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 Post subject: Easy to build capacitor checker A must for checking parts .
PostPosted: Oct Fri 31, 2008 6:00 pm 
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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.
Image
In picture 2 notice the top is removed, and, you see the little circuit board the parts are mounted on.
Image
In picture 3 please note the placement of the parts.
Image

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.
:D Bill Cahill :D

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 Post subject:
PostPosted: Oct Fri 31, 2008 7:26 pm 
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In order to be fully usable as a Hint&Tip, you need to add the schematic and a parts list and parts sources Bill.

ALSO PLEASE NOTE: To all who may build this device,
please be aware that it it connected DIRECTLY to 120 volt AC mains.


There is a potential shock hazard that any user should be made aware of.

Chuck

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 Post subject:
PostPosted: Oct Fri 31, 2008 8:22 pm 
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Chuck Schwark wrote:
In order to be fully usable as a Hint&Tip, you need to add the schematic and a parts list and parts sources Bill.

ALSO PLEASE NOTE: To all who may build this device,
please be aware that it it connected DIRECTLY to 120 volt AC mains.


There is a potential shock hazard that any user should be made aware of.

Chuck


The only safe way to build this is with a fused AC isolation transformer.

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 Post subject:
PostPosted: Oct Fri 31, 2008 8:41 pm 
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As far as that goes, you don't need a dedicated isolation transformer. Just some old power transformer out of an old radio. The current requirements are nil, so if you have one of those old power transformers out of an old VTVM or other small piece of AC operated test equipment, they would be fine and provide the isolation you need.
Curt

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 Post subject:
PostPosted: Nov Tue 04, 2008 11:31 pm 
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I built a capacitor leakage tester, years back, as well. I made it from an article in Poptronics or Experimenters Handbook. It operates similar to yours, but it has additional parts. It uses two diodes as well, but they are configured as a voltage doubler to put ~200VDC on the capacitor under test.
The test leads each have a 100k resistor in series which limits the current making the device much safer to use.
Capacitors are cheap and I replace them in every receiver I work on. My curiosity gets the best of me though, so I test the caps I pull out, before I toss them. What I have found is that all the old caps show a little leakage under power, but some are really leaky and some even shorted. I thought something could be wrong with the tester until I checked a batch of new caps. They all tested normal with no leakage at all.
If anyone is interested in a sketch of this tester, send me your email.

Joe


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 Post subject:
PostPosted: Nov Tue 04, 2008 11:49 pm 
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Before I had my Heathkit IT-28 I made a safe cap checker for real cheap. I snapped 10 9v batts together for a 90v supply. I put my DMM(10 meg) voltmeter in series with the cap and supply. Any cap leakage will show as a voltage after charging. This was for paper and mica caps. Even the slightest leakage will show up and it's very safe and did I say cheap!


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 Post subject:
PostPosted: Nov Wed 05, 2008 2:27 am 
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Here are a couple of scans of the "Capacitor Leakage Tester" that Joe mentioned. The article appeared in the December 1959 issue of Popular Electronics:

Image

Image

I made one. It proved old caps were usually leaky while new capacitors displayed no leakage at all. I gave it to my son to check automotive distributor points capacitors. Time to construct another, I guess.

Ron


Last edited by Ron Reeland on Nov Tue 11, 2008 3:26 am, edited 2 times in total.

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 Post subject:
PostPosted: Nov Thu 06, 2008 9:21 pm 
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Thanks for posting that article Ron. I can't believe almost a half century has passed since I built it. It makes me feel old.

Joe


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 Post subject:
PostPosted: Nov Fri 07, 2008 9:57 pm 
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Threw together the tester Ron posted from Popular Electronics. Worked fine and helped me find some leaky NOS capacitors that I had laying around.

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 Post subject:
PostPosted: Nov Sat 15, 2008 2:35 am 
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When using the Popular Electronics version, are you limited to caps with a voltage rating above 300 VDC?


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 Post subject:
PostPosted: Nov Sat 15, 2008 3:18 am 
Silent Key

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Since it uses a voltage doubler off the AC line, I would have to say yes.
Curt

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 Post subject:
PostPosted: Nov Tue 18, 2008 1:47 am 
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Here are a couple more scans of capacitor leakage testers. Notice the variable voltage version. The simple addition of a potentiometer make it capable of checking lower voltage capacitors.

Image

Notice the 5 watt rating of the pot. It will get a little warm, I'm sure.

Ron


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 Post subject:
PostPosted: Nov Tue 18, 2008 2:54 am 
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Excellent! Okay so I have my cap checker wired exactly like the first diagram on the above response (with the spst switch). Mine does not work, however, at all! I cannot figure what I am doing wrong. I read all the wiring as well as individual components and they all check good. My light is rated for 6.3V at .15A but I am just not getting voltages to my probes. I even plugged it in, turned it on, and tried shorting out the probes to see if I get a spark. Nothing. There is no voltage getting to the alligator clips, yet continuity checks out.
Any ideas?

Thanks,
Chris


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 Post subject:
PostPosted: Nov Tue 18, 2008 1:01 pm 
Silent Key

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That bulb has to be a neon bulb. A regular incandescent bulb will not work in the circuit.
Curt

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 Post subject:
PostPosted: Nov Tue 18, 2008 6:03 pm 
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Is the SPDT switch up there where it says "check/test" making good contact?

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 Post subject:
PostPosted: Nov Tue 18, 2008 10:34 pm 
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Curt...I think that may be it. Ill go pick up some neon bulbs and let you know. Thanks.
Chris


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 Post subject:
PostPosted: Nov Wed 26, 2008 2:26 am 
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Ok sorry if this is a dumb question but here goes.

Can this type of cap tester be used on electrolitics? If not can some explain why please?

Thanks

Just trying to learn something...


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 Post subject:
PostPosted: Nov Wed 26, 2008 4:39 pm 
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ipwizard wrote:
Ok sorry if this is a dumb question but here goes.

Can this type of cap tester be used on electrolitics? If not can some explain why please?

Thanks

Just trying to learn something...


Not really! This tester is for coupling capacitors only.

1st. Safety! The voltage doubler tester puts out a couple of hundred volts DC. Electrolytics are polarity conscious and would have to be connected one way only - or else! You would also have to watch the voltage rating and not connect a lower rated capacitor.

2nd. Since the capacitance rating is thousands of times more with an electrolytic (example .02mfd vs. 50mfd), the time constant of charging in this circuit would take forever. Instead of a blink, the neon would glow brightly, then dim over time as the capacitor charges, all while you are holding the button. I'm talking a long time! Still, the neon will probably never go out completely.

3rd. When you test the electrolytic it will charge and store the voltage over time. After testing, it needs to be discharged before you touch the leads. Letting go of the switch discharges the coupling capacitor, but the large charge on the electrolytic will probably zap the switch.

Joe


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 Post subject:
PostPosted: Nov Wed 26, 2008 6:45 pm 
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Good one.


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 Post subject:
PostPosted: Apr Fri 24, 2009 2:24 am 
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To continue from Joe's answer, there are glow tubes that will take a lot more current than a neon, and they could be shunted with a resistor to speed up charging time. The danger is that electrolytics store enough electrical energy that they can explode. I found this out by doing it.
If anyone is determined to test electrolytics, please build an enclosure that can contain an explosion. I don't know of any way to predict the possible size of "pop" based on the capacitance.


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