Like many electrical and electronic components, MOSFETs have many important characteristics that determine how it operates.

One of these characteristics is the Threshold Voltage (Vth).

So what exactly is the threshold voltage of a MOSFET? The threshold voltage of a MOSFET is the minimum gate-to-source voltage (Vgs) required to turn the MOSFET ‘ON’ (which allows current to start flowing between source and drain).

However, there are variations of  MOSFET each having different threshold voltages.

This article shall take a closer look at the MOSFET, its variations and threshold voltage. 

What is a MOSFET?

Transistors are semiconducting devices that can be used either to amplify or switch electrical signals. 

The MOSFET is part of the family of transistors which shares the same purpose as its peers. But, the way it goes about doing so is different. 

Furthermore, it is a subset of another commonly used transistor, the Field Effect Transistor (FET).

MOSFET stands for Metal Oxide Semiconducting Field Effect Transistor. 

It is a three terminal device which includes a Gate(G), Source (S), and Drain(D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four types of MOSFET seen below;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

Construction of a MOSFET

In Enhancement type MOSFETs, there is no physical connection between the source and drain, hence the broken lines in its symbol. 

On the other hand, depletion MOSFETs have a small semiconducting strip that connects the source and drain terminals. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer (SiO2) is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region. 

The substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. 

Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes.

Regions of operation of a MOSFET

The MOSFET has  three major areas of operation which include;

• Cut-Off region
• Saturation Region
• Linear/Ohmic Region

Cut-off region

When the MOSFET is in the cut-off region the device is considered to be in its ‘OFF’ state. When it is OFF, no current flows through it. 

Just like when a mechanical switch is open and stops the flow of current as well.

Saturation region

Saturation is when the current flow through the MOSFET is a constant value. In this region it behaves like a closed switch allowing current to flow freely. 

Linear/Ohmic region

Last but not least is the Linear/Ohmic region. 

In this region of operation, an increase of voltage across the drain and source sees an increase in current through the MOSFET as well.

The threshold voltage of the MOSFET

When there is no voltage present between the gate and source terminals, the MOSFET is operating in the cut-off region. 

In this region the MOSFET is ‘OFF’ as no current can flow from the source to drain.

In order to allow current to flow freely from the source to drain, we need a conducting path. This conducting path is created when the MOSFET is operating in the saturation region.

But in order to get to the saturation region we need to turn the MOSFET ‘ON’.

To do so we need to apply a sufficient amount of voltage between the gate and source terminals known as VGS. 

The minimum amount of voltage required to enter the saturation region and start conducting current is known as the Threshold Voltage or VTH. 

When VGS > VTH, the MOSFET is now said to be ‘ON’.

Threshold voltage for the different types of MOSFET

We know that there are derivatives of the MOSFET. 

But, are the threshold voltages the same for all four?

The table below summarises the conditions for the different threshold voltages for the various types of MOSFET.

*The information in this table is thanks to https://www.electrical4u.com/mosfet-characteristics/ . For a more in-depth explanation check them out. 

VTH = threshold voltage

VGS = gate-to-source voltage

VDS = drain-to-source voltage 

VP =  pinch-off voltage

What is the threshold voltage required to turn a MOSFET on?  

So, how much voltage does it take to put the MOSFET on? 

This comes down to the two main regions of operation of a MOSFET;

• Cut-Off
• Saturation 

As we just learnt, when the MOSFET is ‘OFF’, it is in the cutoff region.

The saturation region is when the MOSFET is ‘ON’ and current can flow freely from source to drain. This is when the maximum gate voltage is applied and results in the maximum amount of current that can flow. 

To get to this region of operation VGS > VTH.

The exact threshold voltage will differ for the four types of MOSFET, as  there are many factors like materials used, manufacturer, oxide thickness, etc, which will determine the threshold voltage value. 

To find out the exact threshold voltage for a particular MOSFET, you will have to look up its datasheet which will have the necessary values. 

Threshold voltage is usually denoted VGS(TH) in most datasheets. 

Is the threshold voltage of the MOSFET constant?

