Odd effect when measuring cap

[Tom41]

I got some old Fuhjyuus out of the SUNTEK PSU I mentioned in the other thread, and decided to test them with a multimeter. With resistance (ohms) setting, I noticed a value on the display which was slowly increasing until it reached a certain point, then did not go up any further. I assume the cap is charging up from the battery inside the multimeter, but what exactly am I measuring here? General resistance? ESR? Capacitance?

[kc8adu]

Re: Odd effect when measuring cap

meter voltage is trying to charge it.it should go infinite but since it a known badcap its likely leaky.electricly and physicaly.

[whiz]

Re: Odd effect when measuring cap

Really? can we messure leakage with the ohm meter??
i have some caps that going almost infinite and some other that are totally
infinite does this shows they are leaky?

P.S some capacitors when you connect them with the ohm-meter they go up to 1Kohm and they slowly fall...if we have 2 identical caps and there is a difference (at which point the ohm-meter goes and then falls) can we understand something about the caps?!

thanx!

[gonzo0815]

Re: Odd effect when measuring cap

Ypur cords are a powers ource, if in ohm messuring mode. So if the caps is ok, the cap will sink all current untill it is charged. this will result in a low reading wich will go higher till open. If the cap is bad, the leakage current could get high. Thus the cap won`t charge and the reading would not go infinite. If the cap is open, then now low reading would occure, hence the cap isa dried up.
Any way, this is very basic and rough testing. Untill the problems are obviouse, there is not mutch won. The only exact messurment is IMHO an ESR meter and a caüacitance checker. But for SMPS i think ESR meter is all what it needs to find faulty caps.

[willawake]

Re: Odd effect when measuring cap

hmm i thought it would be the cooling down from the desoldering. i didnt know the meter woud charge the caps.

[whiz]

Re: Odd effect when measuring cap

hmm i thought it would be the cooling down from the desoldering. i didnt know the meter woud charge the caps.

You probably don't play alot with your analog multimeter!


@gonzo0815 thank you for your answer! could you please explain me better this?

When we connect a capacitor to the ohm-meter the first indication we get shows low ohm resistance and then starts to fall slowly or fast to infinite resistance...
I understand that if the cap does not fall to infinite, then this shows it is leaky...The first indication (low ohm resistance) what indicates? and how low is it good to go?

do we need our capacitors to touch infinite? if it is slightly higher is it problem?


P.S i understand that this is not the bes method to test caps but why you call it rough test?

[gonzo0815]

Re: Odd effect when measuring cap

do we need our capacitors to touch infinite? if it is slightly higher is it problem?


P.S i understand that this is not the bes method to test caps but why you call it rough test?

With inifnite i mean there should be an open reading, eg. a resistance far beyond the usuall 20Mohm range of standard DVM. It should the same reading if the cords are on air not conected to anything. it is obviousely, that there is nothing wich can go higher than infinite. But ok, if there would be a lower reading e.g. 20Mohm it could be tolerated may be....

Well, with that testing, you can meay be deterime on an old hifi amplifer that the big Al caps would have ben worn out, or are totaly dry, but you can`t decide on a normal behavior in this test, that the caps has a suitable ESR or if the capacitance is in the range. And that basically is the question, wich is important for SMPS circuits, wich is my background for the above stated.
If you messure the time and the voltage during the test with a second DVM, you can may be calculate the rough capacitance, but nothing said about ESR.
I have some very old Jptur caps from an old Enermax, it was shuting down with any seriouse load. But if i tested them like above stated, i would have taken them as good.

Keep in mind, that even new caps or long stored ones could fail tho this test, and could be ok after some time under voltage. i hope you can get my view, why tist test is not good for a definite answer, if the cap is ok for the applikation or if it is worn out.

The ESr meter is different. If the ESr is to high, you know fore shure, if this is a problem (and hence the cap is bad) or not.
Capacitance meter will at least give an idear, if the cap can be used in some circuits, where ESR isn`t that important, but again not good or exact for SMPS.

@willawake

hoock a LED to an DVM and in most cases it would lite a bit (only low power types...normaly only about 3v). You need voltage to measure the current and hence be able to calculate the resitance.

[whiz]

Re: Odd effect when measuring cap

I looked on the internet and i can't find ESR tester...

[dood]

Re: Odd effect when measuring cap

I looked on the internet and i can't find ESR tester...

Then it must not exist!!!

[trodas]

Re: Odd effect when measuring cap

It does exist, however

Google - ESR meter:

http://members.ozemail.com.au/~bobpar/esrmeter.htm






PS. to stay on topic, the charging cap is visible on DMM too, if it got the analog gauge and simply by looking how the resistance increasing. But in real life, most recapped Vcore caps (all in parallel) does start at about 8 ohms and swiftly move to about 28 ohms, where is slowly then increasing the resistance to a little above 30 ohms, where it stop. Who can explain me that...?

[willawake]

Re: Odd effect when measuring cap

I looked on the internet and i can't find ESR tester...

https://www.badcaps.net/forum/showthread.php?t=1287

they shipped this to me in greece fine, nice meter. ask them though if they supply the leads cos chilli_sauce said they did not supply them anymore, though they did when i purchased it.

[whiz]

Re: Odd effect when measuring cap

Thanks guys!

@Trodas i found it on Google too...and some other home made ESR meters! i said that i can't find any, because i thought is is a meter widely used and i expected to find dosens from known brands (or unknown) like any ordinary multimeters...

possibly there is something that i haven't understood!

@Willawake do you think i can order via paypal? i have heard many bad things about paypal...

http://www.paypalwarning.com/

do you think it is safe to make so "big" transaction?

[kc8adu]

Re: Odd effect when measuring cap

It does exist, however

Google - ESR meter:

http://members.ozemail.com.au/~bobpar/esrmeter.htm

ROFLOL!