Power supply filter capacitor measurement in circuit

[Fritter]

Hi
Measuring a 400v 330uf capacitor across the bridge rectifier and in circuit with a multimeter on ohms. Measures 360 ohms both ways when polarity on the meter is changed. If this is normal, I'd like a brief lesson
Thanks

[dmill89]

Re: Power supply filter capacitor measurement in circuit

Hi
Measuring a 400v 330uf capacitor across the bridge rectifier and in circuit with a multimeter on ohms. Measures 360 ohms both ways when polarity on the meter is changed. If this is normal, I'd like a brief lesson
Thanks

What are you trying to test?

If it is the ESR of the capacitors you need a specific ESR meter not a general resistance measurement from a multimeter.

Some ESR meters claim to be able to test in circuit, but everything I've seen indicates they aren't very accurate and the capacitors really need to be tested out of circuit to get a good reading (and personally if I'm going to the trouble of removing a capacitor from circuit it is getting replaced regardless if it is a "problematic" brand or more than 10 years old unless it is something exceptionally expensive/difficult to come by).

In any case if you're trying to test a cap you want to go directly to the leads of that cap and not through other components. While "acceptable" ESR varies by each series/size of cap (you can generally google the brand/series of a cap to get the data sheet), a cap that size should generally be less than 0.5 ohm (some are as low as 0.1 Ohm), and definitely less than 1 Ohm.

[Fritter]

Re: Power supply filter capacitor measurement in circuit

Unfortunately I don't have an ESR meter. I was just checking to see if the cap was charging or not, but get a static resistance reading that is the same regardless of polarity.

[stj]

Re: Power supply filter capacitor measurement in circuit

what your seeing on the meter is probably the diodes in the rectifier
there should also be a discharge resistor across the cap(s) but that os going to be big.
like 200k > 1meg
if it doesnt have that resistor - be very carefull around the cap!

[dmill89]

Re: Power supply filter capacitor measurement in circuit

what your seeing on the meter is probably the diodes in the rectifier
there should also be a discharge resistor across the cap(s) but that os going to be big.
like 200k > 1meg
if it doesnt have that resistor - be very carefull around the cap!

^This, if your meter supports capacitance test you could try that (though you'll likely need to remove the cap from the circuit to get an accurate reading). You could also probe voltages at various points in the PSU (with the PSU plugged in) to see what is getting power (but as STJ noted be careful doing this as the primary side of a PSU is high voltage and can give a nasty shock).

Unless the cap is dead-short or open it should be "charging" assuming it is getting power, but the capacitance (it could be something like .1uf if the cap is dried out) and ESR could still be in question.

[Fritter]

Re: Power supply filter capacitor measurement in circuit

Thanks for the pointers.Definitely needs to come out. (and I need meter!)

[Fritter]

Re: Power supply filter capacitor measurement in circuit

what your seeing on the meter is probably the diodes in the rectifier
there should also be a discharge resistor across the cap(s) but that os going to be big.
like 200k > 1meg
if it doesnt have that resistor - be very carefull around the cap!

Board is out and cap measured 0v. Measures 350 ohms both ways.

[stj]

Re: Power supply filter capacitor measurement in circuit

then it's fucked
it should look really low and then slowly rise to infinity as it charges

[momaka]

Re: Power supply filter capacitor measurement in circuit

Board is out and cap measured 0v. Measures 350 ohms both ways.

Could you double-check your measurements? 350 Ohms or 350 KOhms?

I don't think that cap could fail with 350 Ohms (DC) resistance and not bulge / heat up like crazy when the PSU is plugged it. At 340V DC (230-240V AC rectified), that would essentially imply the cap to be drawing 1 Amp of current... which at 340V is 340 Watts!!! Either the cap would immediately overheat and vent or it would short-circuit and blow the fuse.

Since you don't have an ESR meter, here's a better test:
Find a 18/19.5/20V laptop power adapter or 24V adapter from the likes on an inkjet printer. Then connect (-) on the adapter to (-) on the cap and (+) on the adapter to (+) on the cap. Plug in the adapter to change the cap. Now disconnect the cap from the adapter and measure the DC voltage on its leads. Does it read ~20/24V and hold that value fairly steadily (it will decrease overtime as the multimeter discharges it, but shouldn't decrease too quickly... maybe 10's of mV per second or so.)
Now take a screw driver or some other metal object, put it on one terminal, and then slowly move it over towards the other terminal of the cap to touch it. You should hear and see a somewhat decent spark (though nothing too crazy.) If you do, the cap is probably not dead... though no telling if the ESR is still good or just marginal.

Now, if you do have some kind of power adapter that does more than 20V (30-48V DC would be nice), that would be better. Charging the cap with such would definitely produce a much bigger spark when you short it... and that would be a fairly good indication the cap is at least somewhat healthy. Dead caps tend to loose charge very quickly (due to high leakage current) and tend not to spark well or at all, especially after being left for 10-20 minutes charged up.