You might be wondering if the MOSFET is exactly like a switch, in the sense that when it is closed the maximum amount of current can flow.

The answer to that is no. The amount of current that flows is dependent on the amount of voltage applied to the gate and source (Vgs).

So, when there is more to the question of whether threshold voltage is constant or not.

There is a range of threshold voltages that range from minimum to maximum. 

When the amount of Vgs is greater than Vth, this will allow current to start flowing. But, this is not the maximum amount of current that can flow (like when a switch is closed).

The amount of drain current that can flow is determined by how much voltage is applied to the gate and source and also sits between the range of minimum and maximum threshold voltage. 

Lower values of Vgs will conduct less drain current, while higher values will allow higher amounts of current to flow. 

MOSFETs have a Drain Characteristics Curve that shows the amount of current for different Vgs values. 

Factors that can affect the threshold voltage of a MOSFET

Performance of electronic components can be affected by internal or external factors and the MOSFET is no different.

The threshold voltage of MOSFET can be influenced by a few factors which include;

• Oxide Thickness
• Temperature 
• Random dopant Fluctuation 

Oxide thickness

The type of oxide chosen to construct the MOSFET, as well as its thickness plays a large part in the threshold voltage.

Thickness of the oxide layer shares a directly proportional relationship with threshold voltage which means the smaller the oxide layer, the smaller the threshold voltage and vice versa. 

Temperature

While there is no direct relationship the side effects of temperature do affect the threshold voltage with the variation being -4mV/K or -2mV/K which is dependent on the doping level.

A change in temperature around 30 °C can see a change in threshold voltage of around 500mV. 

Random dopant fluctuation

Random Dopant Fluctuation or RDF is a process where the amount of random dopant fluctuation is reduced by lowering the dopant density. 

The RDF in the channel region of MOSFETs can vary certain characteristics, like threshold voltage.

The post Threshold voltage of a MOSFET appeared first on Electronic Guidebook.

It is a type of solid-state device used to control the flow of current. 

A MOSFET is one of the many varieties of transistors available that perform the tasks mentioned above. 

But, transistors can either be voltage or current controlled devices.

So, is a MOSFET a voltage or current controlled device? The MOSFET is a voltage controlled device. This is due to the fact that the flow of current between its SOURCE and DRAIN terminals is controlled by an input voltage at its GATE terminal.

This article shall take a closer look at why the MOSFET is a voltage controlled device. 

Difference between a voltage and current controlled device?

To better understand why a MOSFET is a voltage controlled device, it will help to learn what the differences are between a voltage and current controlled device. 

So let’s take a look. 

As mentioned at the start, transistors are a semiconductor device used for the amplification, control, and generation of electrical signals most of  which have three terminals (some have four). 

They are active devices that have many applications. One of the most common being the main component in Integrated Circuits (such as microprocessors and microcontrollers). 

Below are the many different types of transistor;

• BJT 
• FET
• JFET
• IGBT
• MOSFET

While each of these types of transistors share similar functions, the way they go about doing it is a bit different. 

Some of them are voltage controlled and the others are current controlled.

What is a voltage controlled device?

A transistor that is voltage controlled, has voltage as the controlling parameter.

What this means is that the flow of current at the output terminals is controlled by a voltage at the input terminal. 

The output current is a function of an input voltage. 

What is a current controlled device?

On the other hand, a transistor that is current controlled, has current as the controlling parameter.

This means that the flow of current at the output terminals is controlled by a current at the input terminal.

Output current is a function of an input current. 

Why a MOSFET is a voltage controlled device

So why is a MOSFET a voltage controlled device?

This comes down to its construction and working principle. Let’s take a closer look at the MOSFET and what makes it a voltage controlled device.

What is a MOSFET?

We learnt earlier that transistors are semiconducting devices that can be used either to amplify or switch electrical signals. 

The MOSFET is part of the family of transistors which shares the same purpose as its peers. But, the way it goes about doing so is different. 

Furthermore, it is a subset of another commonly used transistor, the Field Effect Transistor (FET).

