Easy to build capacitor checker A must for checking parts .

Hi. I just came accross a nice little device I made a number of years ago that's easy to make, and, works well. It's a capacitor checker. It will not properly test electrolytics, but, is good for paper, and, mylar capacitors, as well as mica caps with some tweaking. I thought I'd start out by showing some pictures of it.

In Picture number one, this is the entire outside of the device.

In picture 2 notice the top is removed, and, you see the little circuit board the parts are mounted on.

In picture 3 please note the placement of the parts.


I used a simple little experimenter's box that I picked up at Radio Shack. The bulb is a 120 volt neon panel lamp with a series string resistor in it. I used a couple of old 2A 1KV pip silicon rectifiers that I put inseries in case one shorts out.
I used two corectly colored meter leads with pin connectors, but, leads with aligator clips would work even better.
Then, a power cord, and, plug. I would reccommend using a polarized plug, and, putting the ground side, which is the large slot, on the neagative test lead.
The hot side of the cord goes to the ac side of the first rectifier. You could even connect an on-off switch there to turn it on, and, off, but, I simply un plug it.
The ac side of the second diode goes to the dc side of the first diode.
I accidentally broke a wire openning it up, but, then, one side of the 120 volt neon bulb connects to the positive side of the second diode.
The other side of the neon lamp, and, mind you, you have to make sure that resistor is in series with either side, goes to the red test lead.

Different bulbs will have different amounts of sensitivity.
To check lower value capacitors, including mica, two things are important.
A sensitive neon bulb.
And, a reversing switch so you can rapidly reverse the direction of the capacitor, which is especially helpful with mica capacitors.

This is just the device in its' simplest form. Ohter things can be added to improve it, and, even make it safer.
But, I find that this little device serves me well.
It will show shorts by the bulb lighting full brightness.
If it continually flashes without ceasing, the capacitor is leaky. The faster it flashes, the leakier it is.
It it flashes once, then, quits, the capaccitor is good.
Reversing the direction of the capacitor will test prove this. That's where the reversing switch would come in handy.

If you check any capacitor, always, of course, be sure to discharge it before handling it by shorting the two capacitor leads together. Otherwise, you can get a nasty shock.

Also, the higher value capacitr ratings, such as .25 on up, will take longer to charge up enough to get an acurate reading. But, once fully charged, you will get a very acurate reading of the condition of that capacitor. Always, of course, disconnect one end of the capacitor being tested first, as this will affect your readings.
Bill Cahill

Wow... that is definately my new project!!!

Thanks Bill

That's a good start.
Technically the larger blade on the plug is normally connected to the neutral wire, not ground.

You can also use a multimeter as an alternative to this, (a lot safer for the Newbies. )
Again make sure the capacitor is discharged before starting the Test.
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Note the capacitor is connected in series with the neon bulb and the rectifiers.
Don't try this on low voltage caps. use this on caps rated at least 100 volts and above only. And have fun.

Nope. A multi meter is not sensitive enough to check caps unless they are dead shorted.
This device is only bare bones. I reccommend a momentary switch on red wire for safety, and, any low amperage ac 120 volt power transformer just for extra safety.
Bill Cahill

Hey Bill I have used a Simpson 260 on testing many caps. (Mostly Electrolytics I must admit)The only thing is it will not test are very small caps, but they can be used on some caps, and I'm sure the newbies probably ain't into cap testing unless they know electronics.

Another way to test caps, is with a 2 flashlight batteries a resistor and a LED in series with the cap in question. (safer IMHO)

Your Idea is a good one though and is practical
Do you have the ability to draw the schematic and post it Here?
That may help. Thanks for your input

I can draw a schematic, but, will have to take a camera picture of it.
My printer committed suicide.
Bill Cahill