Re: Electrolytic Cap Leakage revised



"JC" <jcandela@xxxxxxxxxxx> wrote in message
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"Iain Churches" <IainNG@xxxxxxxxxxx> wrote in message
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Hi Jim,

I don't quite follow you.

The 10M is in series with the cap to reduce the charging current.
The meter is across the cap to read the voltage on it.

The second meter is across the 100k resistor to measure the
voltage across that (with the 10M switched out) to allow the
voltage across the 100k to be read and the leakage current
calculated.

http://www.kolumbus.fi/iain.churches/Projects/ElectrolyticCapacitorTester.html

Draw your circuit in the charge mode with the 10 meg resistor between the
power supply and the capacitor. Now draw in another 10 meg resistor across
the capacitor. The capacitor cannot charge to anything beyond 50% of the
power supply voltage, and will be less than that because of the capacitor
leakage. The second 10m resistor is your DVM.

Hi Jim. OK. Now I see your point:-)

You are right about the 3 hours since 5 R-C time constants work out to 183
seconds.

I had not thought about being able to calculate the
charging time.Please elaborate.

In cases like yours I remove the voltmeter to allow for a higher capacitor
charge voltage, and then I connect the DVM later to measure the voltage
even though the voltage will start dropping as soon as the dvm starts
loading the capacitor. Remenber probing a circuit changes the circuit.


Good. I will try that.

I think a series resistance of 100K is more appropriate than either 3m or
10m. You can still measure the voltage across that 100K to measure leakage
current.

It seems to work well.

Having a variable power supply is handy too so you can charge the
capacitor gradually to a given voltage. The old Eico capacitor bridges
with a 1629 magic eye tube are ideal for forming electrolytics in a non
destructive way. Those old Eico's go up to around 450 volts with the turn
of a knob.

I have a Farnell bench supply and so could charge the cap
by switching first to say 50V and leaving it there for an hour
or two, and then to 100V and so on.

I have always understood that a cap, if it is to be reformed,
needs to be charged very slowly indeed, over the course of
many hours. That's how I decided on the 10M resistor for
reforming and a 100k for leakage testing,

Jim. It's nice to discuss in a rational way with someone who
doesn't rant and rave and call one a f*ckw*t. Thanks.

Regards
Iain



.

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