Author Topic: Is this a valid method for checking capacitor leakage?  (Read 570 times)

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Offline watchmakerTopic starter

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Is this a valid method for checking capacitor leakage?
« on: October 10, 2024, 10:02:05 pm »
Leaving aside the obvious safety issues (if this is valid I will make a safe enclosure), is this a valid approach to leakage testing?

After looking at several designs/kits, I realized I had this transistor withstand tester that does in fact put out over 1000 volts DC.  The display voltage agrees with my DMMs.

The cap shown is a 10uF 200 volt electrolytic.  I started with lower voltage caps of course.

If this is deemed valid, then it avoids a host of construction issues and the tester itself cost about $20 on Ali.  Just need a box some jacks, discharge and test switches.



Regards,

Dewey
 

Offline IanB

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Re: Is this a valid method for checking capacitor leakage?
« Reply #1 on: October 10, 2024, 10:06:48 pm »
You put an ammeter in series with the capacitor, and then charge it up on DC to the working voltage, and when it stops charging, measure the residual current?

I think that is a valid method for the DC leakage current.
 
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Offline watchmakerTopic starter

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Re: Is this a valid method for checking capacitor leakage?
« Reply #2 on: October 10, 2024, 10:14:05 pm »
Yes, that is exactly what I did.

THANKS!!  I will take some shots as I put the thing into an enclosure.
Regards,

Dewey
 

Offline IanB

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Re: Is this a valid method for checking capacitor leakage?
« Reply #3 on: October 10, 2024, 10:23:01 pm »
You might have to be a little careful about the inrush current if the capacitor starts out empty. Might be appropriate to put a 100 ohm resistor or so in series with the capacitor? It depends on the DC supply and what it is expecting as a load.
 
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Online edpalmer42

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Re: Is this a valid method for checking capacitor leakage?
« Reply #4 on: October 10, 2024, 10:52:47 pm »
If you're using high voltage (and 200V is high voltage) you need to have some way to discharge the capacitor to a safe voltage before you touch it.  It could be a SPDT switch that goes between 'charge' and 'discharge' with a voltmeter to measure the voltage on the capacitor.
 
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Offline watchmakerTopic starter

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Re: Is this a valid method for checking capacitor leakage?
« Reply #5 on: October 10, 2024, 11:12:11 pm »
If you're using high voltage (and 200V is high voltage) you need to have some way to discharge the capacitor to a safe voltage before you touch it.  It could be a SPDT switch that goes between 'charge' and 'discharge' with a voltmeter to measure the voltage on the capacitor.

Yes, the discharge switch should be emphasized.  I did say in my original post that I would install a discharge switch.  But I like your emphasis in case others want to follow this path.

Until last night and 3 hours of youtube/reading, I did not realize the importance of leakage current; or that capacitors are a primary failure point in electronics.  (As someone taught on tubes, I always thought it was those damned transistors).

This makes the variety of caps less bewildering to me. It is much more than value, tolerance, temp coeff and ESR.  What a fascinating puzzle for such a "simple" device.
Regards,

Dewey
 

Offline IanB

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Re: Is this a valid method for checking capacitor leakage?
« Reply #6 on: October 10, 2024, 11:23:16 pm »
The leakage current is not always "leakage" either.

Electrolytic capacitors are what they say, they have electrolyte inside them. The dielectric is actually an insulating layer formed on the surface of the aluminum foil by electrolysis.

Old capacitors that have not been used for a long time may need "forming" to rebuild the insulating layer. This forming process will consume a certain amount of current while it is occurring.

Furthermore, until the forming is complete, the capacitor will not withstand the full working voltage, and may fail if you increase the voltage prematurely.

Therefore, old electrolytics need to be handled with care, and if they need reforming, this should be done in the appropriate way before trying to test them.
 

Online edpalmer42

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Re: Is this a valid method for checking capacitor leakage?
« Reply #7 on: October 11, 2024, 01:48:16 am »
Another trap with capacitors is dielectric absorption.

Imagine measuring the energy you put into a capacitor to charge it and then measuring the energy that comes out when you discharge it.

"Wait a minute!  I put more energy in than I got out!  Where did the missing energy go??"

That's dielectric absorption.  The real kicker is that the missing energy will slowly come out and recharge the capacitor.  When you touch the terminals or connect it to a test meter, you can get a shock or blow up your meter.  You sometimes see high voltage capacitors stored with the terminals shorted to prevent this.
 


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