Capacitors are charge-storing components that are used in all sorts of electrical devices. They are quite useful but lose their properties with time as their electrolytes have a specific shelf life.
Capacitor failure is one of the main causes of circuit failures. There are many failure modes of capacitors, and each mode has its own testing method. In this article, we have talked about testing a capacitor with a multimeter
. So, let’s find out how to test a capacitor with a multimeter.
Introduction to capacitors
A capacitor is a passive device whose primary function is storing electrostatic energy and releasing it in a circuit. Capacitors store that energy in the form of electric charges, which also create a potential difference across its two plates.
There are two parallel metal plates in a capacitor. These plates are separated from each other, and there is an insulating material in between them. In most cases, this material is air, but it can also be mica, ceramic, or a waxed paper. This insulating material is called dielectric.
Capacitors do not allow the flow of DC current. When you connect it to a DC circuit, the capacitor gets charged but blocks the current flow, when you apply DC voltage. On the other hand, when you connect it to an AC circuit, the current flows with minimal or no resistance.
Types of capacitors and failure modes
It is important to first know what the basic types of capacitors are so that you can test them with a suitable method. There are two main kinds of capacitors.
Polarized capacitors
As the name indicates, a polarized capacitor has implicit polarity. One electrode has a positive charge while the other electrode has a positive charge. You can only connect it one way in a circuit. You should be careful while testing these capacitors. Connect the positive lead of the capacitor with the positive probe of the multimeter. A capacitor’s polarity is usually written on its casing.
You should desolder the capacitor from its circuit board. This way, the adjacent components of the circuit will not interfere with the measurement. After desoldering, discharge the capacitor completely with the help of a metal object or an electrical device.
Be careful not to touch the probes of the multimeter while performing a test. The human body is a good conductor of electricity, and it does offer resistance. This resistance could affect measurements.
Non-polarized capacitors
A non-polarized capacitor does not have negative or positive polarity. You can insert it into a circuit randomly.
Electrolytic Capacitors:
These capacitors contain fluid between their plates. This fluid is an electrolyte solution that often contains up to 15% sulfuric acid. The best part about these capacitors is they are highly efficient and have high voltage potentials. However, they are also prone to failure because of this acidic electrolyte.
Capacitance Meter:
A capacitance meter is used to check whether a capacitor has intact or damaged leads, and this includes the measurement of any potential difference between the two electrodes. There are many types of capacitance meters available to test a capacitor.
Capacitor Terminals:
A capacitor has a positive terminal and a negative terminal. These terminals may also be marked as (+) and (-) respectively.
The test leads are also called probes and they have an alligator clip at its end that touches the conductor. The prods are color-coded as red for positive and black or blue for negative connections.
Failure modes and their specific testing
The failure of a capacitor on a circuit board can occur due to different reasons. Based on these failure modes, a specific testing method should be performed. Whether they are electrolytic capacitors or ceramic capacitors, there are specific tests for each of them.
Check for dielectric breakdown
Dielectric breakdown is a condition in which operating voltage exceeds the breakdown voltage, which leads to electrical stress. When it occurs, the insulating dielectric fails to perform its function and becomes a conductor and now we are talking about a shorted capacitor.
How to Check for dielectric breakdown
In order to check dielectric breakdown, you need to measure resistance. For this, connect the probes of a multimeter with the leads of the capacitor. If there is a dielectric breakdown, the multimeter display will show zero or nearly zero resistance. On the other hand, if the capacitor is operating correctly, the digital multimeter will show a specific resistance value which will increase on display gradually.
Non-polarized capacitors are easy to test because they do not have polarity, and you can connect them to the multimeter in any way. Before measuring resistance, set the resistance of the multimeter to 2 MΩ because non-operating capacitors do not have a resistance value higher than this.
The applied voltage should read under 10 V.
Pointer multimeters are more beneficial while measuring resistance. It is because you can observe the movement of the pointer and tell how the resistance is changing. The power supply of the pointer multimeter will not affect the capacitor under test. Not to say digital multimeters are also ideal.
The separation between lead and plate
Sometimes one or both leads of the capacitor get separated from their respective plates. It is another mode of capacitor failure.
A good capacitor should have low or zero resistance. If you measure a high value of resistance on the multimeter, then it means that it is a faulty capacitor.
So checking the capacitor value is very important when you are installing a new capacitor or replacing the existing one.
How to check separation?
In order to check this separation, connect the probes of the multimeter with the capacitor. The multimeter will show zero capacitance value, and the capacitor will also fail to get charged during resistance testing.
Capacitance reduction
A decrease in the total capacitance of a capacitor is another reason for its failure. The capacitor terminals may get oxidized or corroded due to exposure to moisture, air pollutants, and other chemicals.
You should check the capacitor’s performance with a digital multimeter. Different measurements will help you determine whether the decrease in total capacitance is significant enough to affect the functioning of the circuit.
How to measure capacitance?
To check if capacitor failure is due to the reduction in capacitance, get a digital multimeter and measure the capacitor’s capacitance. Connect the probes or connector pins of the multimeter with the capacitor after setting a specific measurement range. Some multimeters automatically choose the range according to the type of capacitor. The supply voltage should be present in the circuit.
Measuring increase in current leakage
Current leakage occurs when the insulating material between capacitor plates fails to perform its role correctly.
It is insignificant in normal operating conditions though it reduces when current has to increase and when electrical components on the current board lose the capacity to store the charge when you test a capacitor.
