greyshaw: CR50; C.R. Bridge: Testing Capacitors

Testing capacitors is not just measuring the value.

Other aspects are:

1. ESR (Effective Series Resistance): Lower voltage Power supplies, especially SMPSU
2. Dielectric Loss: VHF, UHF and Microwave performance
3. Low voltage Leakage: long time constant filters, integrators, low frequency oscillators
4. High voltage leakage: Anode to grid coupling, Screen grid with high load resistors (DAF96)
5. Breakdown voltage: Correct initial selection. AC is higher peak voltage.
6. Power handling / Ripple Current: PSU, high power amplfiers, SMPSU
7. Microphonic or  piezoelectric effects that may cause noise pickup or distortion.

 1. ESR is generally only an issue on PSUs in equipment since 1980s. Rises with age due to temperature, or poor original parts.

 2. Dielectric loss is matter of the correct material. In use this will not generally change.

 3. Low Voltage Leakage. Again the appropriate type of material is used originally and it is only likely to be an issue with poor quality Tantalum types.

 4. This a major issue with many but not all types of paper capacitors over 20 years old. Waxed card case, cracked plastic case and tar cased. Sealed Metal can types from 1940s and later are often fine.

 5. Breakdown voltage is related to "4". But many modern 50V ceramic and foil types test fine at 300V. An Electrolytic rated at 25V will show very high leakage on a 300V leakage tester!

 6: Capacitors have losses. Don't expect capacitors carrying many amps of  AC to run cool. Incorrectly specified parts that meet the voltage ratings can explode at high AC current!

See Capacitor Leakage tester made from flash gun inspired by this tester. The CR50 puts about 200V  across a capacitor with no leakage.

But read the larger full article by Emilio Ciardiello to understand the issues of capacitor leakage and some simple diagnostics.

Don't automatically replace certain types of capacitors! Always test first if in doubt. 

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