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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