It is a three terminal device which include a Gate(G), Source (S), and Drain(D).

There are two main two groups of MOSFET (Enhancement and Depletion),which can either be N-Channel or P-Channel. This gives us four types of MOSFET seen below;

• Enhancement Mode N-Channel
• Enhancement Mode P-Channel
• Depletion Mode N-Channel
• Depletion Mode P-Channel 

Construction of a MOSFET

In enhancement type MOSFETs, there is no physical connection between the source and drain, hence the broken lines in its symbol. 

On the other hand, depletion MOSFETs have a small semiconducting strip between the source and drain terminals. 

Below is an image that shows the basic construction of the MOSFET.

The Gate terminal is connected to a substrate where an oxide layer (SiO2) is placed. The purpose of the oxide layer is to act as an insulator. 

The general construction of a MOSFET consists of a lightly doped substrate which is diffused by a heavily doped region and the substrate used ultimately determines whether it is a P-type or N-type MOSFET. 

As we saw above, MOSFETs are either Enhancement or Depletion.

N-Channel Enhancement MOSFET

The source and drain in a N-Channel Enhancement MOSFET consist of a N-type semiconductor which is heavily doped and the substrate is a P-type semiconductor. Electrons are the main charge carriers. 

P-Channel Enhancement MOSFET

On the other hand, the source and drain of P-Channel Enhancement MOSFETs are of a P-type semiconductor which is heavily doped, and the substrate is a N-type semiconductor. Holes are the major carriers here.

In enhancement mode, the source and drain are isolated. 

N-Channel Depletion MOSFET

In N-channel depletion MOSFETs the source and drain are connected by a small strip of material that is a semiconductor of N-type. The substrate is a P-type semiconductor. 

The main charges are electrons while the source and drain are heavily doped.

P-Channel Depletion MOSFET

The P-channel depletion MOSFET is the opposite of the N-channel, as the strip of semiconducting material connecting the source and drain is P-type. The substrate is N-type. 

The main carriers are holes. 

Working principle of a MOSFET 

While there are different types of MOSFET, their main function remains the same which is to act as a switch.

The main operation goes a little something like this; when a voltage (VGS) is applied at the Gate (which exceeds VTH also known as the threshold voltage) of the MOSFET, the path between Source and Drain closes allowing current to flow through.

VGS is the minimum amount of voltage required to turn the MOSFET “ON” 

There are some slight differences in the operation of Enhancement and Depletion MOSFETs, but their overall functionality remains the same. 

As you can see, to turn the MOSFET ‘ON’ (allow current to flow through the source and drain), a voltage is required at the gate terminals.

In a BJT the input impedance is finite which results in a small finite base current which controls the output current. 

When it comes to the MOSFET, there is no input current due to the SiO2 layer between the gate and body. The output current is controlled by an input voltage.  

This is why a MOSFET is known as a voltage controlled device. 

How much voltage is required to control the MOSFET?

When the voltage at the input of the MOSFET is zero, no current flows at its output (source and drain). When no current is flowing the MOSFET is said to be operating in its Cut-Off region.

So, how much voltage does it take to the MOSFET on? 

This comes down to the two main regions of operation of a MOSFET;

• Cut-Off
• Saturation 

As we just learnt, when the MOSFET is ‘OFF’, it is in the cutoff region.

The saturation region is when the MOSFET is ‘ON’ and current can flow freely from source to drain. This is when the maximum gate voltage is applied and results in the maximum amount of current that can flow. 

To get to this region of operation VGS > VTH.

The threshold voltage (VTH) is never going to be the same for all MOSFETs as they might come from different manufactuers.

However, this value can be found in the datasheet of the MOSFET.

The post Is a MOSFET a voltage or current controlled device? appeared first on Electronic Guidebook.

The world is filled with many variations of battery each having its own unique characteristics. 

One of the most common batteries used nowadays is Lithium-Ion.

Sooner or later, the Lithium-Ion is going to go dead (lose all its charge), and if it is a rechargeable battery, will need to be recharged.

