Electronics > Beginners
Purpose of a discharge resistor across a smoothing capacitor after a rectifier?
BlastFX:
I like to open things up and while looking at LED light bulbs using a simple dropper (capacitive or even resistive) to drive the LEDs, I've noticed most have a discharge resistor connected across the smoothing capacitor after the bridge rectifier (well, the ones that do have a smoothing capacitor in the first place).
The way I understand it, the diodes should block any current from going from the smoothing capacitor back out the contacts. If one or more diodes fail open circuit, it's still safe. If a diode fails short circuit, you've got bigger problems than possibly getting a zing off the contacts (the kind of problems that cause other things to go open circuit… and also flames).
So what's the point? Am I missing something?
exe:
What's the voltage across the cap? May be it's lethal, that's why they do it.
I used to be a big fan of bleeding resistors. Not anymore as, apparently, electrolytic caps don't like to be discharged, it's bad for oxide layer inside. Somebody said it's good that PN-junction has a voltage drop of ~0.6, this way ICs cannot discharge caps to zero. So, I'd put a diode in series with the resistor so voltage doesn't drop much below 0.6V.
I myself didn't verify this, but what I did notice that after charging caps to their rated voltage and waiting for a few minutes the leakage current starts to drop. So, I can believe that maintaining cap charged helps it.
CatalinaWOW:
Depending on the speed of the bleed it could be just to help get the lights off quickly.
BlastFX:
--- Quote from: exe on June 04, 2019, 07:05:20 pm ---What's the voltage across the cap? May be it's lethal, that's why they do it.
--- End quote ---
It is potentially lethal, it's full peak mains voltage in some cases (minus two diode drops), that's 310V where I'm from, but, as I said in the first post, there's no way I can conceive of for you to get to that voltage unless you disassemble the light bulb (at which point anything that happens is on you).
--- Quote from: CatalinaWOW on June 04, 2019, 07:16:32 pm ---Depending on the speed of the bleed it could be just to help get the lights off quickly.
--- End quote ---
That kinda makes sense. The caps I usually see are in the tens of micro farads range, the voltage across the LEDs is usually from 20 to 80 volts (and you only need to nudge it down by a couple volts to make the LEDs go out) and the discharge resistors are usually half a megohm, so that works out to tens to hundreds of milliseconds, which would be a fairly sharp turnoff, I suppose.
But — specialist applications aside — I don't see why a light bulb trailing off for a couple seconds would be a concern. The glass in halogen bulbs also keeps glowing for a couple seconds after they're turned off and that doesn't seem to bother anyone.
CatalinaWOW:
Just think of CFDs and how minor differences in turn on/turn off from incandescents put off their acceptance. While it really made no difference, perception is everything. If the LEDs would stay on for more than a couple hundred milliseconds in the absence of the bleed it could be the reason for them.
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