Resistor size/value for discharging filter caps

[pjd3]

I approached the Director of Electrical Engineering where I work to ask him how he likes to go about discharging the "super-caps" in the defibrillators.
He asked me why and I told him I will need to start discharging caps in guitar amps that hold 400-500 volts. As it turns out he just took up the guitar and will be building a tube amp with his friend/guitar teacher.

He then got me a Dale 50 ohm, 50 watt resistor. It appears to be one of those power resistors, ceramic wirewound maybe? 4 inches long, 3/4" diameter.
He threw in a nice pair of thick probes with HV cable. Very nice of him I must say.

What surprised me though was the resistance value. 50 ohms just sounded low to me compared to values I've seen others use in video's. I asked him if 50 ohms is a low resistance for this application and he said no worries, we use them here to discharge 2000 volts. I wasn't about to argue with him, he's the one all the Electrical Engineers report to and I'm just a lowly tech.

What value resistors do you feel comfortable using and why? Ha, Uncle Doug used 22K I believe it is. That guys taking no chances I guess.
Thanks all,
PJD3

[pjd3]

If this was steady state DC,

Say,

400Volts/50 ohms = 8A

Power = 8A x 400V = 3200 watts

Ha, better be a quick discharge!

But, how quick can it be without burning something.

I donno.
PJD3

[xtian]

50 ohms? Might as well use a screwdriver!

[pjd3]

xtian, I was thinking the same thing. And he told me to not let the spark scare me. I'm not worried about me, I'm worried about over-currenting a good capacitor!

I can probably switch this cap for a higher resistance next time I go in.

What value are you comfortable using for caps in the 400-500 volt range? 32uF

Thanks,
Phil PJD3

[pjd3]

I meant "switch this resistor" not, "switch this cap".

[pompeiisneaks]

Most people try for at least 10k or more, I use a 22k resistor for that, but anything over is going to take longer to discharge, but will work.

Lower causes sparks that defeats the point of the resistor. I don't know an 'exact' value, but so far my 22k has never sparked and seems good.

~Phil

[xtian]

I use a 470R/10W ceramic resistor, with aligator clips soldered to each end. No sparks or pops, and drains 500v in five to ten seconds.

[Leo_Gnardo]

I use a 470R/10W ceramic resistor, with aligator clips soldered to each end. No sparks or pops, and drains 500v in five to ten seconds.

Similar here, but with a 1000 ohm resistor. Works fine!

[pjd3]

Thanks everyone. I’ll bring the 50 ohm sparking cap beater into work and swap it for at least a 470 ohm resistors or higher if they have it. Or series up one or two or three of them if they are too low. This is very helpful, thanks. My first two builds I decided to hand off too the amp tech to make sure I didn’t install a fatal mistakes but, he said my work was very good with no building issues. The third project coming up now will involve some ground up design portions so I will be constantly be turning it on and off quite frequently. I want to be prepared for that and not introduce stressful electrical events!
Thanks,
Pjd3

[pjd3]

So, did something a little crazy but, I think it will work fine.

Got my hands on a bunch of 3 watt resistors, six 1k and one 5.1K came out to a little over 11K total.

Even at a steady 450 dcv none of the resistors would be seeing much more than 2.6 watts, approx.

Think we're good to go here. The thing looks like a black necklace, 7 resistors in series with a long thick heatshrink. I don't think I'll give it to my wife for Christmas but, it's classy in an "electronicky" kind of way.

Thanks for your input everyone, safe discharge to you all.

Best,
Phil D.

[boots]

Remember that we are not dealing with steady-state DC. The voltage across the cap drops very rapidly as it discharges. If I recall correctly, the voltage drops about 63% every time constant as it discharges. The time constant (in seconds) is equal to R times C, where R is in ohms and C is in Farads.

So the value of the resistor does affect the discharge time, with bigger values taking longer to discharge. But, we are talking about time constants on the order of a couple of seconds. So I don't think the value of resistor is all that critical. Although 50 ohms does seem pretty small.

I use a 100k resistor because it is what I had handy when I made my discharge tool. I have never seen a spark, and the resistor has never become the least bit warm.

[pjd3]

Thanks Boots, And I agree entirely with what you present.

I figured since I had the opportunity to do a "worst case scenario" treatment, and it didn't really cost anything, I'd just go for it.

But yes, I'm certain I could have got away with far less than I did. I suspect these 7 resistors will be laughing at me every time I discharge a cap.

Thanks!
Phil