Letting a battery go fully dead is not an ideal situation, so knowing at what voltage a Lithium-Ion battery loses all its charge will help you extend its lifespan.

So, at what voltage is a Lithium-Ion battery considered dead? The voltage at which a Lithium-Ion is dead is around 3.4 volts. This voltage can vary depending on factors such as the temperature and also its manufacturer. Lithium-Ion batteries should not be used when they are dead.  They contain a cutoff circuit to protect them from being used past the voltage at which they have lost all its charge. 

This article shall take a look closer at Lithium-Ion batteries and its discharge profile. 

Deeper look at a Lithium-Ion battery

There are many batteries that exist in the world today, and while they all share one main goal, which is to provide power to electrical and electronic devices, they differ in many different characteristics.

Characteristics such as;

• Chemical composition
• Nominal voltage
• Current capacity 
• Shape
• Size
• Energy Density 
• Specific Energy density

The main characteristic here that plays a major part in determining many of the other factors (such as voltage, current capacity, energy density, etc), is the Chemical Composition.

Batteries produce electrical power from chemical reactions that occur inside the battery. There are a range of chemicals that are used in different batteries which include;

• Nickel-Cadmium (Ni-Cd)
• Zinc-Carbon 
• Lithium-Ion (Li-Ion)
• Lead-Acid 
• Alkaline 

The chemical composition we are most concerned about for this article is Lithium-Ion (Li-On). 

The battery is constructed using cells where lithium-ions move from the negative electrode through an electrolyte towards the positive electrode. 

Lithium-Ion battery nominal voltage 

To better understand at what voltage a Lithium-Ion battery is dead, it will first help to understand the voltage at which it is operational.

The voltage of the battery is one of the most important characteristics when selecting a battery for a particular application. 

All electrical and electronic devices have a specific voltage rating that they require to operate efficiently and effectively. 

So, you will have to select a battery with the exact voltage (or a bit higher), to satisfy the needs of that device. 

The voltage of a battery refers to the amount of electrical potential it is able to hold, and is given in the standard international unit of Voltage (V).

All batteries have a theoretical voltage, however, the actual voltage (nominal voltage) produced will be lower.

This is due to polarisation and resistance losses, and is largely dependent on the current drawn by the load and the internal impedance of the battery. 

The maximum voltage that a lithium-ion battery is capable of producing is 4.2V, however this will soon drop to its nominal voltage of 3.7V. 

Different types of Lithium-Ion battery

Lithium-Ion batteries come in a variety of shapes and sizes to suit the needs of many different applications, from power tools to RC planes. 

Below are the different shapes available for lithium-ion batteries;

• Small cylindrical (single cell with, solid body, with no terminals)
• Large cylindrical (single cell,solid body, with threaded terminals)
• Flat or pouch (soft, flat body)
• Rigid plastic case (large threaded terminals) 

But, do the  different variety of shapes of lithium-ion batteries share the same voltage? 

Yes, while they vary in size, the batteries share the same nominal voltage of 3.7V. 

What about devices that require larger voltages, and use lithium-ion batteries? To generate a larger voltage, lithium-ion batteries can be connected in series. 

Note, this process is a bit more complicated than connecting other batteries in series, as the impedances of lithium-ion batteries need to be matched. 

So, two lithium-ion batteries connected in series (with their impedances matched of course), will now have a nominal voltage of 7.4V.

Adding more batteries will consequently increase the voltage by 3.7V.

Lithium-Ion batteries are available in packs with these higher voltages. 

Voltage at which a Lithium-Ion battery is dead

There are a couple of voltages that we need to be aware of when using a lithium-ion battery (or any other battery for that matter).

The first being the nominal voltage, which we now know is 3.7V for lithium-ion batteries. 

Another voltage that is of utmost importance is the voltage at which the battery is considered dead, when it has lost all its charge.

It is essential to know this voltage as the battery will need to be recharged back to its nominal voltage to be able to effectively power electronics. 

So, what is the voltage at which a lithium-ion battery is considered dead?

The voltage at which a lithium-ion battery is dead is around 3.4V. 

