DIY Capacitor Leak tester

If you’re like me and attend to salvage caps from discarded circuit boards or even repair electronics it’s great to have a Capacitor Leak Tester. Granted a multi meter and a ESR meter can give you a lot of info about a unknown cap but you want to know if the cap can work under full voltage load.

My little tester I’ve started drawing up plans for is simple and hopefully if all goes to plan I can test caps up to 50 Volts. I hardly deal with anything higher then 50 Volts so this will work fine for my applications.

The tester is simple to make and use. The circuit is basically a DC power supply and a analog meter. The idea is you load the cap with DC and you get the reading. A Capacitor is loaded with current until it is full then stops. On the analog meter the needle will raise until the cap is full then drop near to zero if the cap isn’t leaky. If the cap is leaky then the needle will not go near zero.

Really and truly if you have a bench power supply that can reach 50Volts or higher then all you need is to make a simple adapter, however if you don’t then you’ll have to build the power supply side.

When I was trying to draw this up I wanted to add a couple of features.
The first one I wanted to use momentary switches to control it. This way if the cap wants to blow up I can stop feeding it power quickly. Another switch to the discharge circuit since you don’t want to test the cap and leave it charged. A great feature on it’s own since using momentary switches is less likely of error to happen such as leaving the discharge circuit enabled and trying to get a reading for the cap. If you keep having the discharge bleed resistor enabled in the needle on the meter probably would never move thus giving you a false reading.
The other thing I wanted to make sure of and of-course is way overkill was to use high rated resistors. If m math is right amps times volts (0.5 Amps)(50 Volts) = 25 Watts.
The highest rated resistors I have in my junk box are 7 Watt. The meter I have is from a old car battery charger and another one I could try came from a old amp meter. I just have to find the proper resistor to get the meter to full tilt.

Here is a quick rendition of the circuit.
I just need to find a Transformer that can meet my needs. The voltage doesn’t have to be exact since we don’t use caps at their full voltage rating. So if I can find a transformer that can do 40Volts then I would be happy. However I would need a multi-tap transformer that can do other voltages 5 volts, 12 volts, 20 volts and 30 volts. If anything I could use a Transformer that can handle the 40 or so volts and use this circuit.

Instead of a POT I could use a rotary switch and some fixed resistors to get the proper voltage levels to test different rated caps. For now I just need to find a proper Transformer in my little collection I got.


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