Building a capacitor discharger - what size resistor to use?

[sladef34]

First let me say that I don't know much about electricity, and this is my first amp build.
I understand the safety precautions pretty well.

I want to build a small tool to discharge the capacitors on my Crack as I work on it.

Does someone have a suggestion for an appropriate resistor that can be used for this purpose?
10K Ohm, 5 Watt?
What is the smallest resistor in terms of Watts that I can get away with (RadioShack has a limited selection)?

Thank you,

Slade

 

[Paul Joppa]

For the 220uF/250v capacitors, the peak current is around 1 amp and the peak voltage around 200v, so the resistor must be greater than 200 ohms to avoid damaging the capacitor. If the current were steady, that would be 200 watts dissipation which is unreasonable!

The current isn't steady, but to do an evaluation you'd have to know the thermal time constant of the resistor, which depends on the size and construction of the resistor. Plus you'd have constraints like how long to wait between discharges.

Perhaps we should look for a resistor that will survive even if the circuit is still accidentally powered. Let's say you want to use a 5-watt resistor; then since power equals voltage squared over resistance, the minimum resistance is 8000 ohms. The time constant RC is less than 2 seconds, so if you leave it connected for 10 seconds (5 time constants) the voltage will decay by a factor of 175 (e to the 5th power) - i.e. to a bit over a volt. 10K is probably easier to find.

For a 1 watt resistor, you'd need 40K and it would take 50 seconds to discharge. I'm not that patient, myself.

 

[lextek]

Is there a "better" method of discharging caps before working with them?  Do they discharge on there own after time?

 

[2wo]

I use a 10k, 5 or 10w resistor. I cut a jumper leed in half and soldered the ends to it...John 

 

[Paul Joppa]

Is there a "better" method of discharging caps before working with them?  Do they discharge on there own after time?

Caps are specified for maximum leakage, which translates to "it takes at least this long for it to discharge". The leakage gets less as the voltage drops, very strongly, so it takes a very long time indeed. I did a quick calculation that it takes a minimum of 10 days to lose 10% of the voltage, but the next 10% would take three months, and the next 10% after that, two years. At that point you are at no less than 73% voltage, or 180v on a 250v cap - not a good idea!

 

[Beefy]

Lets not forget that the Crack has a bleeder resistor built in, 270K at 12U to 13U. It won't give you an instant bleed like a specific tool, but it will make it safe to work on fairly quickly after power off.

 

[sladef34]

Thank you very much for the intelligent responses everyone.

I'm headed up to Fry's today to pick up a 10K/5W resistor and plan on making a discharge tool.

Slade

 

[2wo]

You got it ...John

 

[InfernoSTi]

Ha! I use a 10K/5 watt with two alligator clips to discharge.  Read it somewhere, I'm sure....

John