If the battery is still connected and continues to discharge past 3.4V, a cutoff circuitry kicks in around 3V and disconnects the battery for protection purposes. 

What can affect how fast a lithium-ion battery goes dead?

There are a couple of factors that can affect how fast the lithium-ion battery goes dead, with the two major factors being;

• Load
• Temperature

Load

The first obvious factor is the load that is placed on the battery. A great analogy for this is to imagine you are carrying a backpack (which represents the load), and your energy levels represent the battery. 

If you have a lot of items in the backpack, the weight is going to be larger. This means you have to generate more power to carry the load, which is going to cause you to tire faster (lose all your energy).

However, if you only had a few items in the backpack (which meant the weight is far less), you would need to generate less power to carry it. This means you would be able to travel further. 

This concept is similar for batteries. If a greater load is placed on the battery (such as powering a motor), the battery is going to have to generate more power causing it to lose charge faster and go dead.

But, if the battery is connected to a device such as an LED (which consumes far less power than a motor), the battery will last much longer. 

Temperature

The next major factor that influences the performance of a battery is temperature.

Lithium-Ion batteries have a range of ideal temperatures at which they can be charged and discharged at.

The ideal temperature to charge a lithium-ion battery is 32°F (0°C) to 113°F (45°C) and the ideal discharge temperature is –4°F (-20°C) to 140°F (60°C).

However it is not recommended to charge or discharge the lithium-ion batteries at the extreme temperatures (either real cold or hot).

Higher temperatures can have a temporary advantage of greater performance and increased storage capacity, however, the long term side effect is a decreased life cycle.

Every battery has an internal resistance and when they are subject to drastically lower temperatures, the internal resistance increases.

This means that the battery has to do more work to overcome this increase in resistance causing it to lose power and go dead faster. 

What happens when a Lithium-Ion battery is dead

There are two things not to do with a Lithium-Ion battery when it comes to voltage;

• Do not charge them past their maximum safe voltage of 4.2V 
• Do not discharge them below the minimum safe voltage of 3V.

Lucky for you and me, we do not have to worry about constantly monitoring the battery to see if the voltage goes past these two limits.

When it comes to charging, lithium-ion batteries require a special charger to ensure that the maximum voltage is not exceeded.

This means lithium-ion battery chargers do not have trickle charging (which is a common technique used to charge a battery when it has reached full charge). 

Once the lithium-ion battery has reached full capacity, the battery charger stops charging the battery.

For discharging, lithium-ion batteries include a similar protection circuit that is built on the cell (usually at the the top of the battery covered in tape).

This protection circuit will monitor and disconnect the battery once it has gone dead to protect it from damage. . 

Can a lithium-ion battery become dead if it is not used?  

Yes, a lithium-ion battery can go dead if it is not used (even though it is not supplying a load).

All batteries have something that is known as shelf life. 

The shelf life of a battery tells us the time a battery can hold its charge when it is not being used. After that time, the battery will start to lose charge and need to be recharged (if it is a rechargeable battery).

Lithium-Ion batteries have a self-discharge rate of 5% per month at room temperature.

Irreversible capacity loss occurs if the battery is unused for longer than 12 months. 

If the battery is at a voltage of 1.5V or lower, do not try recharging it. Over long periods of time a build of copper shunts can result within the battery which can cause shorts, leading to excessive heating which could result in the worst case scenario of an explosion. 

How to check if a Lithium-Ion battery is dead

The easiest way to check the voltage of a lithium-ion battery to see if it is dead is to use a Multimeter.

A multimeter is an electronic measuring instrument that has a range of functions which include measuring voltage, current, resistance, continuity, diode test, frequency, etc. 

The number of electrical quantities it is capable of measuring solely depends on the complexity of the multimeter. 

However, all standard multimeters will measure the three main quantities which are voltage, current and resistance. 

