ESR to the can?

[Thor__]

Years ago I used to use the can of an electrolytic as a ground point when measuring voltages, it was easy to get at from the top of the board.

I have a couple of crashed Samsung 943BX (2006-8) monitors and went at them with my blue ESR meter. Right off I find two Sam Young caps, 820@25v in parallel so I tested on-board and I get zero for ESR, no bulging and the bung is inplace and it is not bulging.. I unsolder them and I get zero ESR but their capacitance is in the ballpark, I think it was 860 uF. So, I take an ESR reading from positive to the can and I get 24 ohms! Wha? I have the cap I have the meter so I check from negative to the can, expecting zero but I get 24 ohms again, gasp!
Using a DMM gives no reading on the lowest scale, I do not remember if I check for a capacitance reading to the can or not, probably way to low to read with my gear.

I guess to round this up and ask an actual question, should I be reading
ANY ESR from positive to the can or negative to the can? Does a reading to the can from any of the leads indicate a short or leak or what?(by leak I do not mean electrolyte leaking from the capacitor) Does the zero ESR indicate and open cap or maybe a fault in my blue ESR meter?

If you need more info then just ask.

Thanks for your time,
Thor

[budm]

Re: ESR to the can?

ESR meter uses 100KHz AC signal as the source so you are reading AC leakage signal due to capacitive coupling between the can and the cap construction (aluminum foil/insulator sheet/electrolyte).

[Thor__]

Re: ESR to the can?

So does the ac leakage to the can indicate a bad cap or is it bad design? I checked several others and some gave reading where other did not. I am trying to determine if I should always test to the can or not and I haven't seen anything mentioned anywhere about testing to the can.

I didn't do the math yet but I think that zero ESR reading is telling me that cap is bad even if it reads near right on a capacitance meter. Also about the zero reading, that changes to about .01 depending on , I think, the temp of the cap and that is only warming it up by my hand or cooling it by ambient about 72F. Still, .01 ESR for a 820uF Sam Young just seems wrong.

Just to be clear, the capacitors I was measuring were indeed unsoldered and off the board.


Thanks for the reply,
Thor

[goontron]

Re: ESR to the can?

mind you some caps have the ground lead bonded to the can

[Jooo]

Re: ESR to the can?

I didn't do the math yet but I think that zero ESR reading is telling me that cap is bad even if it reads near right on a capacitance meter. Also about the zero reading, that changes to about .01 depending on , I think, the temp of the cap and that is only warming it up by my hand or cooling it by ambient about 72F. Still, .01 ESR for a 820uF Sam Young just seems wrong.

ESR meters aren't really accurate, they give just a estimate of the ESR.
So, it probably means that the capacitor is just very low ESR, below 0.01 ohms. If you can give the series of the capacitor, datasheet would probably confirm this.
I have seen several brand new capacitors measuring 0.00 ohms with my Atlas ESR70.

[Thor__]

Re: ESR to the can?

Sam Young series NXC ESR max 36m ohm/20C @ 100Hz, from the samyoung datasheet that is dated 10/04/07.

So, .036 max versus a reading of .00 or .01, could have been my leads not fully zeroed. These are the brown colored capacitors you typically would see in an older samsung LCD monitor.

I see nothing about reading terminals to the can. To me it looks like there would be one or two capacitors in series or parallel with the actual capacitor. Positive to the can negative to the can and the actual capacitor. Am I just looking at this wrong or does someone know whether or not there should be any readings to the can from either leg?

I put those caps back on the power board and ran it for about an hour and it is now a failed board, I haven't checked any voltages at this time so I do not know where it failed, but do not let that lead anyone astray from my question about measurements to the can.

Thanks for all of the replies,
Thor

[tmiha71]

Re: ESR to the can?

So, .036 max versus

After endurance test, few thousand hours on 105 degC, with rated voltage and ripple current...

[Kiriakos GR]

Re: ESR to the can?

Does the zero ESR indicate and open cap or maybe a fault in my blue ESR meter?

How about mesuring a known common resistor, this will do the trick.

[Thor__]

Re: ESR to the can?

Yeah, I checked with a 10 ohm resistor and my blue ESR shows agreement with my DMM.

I am going to have to settle on what I am seeing as a VERY low/open ESR reading. So low that it is a fault. Along with this I am also seeing the cap is shorted to the can, AC short and not DC. It is a faulty capacitor.

A note to everyone , I will continue to measure ESR to the can. I checked the capacitance of the short to the can and it is 3uF, bad design or whatever, it should not be there. Other caps I also checked to the can were faulty too. I can understand a bonded negative lead bot not a capacitive coupled connection.

Thor

[Kiriakos GR]

Re: ESR to the can?

A note to everyone , I will continue to measure ESR to the can.

A note to everyone , I will continue to measure ESR by a worthy LCR meter of 200$, and I would always desolder the capacitors so to avoid interference by germanium diodes, plus I would inspect every capacitor at the specific frequency noted by the manufacturer in his datasheet.