generated from overload conditions, or excessive shock/vibration. However, some of the time, physical damage might be internal. Here, you will not be able to tell if a resistor has gone bad visually. If it has been damaged but there are no visual clues, you will be able to spot the issue electrically. A circuit would have been designed with specific voltages and currents. If a resistor gets damaged, the voltage across the resistor, as well as other areas of the circuit will change and not match the designed values. 

Also, if the resistance of a resistor has changed, you know that it has gone bad. But how do you check the resistance? We will learn about this in the next section. 

How to test a resistor if it has gone bad?

As discussed above, if you cannot visually tell if a resistor has gone bad, you can still identify issues electrically. One useful and essential tool in the electrical and electronics world is the Multimeter. The multimeter is an electronic measurement instrument that allows you to test and read voltage, current, resistance, continuity, capacitance, and more. 

When it comes to a bad resistor, there are a couple functions of the multimeter that can aid us in identifying the issues. These include the voltage, resistance, and continuity functions. 

Voltage

The first function of the multimeter which will help us identify a bad resistor is the voltage function. It allows us to measure voltage across components or at certain points in a circuit. Since we are dealing with a fixed resistor, the voltage drop across the resistor should be fixed as well. This voltage can be calculated theoretically and measured to ensure that it is correct. If a resistor is damaged, it will either become an open or short circuit. Now, if we measure the voltage across the resistor it would have changed. 

Resistance

Next we have the resistance function. This multimeter function gives us the ability to measure the resistance of any resistor (as long as it is within the multimeter’s range). As we learnt earlier, extreme conditions can affect the resistance of a resistor. If a circuit is acting abnormally, it could be that the resistance of a particular resistor has increased or decreased. But, you would not be able to tell purely on visual inspection. By using the resistance function, you will be able to measure the resistance and ensure it is the right value. 

Continuity

Last up we have continuity. Electrical continuity in electronics is the presence of an unbroken path for current to flow through. For example, a wire that has no breaks between one end and the other is said to have electrical continuity. However, insulation covers most wires and components to protect us from harm. This makes it hard to identify breaks within a wire or component. The multimeter’s continuity function allows us to detect if a wire or component has continuity without having to rip it apart. So, if a resistor has been damaged internally you can use the continuity function to check. However, a note should be made, continuity might not work with higher value resistors. In this instance you will have to use the resistance function and see whether you can measure resistance. A damaged resistor will not have any resistance (or its resistance will not match its resistor colour code). 

Below are steps on how to spot a potential bad resistor;

1. Check for any physical damage visually (burnt, melted, broken, ruptured, etc)
2. Measure voltage across the resistor. If not normal the resistor will be damaged.
3. Check continuity
4. If you cannot check continuity, measure resistance and make sure it matches with its colour code. Note, when checking resistance, make sure the power to the circuit is turned off and all capacitors have been discharged.

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So, does a resistor go before or after a LED?

A resistor can go before or after a LED. The current before and after the LED (and any other point in the circuit) remains the same, so it does not matter whether you place the resistor before or after the LED. The resistor will still limit the current the LED receives so it does not get damaged. 

Why it doesn’t matter if a resistor goes before or after a LED

To better understand why it doesn’t matter whether a resistor goes before or after a LED, we need to learn about current in a circuit.

In electrical and electronic theory, when it comes to closed loop circuits, the current flow out of a battery is equal to the current that flows back into the battery. So what this tells us is that the current at any point along the closed loop is going to be the same. The image below shows a simple closed loop series circuit with a battery source and a LED (let’s assume the battery is supplying the right voltage and current so no resistor is needed). The current before and after the LED (or any point in the circuit) remains the same at 100mA.

Now if we increase the voltage of the battery, we will need to maintain the 100mA of current so the LED does not get damaged. To this we are going  to require a resistor. The main purpose of a resistor is to limit the current in a circuit to protect components like a LED. By adding a resistor to the previous circuit, we are simply dropping the amount of current. 

But, the placement of the resistor does not matter because the current leaving the battery is still going to equal the current re-entering the battery. All the resistor is doing is limiting the amount of current that will flow out of the battery. 

This is why it does not matter whether a resistor goes before or after a LED.

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Effects of using a higher or lower ohm resistor

Knowing whether you can use a lower or higher ohm resistor comes down to the design and needs of the circuit it will be used in. As we saw above, the main use of resistors is to limit the flow of current. Current is the lifeblood of electrical and electronic circuits. Without it nothing would be able to function right. 

