Hi. Hope you are having a good time. Today, I was wondering how I can test a capacitor inside a circuit. To find the answer I started my research. I read people reviews on different forums and blogs, watched a couple of youtube videos, and used my own knowledge to get to the proper solutions. In this post, I share the conclusion of various solutions using which we can actually test a capacitor without desoldering it from a circuit.
Hopefully, you will enjoy and benefit from this article.
Test A Capacitor Without Desoldering It
Let just face it. You just cannot test a capacitor in 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. I write about it in detail in later sections, don’t worry.
We can’t go with capacitance value to test it. So, there left one choice we can use to test a capacitor, and that is by measuring its equivalent series resistance (ESR).
In summery, 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.
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 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 meter like this ESR meter (Amazon Link) selling on Amazon. The beauty of this ESR meter is, it is reliable and comes at a very decent price. You can also see my review of this meter by clicking here.
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 little expensive. The last time I checked its price was around $300, you can see its current price by visiting on Amazon. 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.
Why we cannot test the in-circuit capacitor with a normal multimeter
Multimeter or capacitor meter are used to measure capacitances. They can also tell if the capacitor is bad or good.
If the measured value of capacitance is in 10% deviations from the actual value, it is a good one else it is bad and needs to be replaced. The reason, we cannot go with this when capacitors are inside the circuit, is because inside the circuit there are a lot of other components in series or parallel. And in these situations, the multimeter leads us to a false reading.
Another important thing, sometime a bad capacitor may give you a proper capacitance value on the multimeter or capacitor meter. So, if you want to test a capacitor, in my opinion, you should always test it on ESR meter. Use a multimeter for measuring the capacitance value.
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.
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.
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.
- Learn Basic Electronics For Beginners [Easy Step By Step Guide 2018]
- 3 Insanely Best Beginner Oscilloscopes [That Really Work]
- 3 Best Soldering Station for Hobbyists And Beginners
- Best Resistor Kits On Amazon
- Best Lab Power Supply[Really Affordable]
- 3 Insanely Best Beginner Multimeter [My Personal Favorite]
Thank you and have a good time.