When working on a tube guitar amp, you need to discharge the filter capacitors in the power supply before doing any work because they can still store energy when the amp is not plugged in. I understand that much and don't have a problem with it. But I've read in a few different places (here's one) that even after discharging the caps, if you don't leave a jumper in place to short the terminals, they can regrow a charge. How can this happen?
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3 Answers
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In an ideal capacitor, the dielectric is a perfect insulator and is not affected by nor affects the charge on the plates. In a real capacitor, this is not true.
What can happen is that during operation the dielectric can actually absorb some of the charge from the plates. Once the charge has been drained from the plates externally the dielectric will release this charge back onto the plates. Naturally this re-energizes the capacitors, potentially to dangerous levels.
answered May 22, 2014 at 4:35
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\$\begingroup\$ Thanks, that makes sense because the filter caps are aluminum electrolytic which according to the Wikipedia article have the highest rate of dielectric absorption. \$\endgroup\$– nonexCommented May 22, 2014 at 16:26
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Yes, dielectric absorption will do that.
Also if we relax a bit the "no power receiving": If you ever order big capacitors (big can, like bier can sized), they often come in a box with metalised cover, "conductive" foam cover or simply shorted across terminals. Capacitors are so good these days, they can pick up charge by sitting on the shelve with open terminals. The maximum voltage I measured was 2.5V build up in a week.
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It can happen in a number of ways, here's some suggestions.
- RF interference (those wires are antennas)
- Static build-up
- Ionising cosmic radiation
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\$\begingroup\$ Dielectric relaxation. \$\endgroup\$ Commented May 22, 2014 at 12:23