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Old 2nd Dec 2016, 5:15 pm   #41
julie_m
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Join Date: May 2008
Location: Derby, UK.
Posts: 7,735
Default Re: 'THAT' Capacitor. What is it?

Quote:
Originally Posted by DKW F102 View Post
I bought an E.S.R meter when I started geting my kit together but could not realy understand what it was supposed to be for, so have not used it since.
Equivalent Series Resistance (hereinafter abbreviated) is a property of any electronic component, and represents the fact that the connections are never perfect conductors. An electrolytic capacitor makes contact via the electrolyte, which can be a very imperfect conductor if the capacitor is old and worn out. In this state, its decoupling ability is reduced, since an AC voltage can exist across the equivalent series resistance; and there is now a positive feedback loop causing the capacitor to run warmer than it should, only accelerating its own demise.

An E.S.R. meter is designed to work at a very low voltage, too small to turn on a semiconductor junction, and a high frequency so the capacitive reactance is close to zero; so it can be applied to a capacitor still in-circuit without fear of false readings. It would normally be usedf or testing the capacitors in a switched-mode PSU, which are required to have a low E.S.R. for correct operation. But a high E.S.R. is a problem in any decoupling or smoothing capacitor; and also in a loudspeaker DC blocking capacitor, since it ends up stealing some energy that should have gone into the speaker.

Since a capacitor which has started to go high-E.S.R. probably also has begun failing in other ways, it can be useful for identifying faulty electrolytic capacitors in any circuit. I haven't got one yet, but I would definitely purchase one if I needed to do some repair work on a PSU or amplifier with many electrolytics. (But I probably would not buy one this close to Christmas, in case there was already one in Santa's sack ..... )

Capacitors used for DC blocking can develop an unwanted parallel resistance. This sometimes only begins to conduct at high voltage, so may not show up on a test meter using only a low voltage at the probes -- or even in real life, in a transistor circuit using only low voltages.
Quote:
Originally Posted by DKW F102 View Post
I measure my resistors with my AVO 8 and my capacitors with a Greyshaw CR 50 C/R Bridge. Both the AVO and the bridge have been overhalled /aligned by a kind member of this forum. Am I missing something I should realy be paying attention to ?
No, that's right. The AVO 8 was for many years considered the "gold standard" test instrument, until it was finally displaced in the 1980s - 90s by digital instruments such as the Fluke 77; and an AC bridge measures capacitance in a way that is less likely to be affected by DC leakage resistance than a modern, digital capacitance meter. The latter type work by measuring the time taken for the capacitor to charge enough to produce a certain voltage change, at a known current. A parallel resistance will take some of the energy that was meant to get stored in the capacitor; meaning it will charge more slowly than expected, and discharge more quickly. Most digital capacitance meters only time the charging phase, and ignore the time taken to discharge; so they will tend to read artificially high when testing leaky capacitors. An AC bridge is measuring the capacitive reactance (which is inversely proportional to capacitance) directly.
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