In this tutorial, we will see how to test a Capacitor and find out whether the capacitor is working properly or it is a defective one. A Capacitor is an Electronics/Electrical component that stores energy in the form of Electric Charge. Capacitors are often used in electronics circuit boards or few electrical appliances and perform a variety of functions.
Why do we need to Test a Capacitor?When a capacitor is placed in an active circuit (a circuit with active current flowing), charge starts to build up in the capacitor (on one of its plate) and once the plate of the capacitor can no longer accept any more charge, this means the capacitor is fully charged.
Now, if the circuit demands this charge (like a bypass capacitor, for example), the capacitor releases the charge back in to the circuit and it continues until the charge is completely released or the circuit stops demanding. These action are called as Charging and Discharging of Capacitor.
Basically, capacitors can be classified in to Electrolytic and Non – Electrolytic Capacitors. Like all electrical and electronics components, a capacitor is also sensitive to spikes and such voltage swings can damage the capacitors permanently.
Electrolytic Capacitor often fail due to discharging more current in a short period of time or cannot hold charge due to drying out over time. Non – Electrolytic Capacitors on the other hand fail due to leakages.
There are different methods to test whether a Capacitor is functioning properly or not. Let us see some of the methods to test a capacitor.
NOTE: Some of the methods mentioned here may not be the best ways to test a capacitor. But we included those methods just to specify the possibilities. Be very careful.
How to Discharge a Capacitor?Before proceeding further and looking at different methods of testing a capacitor, let us understand how to properly discharge a capacitor. This is very important because capacitors can hold the charge even if the power supply is disconnected. If the capacitor is not discharged properly and if you accidently touch the leads of the capacitor, it will discharge through your body and cause an electric shock.
There are a couple of ways in which you can discharge a capacitor. There will be a dedicated tutorial on how to discharge a capacitor but for the time being, let us go through both these methods very briefly.
Using a ScrewdriverWARNING: This method is not preferred (especially if you are beginner) as there will be sparks generated while discharging, which might cause burns or other damage. Use this method as a last resort.
If the capacitor is in the circuit (on the PCB), then properly de-solder it and make sure not to touch the terminals of the capacitor. Now, take an insulated screwdriver (with a longer handle) and hold it in one hand. Take the capacitor in the other hand and touch the metal part of the screwdriver to both the terminals of the capacitor.
You will see sparks and hear some crackling sound as an indication of electric discharge. Repeat a couple of time to make sure that the capacitor is completely discharge.
Using a Discharge Resistor (Bleeder Resistor)Now, we will see a safe way to discharge the capacitor. This method is often used in power supplies and other similar circuits where a Resistor, known as a Bleeder Resistor, is placed in parallel to the output capacitor so that when the power is turned off, the remaining charge in the capacitor is discharged through this resistor.
Take a large value resistor (usually, few kilo Ohms) with a high power rating (like 5W) and connect it across the terminals of the capacitor. Instead of directly connecting, you can make use of wires with crocodile clips on both the ends. The capacitor will slowly discharge and you can monitor the voltage across the terminals of the capacitor with a multimeter.
There is an easy to use “Capacitor Safety Discharge Calculator” from Digi-Key. Use this tool as a starting point.
For example, assume we have a 1000µF Capacitor rated for 50V and we want to discharge this capacitor down to 1V. Using a 1KΩ Resistor, it will take almost 4 seconds to discharge the capacitor. Also, the power rating of the resistor should be at least 2.5W.
NOTE: High Power Rated Resistors are usually expensive when compared to normal resistors (1/4 Watt or 1/2 Watt).
Method 1 Checking a Capacitor using Multimeter with Capacitance SettingThis is one of the easiest, quickest and accurate way to test a capacitor. For this, we need a Digital Multimeter with Capacitance Meter feature. Most of the mid and high end Digital Multimeters include this functionality.
The capacitance meter on the digital multimeters often display the capacitance of the Capacitor but few meters display other parameters like ESR, leakage, etc.
Using this method, capacitors ranging from few nano Farads to few hundreds of micro Farads of Capacitances can be measured.
Method 2 Checking a Capacitor using Multimeter without Capacitance SettingMost of the low end and cheap Digital Multimeters do not include Capacitance Meter or Capacitance Settings. Even with these Multimeters, we can test a Capacitor.
This method of testing the capacitor might not be accurate but can differentiate between a good and bad capacitors. This method also doesn’t give the capacitance of the capacitor.
Method 3 Testing a Capacitor by measuring the Time ConstantThis method is applicable only if the capacitance value is known and if we want to test whether a capacitor is good or dead. In this method, we measure the Time Constant of the Capacitor and derive the capacitance from the measured time. If the measured capacitance and the actual capacitance are similar, then the capacitor is a good one.
NOTE: An oscilloscope will be a better tool for this method than a multimeter.
Time Constant of a Capacitor is the time taken by a Capacitor to charge to 63.2% of the applied voltage when charged through a known resistor. If C is Capacitance, R is a known Resistor, then Time Constant TC (or Greek Alphabet Tau – τ) is given by τ = RC.
The discharge time can also be calculated. In this case, the time take by the capacitor to discharge to 36.8% of the peak voltage can be measured.
Method 4 Test a Capacitor with a simple VoltmeterAll capacitors are rated with a maximum voltage that they can be applied with. For this method of testing a capacitor, we will use the voltage rating of a capacitor.
Only the initial reading on the Multimeter must be taken in to account as the value will slowly fall down. This is normal.
Analog Multimeters, like Digital Multimeters, can measure different quantities like Current (A), Voltage (V) and Resistance (O). In order to Test a Capacitor using Analog Multimeter, we are going to use its Ohmmeter functionality.
This test can be applied to both through hole and surface mount capacitors.
Method 6 Shorting the Leads of Capacitor (Traditional Method – only for Professionals)The method described here is one of the oldest methods to test a capacitor and check whether it is a good one or a bad one.
Warning: This method is very dangerous and it is for Professionals only. It must be used as a last option to test the capacitor.
Safety: The method is described with respect to 230V AC Supply. But for safety reasons, a 24V DC supply can be used. Even with 230V AC, we need to use a series resistor (high wattage rated) to limit the current.
This method can be used for capacitors with smaller capacitances. This method can only determine if the capacitor can hold charge or not.
ConclusionA complete beginner’s guide on different ways to test a capacitor. Learn how to test a capacitor, how to properly discharge a capacitor before testing, what methods are safe to use by beginners.
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