ESR Meter for Testing Polyester Capacitors

[Ingenieur]

Hey guys, I just had a quick question- I am considering buying an ESR meter (specifically a MESR-100 v2) to work on an electronic ballast for an underhood light I own that no longer works. I am sure I will need it in the future regardless so I am buying an ESR anyway, but this particular ballast uses a lot of of polyester capacitors ("green caps")- will an ESR meter work on polyester caps or will it work only on electrolytic capacitors?

I have searched the forums and I can only find one post which says it will (albeit you run into problems due to the polyester caps tending to be such a low capacitance rating) but I just want to confirm beforehand.

Thanks in advance.

-Techniker

[budm]

Re: ESR Meter for Testing Polyester Capacitors

Electrolytics only. Instead of spending money on the meter now, recap the whole board with good caps. Polycap you can just use capacitance meter and Ohm meter for resistance leakage.
I have Version 1 that I purchased for our repair department over a year ago to use, so far the techs are able fixed many boards due to caps with high ESR.

[Ingenieur]

Re: ESR Meter for Testing Polyester Capacitors

Electrolytics only. Instead of spending money on the meter now, recap the whole board with good caps. Polycap you can just use capacitance meter and Ohm meter for resistance leakage.
I have Version 1 that I purchased for our repair department over a year ago to use, so far the techs are able fixed many boards due to caps with high ESR.

Thank you for the response! And I'm glad to hear that your shop likes the MESR-100- I need one anyway so I suppose I will be buying one.

Just for my own edification, why will the ESR meter not work with polycaps?

I also have a question about the ohmmeter with a polycap- won't that only indicate if a polycap is shorted? I am under the impression that an ohmmeter will not work with a capacitor since capacitors do not pass direct current. Also- can this test be done in-circuit? I know the capacitance measurement cannot be done in-circuit.

Thanks again,
Techniker

[budm]

Re: ESR Meter for Testing Polyester Capacitors

Poly cap can fail open (no capacitance) or has leakage resistance using Ohm meter on the highest scale (note that since Ohm meter does not put out the working voltage rating of the cap which can breakdown when apply rating working voltage to it, Ohm meter just give you some idea if it has breakdown or not), then you use cap meter to check the capacitance. I always test the component off the board. Safety cap will fail in open mode.

[ben7]

Re: ESR Meter for Testing Polyester Capacitors

Poly cap can fail open (no capacitance) or has leakage resistance using Ohm meter on the highest scale (note that since Ohm meter does not put out the working voltage rating of the cap which can breakdown when apply rating working voltage to it, Ohm meter just give you some idea if it has breakdown or not), then you use cap meter to check the capacitance. I always test the component off the board. Safety cap will fail in open mode.

Yep indeed.

The poly capacitors are self 'healing'.

When a dielectric breakdown occurs, a small spark (short circuit) occurs, and that vaporizes the metal electrodes close to the hole in the dielectric. This, over time, will result in a loss of capacitance.

[Ingenieur]

Re: ESR Meter for Testing Polyester Capacitors

Yep indeed.

The poly capacitors are self 'healing'.

When a dielectric breakdown occurs, a small spark (short circuit) occurs, and that vaporizes the metal electrodes close to the hole in the dielectric. This, over time, will result in a loss of capacitance.

I'm trying to learn here so I would appreciate any feedback on this: From what I have found, and my interpretation, ESR for anything other than electrolytic capacitors tend to be not only be very low but also relatively stable throughout their entire life (I assume due to this self-healing property which also leads to a rather insidious decline of capacitance) and our current measurements for ESR just are not sensitive enough to be able to easily declare a film capacitor "bad" or "good".

Alternatively then, could one use tan d (the dissipation factor) which is a function of ESR over capacitive reactance (a function of capacity and frequency) to evaluate a capacitor in-circuit? It seems to me that mathematically since the DF is inversely proportional to capacitive reactance and capacitive reactance is in itself inversely proportion to capacitance, that DF (tan d) would therefore be directly proportional to capacitance, regardless of what type of capacitor it is and therefore be better able to give an idea of the health of the capacitor.

Sorry for the run-on sentence.

-Techniker

[budm]

Re: ESR Meter for Testing Polyester Capacitors

Yes, you can use math using known data and measured values, but to do that in circuit, it will not be correct since you will have other components in parallel with what you are testing. You also need to test the leakage current at working voltage.