Components all have voltage and current ratings which indicates the sufficient amount of current and voltage needed to operate it effectively. Supplying a current or voltage considerably less than this means the component will not operate as intended (or not operate at all).

Also, all components have maximum voltage, and maximum current ratings. These ratings let us know what are the maximum levels that they can operate at. Going past these levels could lead to permanent damage. So, resistors are used to help limit the current to levels that sit below these maximum current ratings. 

Note, when designing a circuit, all components will be chosen with the same voltage and current ratings. 

So, knowing the current limits of your circuit (which includes all the components) where you will be replacing a resistor with one with a lower value one is essential. But, how does lowering or increasing the ohm value of a resistor affect the overall workings of a circuit? This comes down to Ohm’s Law. 

Ohm’s law states; “that the current through a conductor (between two fixed points), is directly proportional to the voltage across those two points.” This law is summarised by a formula seen below.

When we rearrange the formula to make Current (I) the subject, we see that current is voltage divided by resistance. This shows us that current and resistance share an inversely proportional relationship. What this means is that increasing the resistance will decrease the current, and that decreasing the resistance will increase the current. So, by increasing or lowering the ohm value of a resistor you will be directly affecting the current level.

Note, ohm is the SI Unit of electrical resistance. So, when ohm is mentioned, we mean the resistance of the resistor. 

When can you use a lower ohm resistor?

You can use a lower ohm resistor only when the value of the resistor you will be using does not drastically increase the current levels in the circuit.  As we saw, decreasing the resistance will increase the current (according to Ohm’s Law). If the lower ohm resistor increases the current past maximum current ratings of components used in your circuit, you could potentially damage them permanently. So, you will need to select a resistor with a lower ohm value that does not increase the current past these levels. 

An example will help you better understand. 

Say we have a simple circuit of a battery source, resistor, and lamp (as seen below). In this scenario the lamp is rated for a voltage of 12V and current of 1A (these are just arbitrary values). Using these values and the ohm’s law formula we can calculate the resistance of 12 ohms (12/1). 

Also, let’s say this lamp has maximum ratings of 12.5V and 1.5A. So, operating at or going above these values risk the permanent damage of the lamp. Let’s see what happens when we lower the resistors ohm value by 10% vs 50%. Which will have a greater effect on the overall current. 

When we lower the resistor’s resistance by 10% we end up with a value of 10.8 ohms. Then using this value we can calculate the resulting current which ends up being 1.1 A (12 / 10.8). 

Now, if we lower the resistance value by 50% we get a value of 6 ohms. Using this value we get a new current value of 2A (12 / 6).

As you can see, if we lower the resistor ohm value by 10% we are still below the maximum current rating levels which means no damage will be done to the lamp. However, when we decrease the resistance by 50% the current jumps up to 2A, well outside the maximum current ratings of the lamp. 

So, in this scenario, lowering the resistance by 10% has no real effect on the overall workings of the circuit, whereas lowering it by 50% can be detrimental. 

When can you use a higher ohm resistor?

You can use a higher ohm resistor as long as the new resistor does not drastically lower the current in the circuit. According to Ohm’s law, increasing the resistance will decrease the current. If the levels of current drop below the current ratings of components, they will not operate as intended. 

Let’s take a look at the same example from above. However, this time instead of lowering the ohm value, we shall increase it. Again, let’s increase it by 10% vs 50%. 

An increase by 10% gives us a resistance of 13.2 ohms which ends up with a current of 0.9 amps. 

But, an increase of 50% gives us a resistance of 18 ohms, which ends up with a current of 0.6 amps. 

While both are below the levels of current necessary to power the lamp, the 50% increase in resistance shows a far steeper decrease in current compared to the 10% increase. So, increasing the resistance by 10% shouldn’t see much of a drop in the overall performance. 

Rule of thumb for selecting a higher or lower ohm resistor

As we saw above, drastically lowering or increasing the resistance (say by 50%) can have more impact on the current in a circuit. An impact that could damage components, or not sufficiently power it. As a rule of thumb, a better option would be to increase or lower the resistance by 5% – 10%. This way you are not going to have any drastic changes in your circuit. 

When we covered the example above, we were just going through theoretical resistance values. However, resistors do not come in all available values. They come in a set of standard resistor values which are known as the E-Series. The E-12 series is the most commonly used, which uses 12 standard resistor values for each decade. The 12 resistor values are; 1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, and 8.2. 

