Discharging Capacitors ?

[Andy54]

Can I use a lamp limiter to discharge capacitors ? I was thinking that if the L/L was connected to the amps power cable and then completed the circuit that the all capacitors could be discharged.

If not what is the correct / best practice way to go about surviving around them ?

[Enzo]

I'll give you credit for clever thinking, but sorry, no, that is not remotely correct.


Your caps are all charged up by rectified DC coming through rectifiers - diodes.  The primary characteristic of diodes is that they only conduct in one direction.  In this case from the transformer winding to the capacitor.  To discharge, the current would have to flow through the diode the reverse direction, which unless the diode is shorted does not happen.   So ther is no way for your caps to discharge through teh rectifiers and transformer.

But wait there's more.  Even if your diodes magically worked both directions, and knew when to do it, you wind up with a DC voltage - the charge in the filter caps - wanting to go through the transformer.   Transformers do not work on DC voltage.   Only AC voltage will go through a transformer.

Well, technically the voltage doesn;t "go through."  The voltage/current through one winding induced voltage/current into another.  Ther is no direct connection.

SO that is another reason your charged up caps cannot get to your bulb limiter.

It isn't what you suggested, but you COULD connect the bulb directly across the caps to discharge them.  After all, the bulb is just a resistor that gets hot enough to glow.  But we probably would prefer a higher resistance.

And that is how you do discharge a cap, with a resistance.   You could use a very LOW resistance, like a screwdriver blade, but you risk damaging the cap with the sudden current flow.  (The little tinsel wires inside the cap can burn out like a fuse.)   On the other hand you don;t want the discharge to take all week.

I use a 1k resistor.  The value is not critical, I just have a bunch of them.    I use a large 10 watt one.  I don;t need that much power handling, but I find the 10 watt size convenient to handle and keep my fingers off the wires.

May I suggest if you work on stuff much, take a resistor and solder a clip wire to each end.  Now you can clip your resistor to ground, and then the other clip to the charged cap.  The resistor then neatly discharges the cap.

A lot of circuits discharge their own caps already, and a simple voltage check with a meter will tell you that.

[Andy54]

Thanks Enzo, I did a search on the forum but nothing came up.

Sounds like I need to make myself something like a 10 watt resistor with some shortish leads with alligator clips.

Seems like you pro's just know what *not* to touch just like the electricians that have fixed things for me without flicking the power off. They seem to like to work without a net.

[Andy54]

Just found this idea which is basically what Enzo is suggesting.

http://www.ifixit.com/Guide/Constructing+a+Capacitor+Discharge+Tool/2177/1

[DrGonz78]

So many times I have used my multimeter to test for voltage stored on caps and found it to be only in mv's. Most modern SS amps discharge pretty quick and don't even require any method to discharge the filter caps. Now that still does not stop me from testing filter caps before I begin work on the board. Either way that link that posted is exactly what you need to make for those times that you need to discharge caps. Every tech should have a tool like this one lying around their work bench. Never give up the fear of capacitors or you will get zapped one day! Even my cousin told me about a time he got ZAPPED! He is the most level headed person I know and that made me be even more cautious working on amps.

Also, Enzo again has made my brain grow even more with his great explanation on this subject. Each time he does this I find myself understanding power supply concepts (in this case) much more intensely. It's as if he just knows how to communicate as if he is an Amplifier!! Hats off to you Enzo!!  :tu: You make learning very interesting!  :dbtu:

[Enzo]

Yes, ANdy, the project you found is exactly what I was talking about.  In my vision there are alligator clips on the wire ends.  But they don;t need to be.


And Gonz is right too, most stuff discharges itself right away anyway.   I concern myself mainly with tube circuits.  A 500v capacitor can store a high voltage charge for quite a while.   I don;t wory that a 30v rail will be lurking there.  SOme more powerful solid state amps have 80 or 100 volt rails.  Now those can shock you.   And when I unplug the power amps from some Behringer switching power supply and fire it up, the +/-80v supplies in that power supply do retain their 80v for a considerable time.

[Andy54]

Yes DrG, Enzo did give me a good answer with a nice slab of food for thought. I suppose I've come to this forum thinking I could just Part A to Part B problems but now I realise I've got to bite the bullet and start asking myself what is actually going on in a circuit.

I really hadn't thought about what was going on with the physics of for example a transformer or a diode too much. Enzo has made me realise I have some catching up to do.

I think I need a copy of Electronics For Dummies to bone up on and conceptualise a simple amp circuit  :dbtu:

[Roly]

You can, and I do, 'tho I work mostly on valve/tube amps these days, but even a low wattage mains globe is a kinder way to discharge any caps than banging a screwdriver across them.

There are a couple of special cases; with valve amps where you have had to kill the power before the valves warm up.  If it has a solid state rectifier (most) then the HT line will be charged to peak volts and there will be no path to discharge it (unless it is one of the few classic era amps that had a bleeder resistor).  It can stay charged like that for days.

Similarly with solid state amps if you have a faulty one.  If the output stage has blown open rather than short you will have a similar situation, except the energy stored in the filter caps of a big solid state rack amp can be quite considerable, and attempting to discharge these caps using a screwdriver will result in a large chunk chewed out of the screwdriver and sprayed in all directions, white hot, at high speed.

[IndianScout]

I use a $9 test light to discharge my stuff

[joecool85]

I use a $9 test light to discharge my stuff

That's a good idea, I think I might have a test light that I never use hanging around...maybe I've found a use for it!

[Enzo]

As long as it isn;t neon.  Neon lamps have a firing voltage, and if the potential across the lamp drops below that, it stops conducting.  SO it would discharge a cap down to the 30-40-60-80v or whatever that lamop needs, and discharge no further.   An incandescent lamp tester would act like a resistor and go the whole way.