How to test capacitor without desoldering [incircuit testing]

test capacitor without desoldering

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A circuit board usually has resistors, capacitors, inductors, ICs, connectors, and few other components. It is common, these components get burn and need replacement.

The components that have higher chances of burning out are resistors, capacitors, and rarely the ICs. The reason is, mostly resistors and capacitors are in front end of any board. And sometime over voltage burn them out.

When it comes to resistor and IC, you can identify the bad one by just looking at it in the board. A burnt IC or resistor is busted open and you can find it in seconds on the board.

However, this is not the case for capacitor.

In case of capacitor, things are little different. If you are lucky, you will find the bad capacitor just by looking at its top, it will be busted open.

But what if you are not that lucky?

The real problem you would face, a normal looking capacitor can be a bad one. So you have to desolder all the capacitor from board, test each one, find the bad guy, and resolder each and everyone back on the board. Which is not a good way and no one wants to do it.

Don’t worry.

In this post, we will definitely discover a way to how to test a capacitor without deoldering it from the broad.

Hopefully, you will enjoy and benefit from this article.

Test a capacitor without desoldering it

Let just face it.

You just cannot test a bad capacitor inside or outside a circuit board by measuring its capacitance value with a capacitor meter or a multimeter. Because in such situation mentioned devices lead you into false reading and you may not be able to actually tell if the capacitor you tested was actually a bad or right one.

Why?

  • The reason is, when a capacitor is inside the circuit board, there are lot of other components in series or parallel with it. So you get the equivalent reading, not the actually one.
  • When the capacitor is outside the board, sometime a bad capacitor may give you a proper capacitance value on the multimeter or capacitor meter.

No doubt, multimeter or capacitor meter are used to measure capacitances. They just cannot be trusted to tell you if the capacitor is bad or good whether it is outside or inside the circuit board.

So, how can I test this bitch?

There left one choice we can use to test a capacitor, and that is by measuring its equivalent series resistance (ESR).

In summary, the best solution to test a capacitor without actually desoldering it is either using an ESR meter or smart tweezers. Both work the same and are fine to use. But the ESR meter is preferred for through-hole capacitors, and the later one is preferred to test SMD capacitors.

In the rest of the article, I give more info on what are the mentioned devices, and how they test in-circuit capacitors.

The 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. A high ESR value is the first indicator of a capacitor’s failure.

The increase in the ESR value increases both voltage drops within the capacitor and the heat. The heat produces in the capacitors is due to resistive heating and this heat 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.

How to test capacitor 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.

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 ESR meter. You can find a good ESR any where.

Just search for it.

I recommend and like this ESR meter (Amazon Link). The beauty of this meter is, it is reliable and comes at a very decent price.  If you like this one, buy it.

Smart Tweezers

Usually, an ESR meter can do all the job for you but when it comes to SMD components it doesn’t come as handy as smart tweezer. If you decide to go with ESR, you will be fine but smart tweezer (Amazon link) is fun and a wonderful tool to have in your lab, in my opinion.

The real problem with smart tweezers is, they are expensive. The last time I checked its price was around $300. But besides using it just to test capacitors, it can be your awesome portable LCR meter too.

All the measuring steps are the same as I discussed above for ESR meter.

Visually seeing a bad capacitor

Instead of using ESR meter or tweezers we can also test a capacitor without desoldering it by general inspection.

A bad electrolytic capacitor gets swallow at the top side, you see such one in the circuit; just replace it without wasting your time testing it.

The capacitance value may be in good value range when you test it off the circuit with a multimeter or capacitor meter, but still, it is a bad one.

Conclusion

You just cannot test a bad capacitor both inside or outside a circuit board by capacitance meter or multimeter. The reason is. they both may lead you into false results.

The only solution to test capacitor without desoldering is by measuring its equivalent series resistance (ESR). This value is measured by ESR meter.

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.

You get a drifted ESR value after comparing it with a ESR chart, you have a bad capacitor.

And well, there you have it. Now, if a reader like me reading the conclusion first. You read it. Its time to go to the beginning. But you are the reader making this far. I hope you enjoyed it.

Thank you and have a good time.


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