How to test a capacitor

What Is a Capacitor?

The basic definition of a capacitor is a device that’s used to store an electric charge. Most pool pump motors utilize a start capacitor and/or a run capacitor. They both look similar to a large battery. Start capacitors are usually located at the back of the motor and the run capacitor is located at the top of the motor.How To Test A Pool Pump Capacitor

Testing the Capacitor with a Multimeter

  1. Remove the cover at the back of the motor or on top of the motor to expose the capacitor.How To Test A Pool Pump Capacitor
  2. The capacitor will store an electrical charge. It will need to be shorted out by placing an insulated screwdriver across the leads of the capacitor. CAUTION: Capacitors have been known to explode when shorted out. For this reason, wear eye protection and place a cloth over the capacitor before shorting it out.
  3. Remove the leads to the pool pump capacitor. Note the placement of the leads so that you can restore them to their correct positions when you reconnect them.
  4. Set the multimeter on ohms to 1K.
  5. Slip a heavy piece of paper between the points on the switch.
  6. Attach one probe to each terminal. On an analog meter the needle should move rapidly to right then slowly drift to the left. If a digital meter is used, readings should start low and rapidly increase to maximum value. If the needle or number starts at zero and doesn’t move, the capacitor is bad and  therefore needs to be replaced.

4) Checking A Capacitor Using An Analog Multimeter:

If you have an AVO meter (Ampere, Voltage, Ohm meter) you can also test the functionality of your capacitor. Make sure that you fully discharge your capacitor and the multimeter istest-Capacitor-easily-by-using-6-Methods-Electronics-Engineering-Technology on its resistance mode. Then connect the multimeter leads to the terminals of the capacitor (positive lead to the longer leg and the negative to the shorter leg). Take an assessment of the reading accordingly: 

 A Shorted capacitor will not show any resistance so it must be replaced.

 An open capacitor will not show any deflection

 If at first the reading is on zero resistance and gradually starts moving towards infinite resistance then the capacitor is not damaged.

This method is not as safe for Electronics Engineering Technology students the above-mentioned methods so only use it if you’re a professional or if you’re in dire need. 

If you’re using a DC supply then you can use both polarized and non-polarized capacitors (for the former case keep in mind to check the polarity).

Use a 24V DC supply and charge the capacitor for a few seconds and remove it from the charging source. After removing the capacitor, short both of its terminals using a metal contact (have some type of insulation for yourself). If the capacitor is in good condition then strong sparks will be originated, otherwise, the capacitor has gotten bad.

6) Testing A Capacitor By Measuring Its Time Constant:

If you know the value of the capacitor you are using, you can use the time constant method to test the capacitor. The time constant of a capacitor is the time which it takes to charge up to 63.2% of the applied voltage. You also need a resistor of known value.

Time constant T = R X C

Connect the resistor in series with the capacitor and connect that circuit to a power supply on a fixed voltage. Start measuring the time it takes for the capacitor to charge up to 63.2% of the applied voltage. 

Now from that time and the value of the resistor you can find out the value of the capacitor.

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ESR Meter

The term ESR stands for equivalent series resistance, measured in Ohms, meaning ESR meter is a device used to determine the equivalent series resistance of a real capacitor without desoldering it from a circuit. This device cannot measure the capacitance, and can only be used to test a capacitor.

An ideal capacitor has ESR value equals to zero, but in reality, it is very very less; close to the ideal value. The increase in the ESR value increases both voltage drops within the capacitor and the heat. The heat produces in the capacitors due to resistive heating and causes the capacitor to leak out. If you do not test the electrolytic capacitor for ESR value with an ESR meter, you may not be able to tell if the capacitor is a good or bad one. A high ESR value is the first indicator of a capacitor’s failure.

How to test capacitor without desoldering with ESR Meter

Following are the quick steps to test any in-circuit capacitor with ESR meter.

  • Discharge the under test capacitor first. This is so important and crucial, if you accidentally forget this step, you may end in destroying your ESR meter. For extra knowledge, always discharge a capacitor before measuring any parameter of it.
  • Discharging of a capacitor can be done by shorting its legs by any available means. But don’t just short the legs together with low resistance wire, a good practice is to use a high resistance material.
  • Turn on the ESR meter, and short its leads until you get 0 reading on its screen. If the screen is already showing 0 reading, then there is no need to short the leads.
  • Place the red lead of ESR meter to a positive and black lead to the negative terminal of the capacitor under test.
  • Note the readings on ESR meter.
  • Compare the reading with the table given on the body of ESR meter. If the ESR value is in the given range the capacitor is good and need no changes, if not then it is a bad one and needs replacement.
  • If the body of ESR is not giving any table, use the datasheet of the capacitor to read its ESR value.
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The datasheet of every capacitor has listed its ESR value at the frequency of 100kHz and specific voltage rating. Deviation from this value helps us to decide either the capacitor needs to be changed or not. Usually, the ESR of a bad capacitor increase. Further more, a good capacitor would measure almost like a short circuit, and all other parts connected in parallel with it will have minimal influence on the end measurement. This is the feature that makes an ESR meter an irreplaceable tool for troubleshooting electronic equipment.

So, if you really want to find out and fix your bad capacitors in your devices you need a decent meter like this ESR meter selling on Amazon. The beauty of this ESR meter is, it is reliable and comes at a very decent price.

Final words on testing a capacitor in a circuit without desoldering it

An ESR meter sends a 100kHz frequency alternating current into the capacitor under test. The current produces a voltage across the capacitor and then using some mathematics the ESR is calculated and displayed on the screen.

I cannot tell you how much beautiful and handy device a smart tweezer is. I personally just love it and has decided to have one in the near future.

That is all I want to share with you on how you can test a capacitor without desoldering it from a circuit. Hopefully, this post gave you some value.

One last thing, all the given amazon links are affiliated one meaning if you buy these products from these links within 24hrs I get a small commission, which would really help me and I would appreciate this kind act from the heart.

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You can also check my other following articles too: If you are beginners in electronics and want to have a complete short guidepost to tell you what you really need to know about the electronics then in the following list the first post is for you.


  • Wikipedia
  • EEVblog #365 – ESR Meter Bad Cap Monitor Repair
  • Ifixit
  • Ludens

Thank you and have a good time.

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