To measure the voltage of a lithium-ion battery, follow the steps below;

1. Set the multimeter to voltage mode (ensure the voltage of the battery you are measuring is within the range of the multimeters capability) 
2. Connect the positive (red) lead of the multimeter to the positive terminal of the battery
3. Connect the negative (black) lead of the multimeter to the negative terminal of the battery
4. Note the voltage that the multimeter displays

Should you continue to use a Lithium-Ion battery when it is almost dead? 00

If you measure the voltage of a lithium-ion and it happens to be nearing its dead voltage of 3.4V, should you continue to use it?

No, the best option here is to recharge the battery.

Using a battery when it is almost dead can drastically reduce its lifespan.

The post At what voltage is a Lithium-Ion battery dead? appeared first on Electronic Guidebook.

Whether you are an experienced engineer, electrician, DIYer, electronic hobbyist etc, having a Multimeter is going to benefit you in many ways.

A multimeter is capable of more than one measurement which can include; Voltage, Current, Resistance and others depending on the complexity of the multimeter.

Of the many measurements, Voltage is one of the most common parameters it is used to measure.

But, the multimeter is not a perfect measuring instrument. 

Sometimes it might read the wrong voltage. 

You might be reading this because your multimeter is reading the wrong voltage. 

So, does that mean you need to dispose of your multimeter and get a new one?

Of course not! 

Below are some possible reasons why a multimeter is reading the wrong voltage:

• Low battery
• Faulty Leads
• Incorrect placement of probes
• Component failure
• Fuse blown 

For a more detailed description for each reason, read on. 

Note, make sure to always first try the standard procedure of turning your multimeter off and on and then checking to see if the wrong voltage is still being read. 

5 reasons why a multimeter is reading wrong voltage

Let’s take a look at each of the reasons why a multimeter is reading the wrong voltage in more detail.

Reason #1 a multimeter is reading the wrong voltage: Low battery

The first and most possible reason why your multimeter is reading the wrong voltage is because its battery has decreased below its nominal voltage. 

Electronic components, devices all work within a specific voltage range. 

Whether it be your mobile phone, calculator, toaster, Television and so on.

However, some of these devices have the luxury of being plugged into a wall outlet and being powered ‘infinitely’ (or until you stop paying your bills).

Other devices which are mobile in nature run of a limited power source such as a battery. 

A multimeter is a device that is mobile because it needs to be carried and used in different locations. 

Therefore it needs to be powered by batteries. 

This can cause issues with reading voltages when the batteries start to drop in power. 

A drop in battery voltage can cause the internal reference voltage to drop which then could cause the meter reading to be high.

So, if you are not getting normal voltage readings on your multimeter, replace the current batteries with newer ones and then check again. 

If you get normal readings, you know the old batteries have dropped past their nominal voltage. 

Even if you haven’t used your multimeter in a long time, the batteries could still potentially drop in voltage. 

So beware of that.

Reason #2 a multimeter is reading the wrong voltage: Faulty Leads

If you replaced the batteries with newer ones, and you are still getting the wrong voltage readings, then the next possible issue you could have is faulty leads.

To test your multimeter leads, set the multimeter to read resistance and then touch the probes together.

The resistance that should be displayed for multimeter leads that are functioning properly should be zero.

If for some reason the resistance reading is above one, or all over the place, your multimeter leads are faulty and could be the reason your multimeter is reading the wrong voltage. 

Try replacing the leads to see if that rectifies the issue.

Another issue could be that the probes are not connected properly to the multimeter. 

If there isn’t a proper connection, there won’t be a proper electrical conduction and therefore the wrong voltage will be displayed. 

So, make sure your multimeter probes are connected properly. 

Reason #3 a multimeter is reading the wrong voltage: User error

Ok, this reason comes down to user error.

User error involves the user (the person using the multimeter) not placing the multimeter leads on the right points of a circuit or battery. 

This might seem like a silly reason as to why a multimeter is reading the wrong voltage, but it is entirely possible and has happened to me plenty of times.

This could be caused by not reading the schematic right, or, placing it in the wrong place by mistake. 

Either way, double check that you are measuring the right parts of the circuit if your multimeter is reading the wrong voltage. 

