Electronics > Beginners
Purpose of a discharge resistor across a smoothing capacitor after a rectifier?
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dcbrown73:

--- Quote from: Psi on June 14, 2019, 01:29:21 pm ---
My 2600 farad maxwell supercaps have sat on the shelf for a year and only self discharged from 2.5V to ~1.7V.
Which is pretty amazing.

They drop pretty quick from 2.5V down to 2.2V but after that it's very slow

--- End quote ---

By on a shelf, you mean outside of a circuit?
John B:
Polarised electrolytic caps are also used in audio coupling where there is either (almost) no voltage across the cap, or even a slight AC voltage going into reverse polarity. They can last decades being used like that.
ejeffrey:
The 0.6 V for a pn junction is at high current.  A diode will still conduct at any nonzero voltage.  An ideal didoe will have forward conduction about equal to it's reverse leakage at 25 mV.  This isn't much but will discharge ordinary size capacitors in a few minutes.
ArthurDent:
It depends a lot on the capacitor type and voltage rating. Where I worked we did some very high voltage testing and used large high voltage oil filled capacitors. When these were being stored out of circuit these capacitors always had shorting wires across the terminals to prevent charge build-up that could be dangerous. I generally store some of my not-so-big HV caps the same way. Here's a post explaining what happens:

For large, high voltage capacitors, storage with shorted terminals ensures that no lethal voltage is present across the terminals, and therefore no unfortunate accidents happen. For well constructed capacitors in dry climates the voltage will not decay adequately on its own even after months of storage. In fact, a capacitor can be discharged to zero volts, but without the wire the voltage will slowly build up again in a phenomenon called dielectric absorption. It turns out that a fraction of the capacitor charge soaks deep into the dielectric and can slowly raise the voltage back to potentially hazardous levels over days or weeks . The continuous presence of a shorting wire during storage is a wise safety measure, and often required by law.""

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