So when replacing a resistor to use a lower or higher ohm one, you will have to use one of these values. Sometimes you might not get the exact value you want, but as long as it’s near to your calculated value you should be fine (make sure to calculate the resultant current values with the new value of the resistor chosen). 

For example, if you want to replace a 150 a resistor with a lower one, and we go by our rule of thumb (let’s say 10%), we get a calculated value of 135 ohms. However, there isn’t a 135 ohm value resistor available. So the next best option would be a 120 ohm resistor.

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Polarity and Continuity are terminologies that are associated with electrical and electronic components, and questions always arise whether resistors have polarity and continuity. This article shall delve deeper into these two concepts and see if resistors have polarity or continuity. 

What is polarity?

Polarity is a term that is used in many branches of science and technology such as electricity, magnetism, and chemistry. It is described as ; “a state or condition of an atom or molecule that exhibits opposite properties or powers in opposite parts or directions.” For example, a magnet is known for having North and South poles. Here magnetic polarity refers to the orientation of these poles in space. 

But, the polarity we are concerned about in this article pertains to electricity. So what exactly is electric polarity? 

Electric Polarity

Voltage can be defined as an Electromotive Force (EMF) between two points. When we are talking about these two points, we are looking at which point has more electrons than the other. The point that has more electrons is the Negative Pole, making the other point a Positive Pole.

Now when a conductor (such as wire) is used to close the path between the two poles, electrons flow from the negative pole to the positive pole (this flow of electrons is known as current). An example of this is a battery which has negative and positive terminals (or poles). Connecting a wire across the two terminals will cause electrons to flow from the negative terminal to the positive.

Components and polarity

So how does polarity relate to electrical and electronic components? They can be divided into two categories when it comes to polarity;

• Polarised or,
• Non-Polarized

Non-Polarised components do not have any polarity. These components can be placed in a circuit in any orientation without altering their functionality. 

Polarised components on the other hand have polarity. This means they have been constructed in a way where one of their terminals has more electrons than the other. They will have one negative and positive terminal. Therefore they will have to be placed in a circuit in the right orientation (their terminals must match the terminals of the voltage source). A common example of a polarised component is a battery. 

Do resistors have polarity?

No, resistors do not have polarity. They do not have positive or negative terminals which means that they can be placed in any orientation when connecting them up in a circuit. The main job of a resistor is to provide resistance to limit the flow of current. They are designed uniformly so that one terminal will not have more electrons than the other. The result being no matter what their orientation, they will still provide the same resistance.

A common analogy to better understand this is a road with bumps. Just like a resistor, the bumps on the road provide resistance when you drive past them. And just like a resistor, it doesn’t matter which way you are travelling, the bumps on the road are still going to provide the same resistance.

What is continuity?

Continuity is a term used in many aspects of life. It is defined as “the unbroken and consistent existence or operation of something over time.” When it comes to electricity the concept remains the same. Electric continuity refers to the presence of a continuous unbroken path for current to flow. 

Imagine we had a closed circuit with a battery source as seen below. 

The image below highlights two scenarios. Scenario A has a  wire that connects the positive terminal to the negative terminal without any physical breaks in it. Therefore it is said to have continuity as current can flow from the negative terminal to the positive terminal without interruption.

In Scenario B we see there is a physical break in the wire. In this instance current cannot flow from the negative terminal of the battery to the positive as there is no ‘continuous path’. Therefore the wire has no continuity.

Do resistors have continuity?

Yes, resistors do have continuity. Even though they might limit the flow of electrons (by providing a resistance), they still provide a continuous path for the electrons to through. 

Testing the continuity of a resistor using a multimeter   

Knowing if a wire or component has continuity or not is essential when it comes to circuit analysis. Wires and components are insulated to protect us from potential harm. But, if damage occurs it might be hard to detect it by just looking at it as the insulation might obstruct the damage. 

A multimeter is an electrical/electronic measurement tool used to measure things like voltage, current, resistance, etc. It also has the ability to test continuity. The multimeter has two probes that get  placed on two ends of wire for example. It then sends a current from one probe to the other (via the wire). If there is a break in the wire the multimeter will remain silent indicating no continuity. But, if the other probe receives the signal, this indicates that there is no break in the wire (hence it has continuity), and the multimeter will inform you with an audible beep. 

However, when testing the continuity of resistors, some issues arise. The multimeter’s continuity function works only below a certain threshold value of resistance. Anything above this threshold and the multimeter won’t be able to detect if there is continuity or not. So, even if a resistor with a high value of resistance has continuity, the multimeter might not pick it up.

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