Reason #4 a multimeter is reading the wrong voltage: Fuse blown

Fuses are used in the electrical and electronic field as a way of providing safety to currents that exceed the normal threshold of a device. 

This threshold can be different for different devices so there are fuses that can handle different levels of current.

When that current threshold is exceeded, the metal wire inside the fuse melts, disrupting the flow of current. 

Multimeters have a maximum current that they can handle.

So, to protect them and you from overcurrents, they are fitted with fuses. 

If you happen to use the multimeter to measure the current or voltage outside its maximum threshold, the fuse is going to break.

If the fuse is blown, your multimeter is not going to function properly and therefore display incorrect voltage values. 

If you want to know how to check if your fuse is blown and how to replace it watch the video below.

Reason #5 a multimeter is reading the wrong voltage: Component failure

The last possible reason why your multimeter could be reading the wrong voltage might not be an issue with the multimeter itself.

The issue could be with the electronic/electrical component that you are testing.

Electronic components are not perfect. They too are subject to failure. 

Below are a couple ways electronic components could fail;

• Exceeding the current or voltage rating of the component
• Electrostatic Static Discharge

If a particular component has failed, this could cause a wrong voltage reading on the multimeter when testing another part of the circuit (depending on the circuit itself of course). 

How to be certain your multimeter isn’t reading the wrong voltage

So, the multimeter displays a voltage that you are not expecting.

You might know what voltage it should be displaying, but how can you be certain that something is wrong with the multimeter?

If you have a power supply, set it to a voltage (that is within the range of the multimeter) and then use the multimeter to test the output voltage. 

If the voltage on the multimeter matches the power supply’s voltage, great! 

However, if the voltages do not match, you know you have a problem.

You might not have a power supply lying around though.

That’s fine.

Get yourself a new battery (AA, AAA, D-cell, 9V etc), and test the voltage. Since it is a new battery, the voltage when testing should be around the full capacity value of the battery. 

Again, if the multimeter is displaying the wrong voltage, you know you have a problem and can cycle through the 5 possible reasons to find the problem. 

How to avoid reading the wrong voltage with a multimeter

It can be quite a waste of time having to go through all the steps of figuring out why your multimeter is reading the wrong voltage. 

You might know the saying, “Prevention is the best cure”.

This is having habits or procedures in place that prevent a problem from arising in the first place.

Below are some things you can do to avoid the situation of your multimeter reading the wrong voltage for each of the 5 possible reasons mentioned earlier. 

Low battery

You cannot really control how your battery performs. 

But, you can control the quality of the batteries you buy for your multimeter. 

Invest in good quality batteries that will last longer. This will save you from always having to change the batteries of the multimeter (which will save you time and money).

Faulty Leads

Depending on how often you use the multimeter, and the way you use them, you will be twisting, turning, stretching them in every direction. 

This is going to cause some wear and tear on the multimeter leads which will no doubt cause them to fail in time.

So, to ensure longevity, make sure to handle the leads with care when using the multimeter.

User Error

Mistakes are going to be made.

We are human after all.

But, we can reduce the frequency at which we make errors when reading voltages on the multimeter with a few things we do before testing.

Never assume anything. Always make it a habit to read the schematic of the circuit you are reading to identify the right points where to place the multimeter leads.

Test more than once to see if you are reading the same voltage. 

You might have got it wrong the first time, so testing more than once will eliminate any doubt. 

Also, make sure you are making a proper connection with whatever you may be testing. 

Fuse blown

To avoid blowing the fuse of your multimeter, get to know your multimeter.

All multimeters have different maximum current and voltage ratings.

Read your multimeter’s manual, and familiarise yourself with its maximum voltage and current ratings. 

This will prevent you from testing any currents and voltages outside of the limits of your multimeter.

Component failure

Similarly with the multimeter, make sure to stay within the voltage and current limits of the components in your circuit.

Electronic components are also susceptible to damage via electrostatic discharge when you are handling them.

To prevent this follow the points below

• Keep electronics away from blowing air
• Keep electronics away from plastics and synthetic materials 
• Invest in an ESD mat (which is designed to drain static discharge away from you)

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