In most electrical and electronics troubleshooting and repairing works, we face common trouble that how to test and check a capacitor. Is it good, bad (dead), short or open?
Here, we can check a capacitor with analog (AVO meter i.e. Ampere, Voltage, Ohm meter) as well as digital multimeter either it is in good condition or should we replace it with a new one…
Note: To find the value of Capacitance, you need a digital meter with Capacitance measuring features.
1. Traditional Method to Test & Check a Capacitor
Note: Not Recommended for everyone but professionals only. Please be careful to do this practice as it is dangerous. Make sure that you are a professional electrical engineer/electrician (you really know what are you doing or check the warnings before applying this method) and there are no other options to check the capacitor because serious damages may occur during this practice). If you are sure, go ahead, otherwise, go to methods 2 – 6 as alternative methods to a capacitor.
Suppose you want to check the Capacitor (for example, fan capacitors, room air cooler capacitors or tinny capacitors in a circuit board / PCB, etc.)
Warning & recommendations for testing a capacitor
For better safety, use 24V DC instead of 230V AC. In case of the absence of the desired DC 24V system, you may use 220-224V AC, but you have to make a serial of resistors (say 1kΩ~10kΩ, 5~50Watts) to connect between a capacitor and 230V AC supply. So that, it will reduce the charging and discharging current. Here is the step-by-step tutorial that how may you check a capacitor by this method.
- Disconnect the suspected capacitor from the power supply or make sure at least one lead of the capacitor is disconnected.
- Make sure that the capacitor is fully discharged.
- Connect two separate leads to the capacitor terminals. (Optional)
- Now safely connect these leads to 230 V AC Supply for a very short period (about 1-4 Sec) [or for a short time where the Voltage rise to 63.2% of the Source Voltage] .
- Remove safety leads from the 230 V AC Supply.
- Now short the capacitor terminals (Please be careful to do that and make sure that you have to wear safety goggles)
- If it makes a strong spark, then the capacitor is good.
- If it makes a weak spark, then it is a bad capacitor change it immediately with a new one.
2. Test a Capacitor By Analog Multimeter
To check a capacitor by AVO (Ampere, voltage, Ohm Meter ), follow the following steps.
- Make sure the suspected capacitor is fully discharged.
- Take an AVO meter.
- Select analog meter on OHM (Always, select the higher range of Ohms).
- Connect the Meter leads to the Capacitor terminals.
- Note The reading and Compare it with the following results.
- Short Capacitors: Shorted Capacitors will show very low Resistance.
- Open Capacitors: An Open Capacitor will not show any movement (Deflection) on OHM meter Screen.
- Good Capacitors: Initially, it will show low resistance, and then gradually increases toward the infinite. It means that Capacitor is in Good Condition.
3. Check a Capacitor By a Digital Multimeter
To test a capacitor by DMM (Digital Multimeter), follow the steps given below.
- Make sure the capacitor is discharged.
- Set the meter on the Ohm range (Set it to at least 1000Ohm = 1k).
- Connect the meter leads to the capacitor terminals.
- The Digital meter will show some numbers for a second. Note the reading.
- And then immediately it will return to the OL (Open Line). Every attempt of Step 2 will show the same result as in step 4 and Step 5. It means that Capacitor is in Good Condition.
- If there is no Change, then Capacitor is dead.
4. Checking Capacitor By Multimeter in the capacitance Mode
Note: You can do this test with a multimeter if you have a Capacitance meter or if you have a multimeter with a feature to test the capacitance. Also, this method is good to test the tiny capacitors as well. For this test, rotate the multimeter knob to the capacitance mode.
- Make sure the capacitor is fully discharged.
- Remove the capacitors from the board or circuit.
- Now Select “Capacitance” on your multimeter.
- Now connect the capacitor terminal to the multimeter leads.
- If the reading is near to the actual value of the capacitor (i.e. the printed value on the Capacitor container box).
- Then the capacitor is in good condition. (Note that the reading may be less than the actual value of the capacitor (the printed value on the Capacitor container box).
- If you read a significantly lower capacitance or none at all, then the capacitor is dead and you should change it.
5. Testing a Capacitor By Simple Voltmeter.
- Make sure to disconnect a single lead (no worries if Positive (long) or negative (short)) of the capacitor from the circuit (You may fully disconnect as well when needed)
- Check the capacitor voltage rating printed on it (As shown in our below example where is the voltage = 16V)
- Now charge this capacitor for a few seconds to the rated (not to the exact value but less than that i.e. charge a 16V capacitor with 9V battery) voltage. Make sure to connect the positive (red) lead of the voltage source to the positive lead (long) of the capacitor and negative to negative. If you are unable to find it or not sure, here is the tutorial on how to find the negative and positive terminal of a capacitor.
- Set the value of the voltmeter to the DC and connect the Capacitor to the voltmeter by connecting the positive wire of the battery to the positive lead of the capacitor and negative to the negative.
- Note the initial voltage reading in the voltmeter. If it is close to the supplied voltage you gave to the capacitor, the Capacitor is in Good condition. If it shows far little reading, Capacitor is dead then. note that the voltmeter will show the reading for a very short term as the capacitor will discharge its volt in the voltmeter and it is normal.
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6. Find the capacitor value by measuring the value of the Time Constant
We can find the value of a capacitor by measuring the Time constant (TC or τ = Tau) if the value of capacitance of a capacitor is known in microfarad (symbolized µF) printed on it i.e. the capacitor is not blown and burnt at all.
In brief, the time taken by a capacitor to charge about 63.2% of the applied voltage when charged through a known value of the resistor are called Time Constant of Capacitor (TC or τ = Tau) and can be calculated via:
R = Known Resistor
C = Value of Capacitance
τ = TC or τ = Tau (Time Constant)
For instance, if the supply voltage is 9V, then 63.2% of this is around 5.7V.
Now, let’s see how to find the value of a capacitor by measuring the Time Constant.
Make sure to disconnect as well as discharge the capacitor from the board.
Connect a known value of resistance (e.g 5-10kΩ Resistor) in series with the capacitor.
Apply the known value of supply voltage. (e.g 12V or 9V) to the capacitor connected in series with a 10kΩ resistor.
Now, measure the time taken for the capacitor to charge about 63.2% of the applied voltage. For instance, if the supply voltage is 9V, then 63.2% of this is around 5.7V.
From the value of the given resistor and measured time, calculate the value of capacitance by Time Content formula i.e. τ = TC or τ = Tau (Time Constant).
Now compare the calculated value of capacitance with the value of capacitor printed to it.
If they are the same or nearly equal, The capacitor is in good condition. If you find a noticeable difference in both values, time to change the capacitor as it is not function well.
The discharge time can also be calculated. In this case, the time taken by the capacitor to discharge to 36.8% of the peak voltage can be measured.
Good to know: The time taken by a capacitor to discharge about 36.8% of the peak value of the applied voltage can be also measured. The discharge time can be used as the same in the formula to find the value of the capacitor.