You need to charge the capacitor at the power source and measure the voltage at regular intervals, starting from the initial voltage reading.
If leakage is low, then it will not have much effect on a capacitor’s performance. But when it increases, the capacitor’s plates fail to store charge. The stronger the supplied voltage drops the larger the leakage. So current leakages can be evaluated whenever you test a capacitor.
An ohmmeter can be used to measure leakage, but it is not sensitive enough for the task. You can use a digital capacitance meter for the initial test of a capacitor. Digital multimeters can be used for measuring the voltage of a capacitor over a wide range.
Measure the peak voltage
Most dielectrics used to make capacitors have a specific breakdown voltage. Exceeding this voltage will cause the dielectric material to become conductive. How do you check it?
You should connect the capacitor with the capacitance meter and measure resistance for some time so that you can see if there is any change in resistance, which indicates whether the capacitor terminals are properly connected or not.
This is done to test a capacitor and see whether the constant power supply exerts any effects of its or is there some issue in the circuit that is causing voltage swings.
The voltage rating should be checked with a digital multimeter and the capacitor should be disconnected to avoid any damage.
When the capacitor is charged, you may see some increase in voltage. This is due to stress caused by the high voltage applied. A good capacitor will have a constant voltage when it is charged under normal conditions. While faulty capacitors will show fluctuating voltage values that are a result of high current leakage.
How to test increased current leakage?
You can measure current leakage with the help of a conventional multimeter. If you want to get precise readings, it is best to use a multimeter whose input impedance is at least 10 MΩ. To measure current leakage, first, charge the capacitor fully, then check its voltage after regular intervals. If the voltage decreases speedily, it means the leakage is great. The power supply units can easily cause defects in capacitors.
Equivalent series resistance
Sometimes, a capacitor apparently looks perfect yet fails to operate. Under such situations, you can either replace it with a new device or measure the current capacitor’s equivalent series resistance. Use a digital multimeter to measure ESR because most of the high-quality analog multimeters will not be able to measure it correctly, even when the power supply is moderate.
It is best to discharge the capacitor fully before measuring the equivalent series resistance. You can do so by using a potentiometer and a pair of voltmeters or simply by connecting a resistor between one lead from the capacitor and the ground.
How to test an increase in equivalent series resistance?
To measure ESR, you need a high-quality RLC meter. An increase in the ESR makes the capacitor overly heated, which can reduce its lifespan. Set frequency according to the device before performing the test. You can compare the ESR value of the capacitor with the ESR value of a new capacitor to check whether it is acceptable or not. You can also use Bob Parker’s ESR Chart for the same purpose.
Initial check without testing equipment
For an initial test, you can easily perform it without testing and measuring tools. Just disconnect the capacitor and place it outside the circuit for some time. If it fails to work after this, then you can be sure that its failure is not due to high temperature or rough handling. The dc voltage should then be measured to check whether it is within the acceptable range. You should then charge it, in case it has failed due to high temperature or rough handling.
If you need to replace electrolytic capacitors-whether it’s due to being discharged too quickly or dried out over time, then do your research before buying one.
The capacitor leads are coated with a lacquer or plasticizer resin. This prevents corrosion and acting as insulation between the leads. However, over time this becomes brittle and will crack easily.
A capacitor is said to be ‘dried out’ if its dielectric property has been affected due to an increase in its temperature level or humidity level.
The negative terminal and positive terminal of a capacitor are always in close contact with each other, which is why they cannot be separated. Therefore, you should only handle a capacitor from its insulated end or outer casing.
The test capacitors can be connected to the leads of a potentiometer or voltmeter to keep track of their leakage. An ESR meter needs to be installed in place of the potentiometer or voltmeter while measuring the ESR real capacitors.
The value of resistance depends on the dielectric material used for this purpose. Remember that in an open circuit, the resistance is infinite.
The voltage source needs to be functioning properly along with the circuit being closed whenever you test a capacitor with a multimeter. Other components must be removed from the circuit for safety purposes.
In order to keep the multimeter leads in place and the voltage to flow uninterrupted, it is important that you connect those leads to a dc voltage source.
Using a screwdriver
If a capacitor is on a circuit board then properly remove it and make sure not to touch the connectors. You can smell sparks and hear a noise as an indication. Repeat a few times until there’s clear discharge.
This method is not preferable – especially for beginners – as it has sparks created during the discharge – which might cause burns or other damage. In some instances. It is the last resort. We recommend you to contact us via the link.
Summing up
Capacitors have a lifespan and fail after a certain period of time. They need to be replaced or repaired in order to make the circuit work again. There are different modes of capacitor failure. You should perform the above-mentioned tests to find the underlying cause behind capacitor failure. The capacitor leads become brittle due to aging and would crack with increased tension, which results in the capacitor’s failure.
To test a capacitor, you can choose a capacitance meter. This lets you perform different tests and provides the user with useful information.
You can consider checking a new capacitor before installing it in your circuit. Once installed, you cannot check its performance effectively without the help of a multimeter.
An ESR meter will be used whenever you are trying to check the ESR real capacitors. The ESR meter helps you in determining a capacitor’s leakage by measuring the amount of current it can handle.
Using the traditional method, you can test a capacitor easily. Make sure that the voltage rating and tolerance meet the required specifications. Examine its leads and make sure that they are properly insulated from each other.