Capacitor Drain Lead

[Jack Sparrow]

I was about to put together a capacitor drain cable from a pair of test leads and and a resistor and wonder if this will work:



The resistor acts like a bleed resistor in some circuits so testing while the cap is draining shouldn't be any more of a problem here than in one of those circuits, right? Or am I missing something? I feel better about watching the current drain off and knowing that the cap is fully discharged without switching back and forth between a drain cable and test leads. Since I'm going to make a cable anyway, this isn't much more work as long there isn't some hidden danger I'm not considering.

Thanks!

Rats! I meant to show a 10K 10W resistor.

[Steve Conner]

Looks like a great idea!

[Enzo]

And you could even use one of those $9/$2/FREE meters from Harbor Freight dedicated to this use.

[Resonator Guy]

I am not trying to hijack this thread; I think it is a real neat idea. Do you think this old voltmeter I picked up in an "antique" shop would work. The three posts on the side read 20, 10, and a symbol like an H with an extended centre bar that I figure means ground. The dial only reads to 20. I haven't tested it at all yet. I bought it just because it looked good and cosmetically was in perfect shape. If I could use it for something, that would be a bonus.

[JoeM]

That meter shows AC on the dial. Maybe it cant read DC, which is what you need to measure if you want to monitor the voltage on the caps.

[Resonator Guy]

Thanks JoeM. The AC is painfully obvious but I hadn't noticed it. The meter looks good on my shelf anyway.

[defaced]

And you could even use one of those $9/$2/FREE meters from Harbor Freight dedicated to this use.

Best use of them. When I'm in an amp, I have one clipped across the caps so I don't do something stupid.

[R.G.]

I put red LEDs in series with the bleeder resistors on the Workhorse amps. LED on = voltage on the cap. The turn off is sloppy and indistinct. One other thing that could be done is to put a miniature neon and high value resistor across the caps. When the voltage gets under the neon's threshold of 60? 70? volts, the neon goes out.

[Steve Conner]

I've seen a mysterious neon lamp installed inside a large switched mode power supply. As far as I could see, it was just there to warn the service guy that the DC bus capacitors were charged. Bob Pease recommends it in one of his troubleshooting books, he argues that a neon lamp is more scary than a LED.

Hardly any voltage on V1 plate pin by the time you get to it? That's good. Nowadays it's considered bad form to shock the service guy. Newer designs (for tube amps, post 70s?) include bleeder resistors so that the capacitors will discharge themselves in a minute or two.

[loudthud]

Note that neon bulbs want to oscillate. The current limiting resistor and the stray capacitance across the bulb terminals form a relaxation oscillator. The frequency can be pretty high and shouldn't affect audio circuits. (Where have we heard that before?) Tektronix discovered this when they used neon bulbs as panel indicators in 5000 series scope plug-ins.

[R.G.]

Good point.

Of course, neon indicator lamps have been used on amplifiers for a long time as power indicators.