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Megger for checking electrolytic cap leakage




 
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Author Topic: Megger for checking electrolytic cap leakage  (Read 662 times)
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W4AMV
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« on: January 13, 2020, 07:44:47 PM »

Sorting some capacitors and weeding out the not so good ones. I have a hand crank megger which at full crank produces 500 V. Testing ceramic disks it appears to perform fine. Checked out large R values for HV application, fine as well.

However, in testing some 330 uF/450 V electrolytic caps, it gives near ZERO ohms and a false BAD cap reading. As I have taken the same cap, applied near rated V from a supply and see nearly zero current. Yes, as the supply V is dialed up, the current spikes and then quickly steady states to zero. I have placed a series diode in line with the cap under test. No change, indicates faulty cap. Any ideas?

I should mention, the unit I have is noted as a

O H M M E T E R
A N / P S M - 4 3
INCLUDING
ZM-21C/U

SO... it is called a OHMMETER but in every respect operates like a hand crank megger.

Manual attached with the clever meter schematic.

Thanks, Alan

* ZM21C_U instrunctions.pdf (303.02 KB - downloaded 30 times.)
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W4AMV
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« Reply #1 on: January 14, 2020, 01:00:29 PM »

After more careful review, I suspect this unit does not have the energy to sufficiently deliver the required charge to a large capacitor. See schematic. Fine for the measurement of insulation resistance. Hence, not all megger units are comparable for function. For small disc and mica units, probably fine.
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iw5ci
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« Reply #2 on: January 14, 2020, 04:19:17 PM »

While the megger is fantastic for finding bad paper caps , ceramic, black beauties and and Kind of non polarized caps, it is not suitable for detecting leakage in electrolitics. You can damare low voltage caps, if you crank it fast. I used a loro my megger, but Now i prefer a vintage heathkit capacitor  tester with neon lamp for detecting leakage.
It works fine with elrctrolitics. I also use a small chinese component analizer for measuring esr.
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W4AMV
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« Reply #3 on: January 15, 2020, 10:56:22 AM »

Thanks for your reply. Agree! Yes, that is in line with my observation and a different instrument is required.
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WU2D
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« Reply #4 on: January 18, 2020, 10:50:49 AM »

I got this military surplus job from a ham who was cleaning out his basement. I use it to death. Of course it is a capacitance meter.  And does leakage in two ranges, 0-60V and 0-600V. And other interesting capacitor measurements.


* CapMeter.jpg (716.05 KB, 4527x2846 - viewed 74 times.)
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These are the good old days of AM
KD6VXI
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« Reply #5 on: January 18, 2020, 11:06:42 AM »

I got this military surplus job from a ham who was cleaning out his basement. I use it to death. Of course it is a capacitance meter.  And does leakage in two ranges, 0-60V and 0-600V. And other interesting capacitor measurements.

Recently calibrated, too!  👌😎

--Shane
KD6VXI
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AG5UM
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« Reply #6 on: January 18, 2020, 07:55:54 PM »

I use the 1950's Sprague Tel-Ohmike, TO-4, which I really like.
Your Sig-Corp ZM-3/U  looks great, I didn't know they made that, very COOL.
AG5UM
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