Measuring ESR of 100v capacitors

[Mp3]

Hi everyone

I have two 22k uF 100v capacitors i want to test the ESR on.
I have a Dick Smith design ESR meter but i dont want to fry it with 100v.
How can i safely test it?

PS can i do any useful tests with my fluke 101 on it? I thought testing DC voltage but my meter only reads something like 5k uF maximum.

[Ian.M]

Discharge the capacitors before testing. 

[Mp3]

Discharge the capacitors before testing. 

Isn't it potentially dangerous to short out a 100v capacitor??

[ArthurDent]

Ian.M said discharge, not short. It is common to use an appropriate value resistors to drain the charge from a capacitor without putting a direct short across it. You could even use an incandescent 120 volt light bulb for a 100 volt capacitor. 

Keep in mind that if you don't leave the load across the capacitor for a long enough time, dielectric absorbtion can cause some voltage to redevelop. High voltage low leakage capacitors are stored with wires across the terminals for safety.

[Ian.M]

Yes.   If you short a large capacitor the high transient current can even vaporise what you shorted it with or its terminals or internal connections.  However, as Arthur just pointed out,  discharging a capacitor can be done without shorting it - just use a suitable wirewound resistor that can handle the peak power.  As a rule of thumb, the peak wattage (from V²/R) shouldn't exceed 10x the resistor's continuous rating.

A 22uF 100V cap only stores 0.11 Joules of energy when fully charged.  Discharging it by shorting it  directly across its pins ONCE is unlikely to damage it.   Repeated shorts when fully charged, or shorting a larger or higher voltage cap with more stored energy would not be advisable.

[Mp3]

Ian.M said discharge, not short. It is common to use an appropriate value resistors to drain the charge from a capacitor without putting a direct short across it. You could even use an incandescent 120 volt light bulb for a 100 volt capacitor. 

Keep in mind that if you don't leave the load across the capacitor for a long enough time, dielectric absorbtion can cause some voltage to redevelop. High voltage low leakage capacitors are stored with wires across the terminals for safety.

Thank you for this post as i've forgotten that discharge is safe to do 

I just hooked up a 120v light bulb across the terminals and will be checking it later today Thank you both!

(I just read Ian's comment about how few joules are actually stored in it, but better safe than sorry i guess.)

[Ian.M]

If you don't want to be spending far too much of your time repairing ESR meters, stick a strip of heavy brass or copper to the meter with double sided tape, and *always* touch down the capacitors leads on the metal immediately before connecting them to the meter.  If you forgot to discharge it safely or its regained charge, you may get a bit of a bang but the habit will save your meter from damage,

Any cap with the potential to store more than a Joule should be treated with respect.  If it can store tens or hundreds of joules treat it with extreme respect - its potentially lethal.