100 ohm resistor jumped to chassis to drain filter caps.

  • Thread starter Thread starter Chester Nimitz
  • Start date Start date
1/4 watt? You're joking right? 10k 2 watt is bare minimum. A 1 watt may be ok but I would not go lower than 2 watt.
 
Ive already checked with other people and this resistor is fine but prolly going to order others.
 
It's only necessary for some amps. Often flipping the Power switch OFF with Standby still ON and waiting a few minutes (if that) will drain them enough. Always measure them anyway so you know they're drained.
 
Just did this a few hours ago with my 100w Plexi. You can put a clip on pin 1 of V1 and then connect the other end to the chassis. I then used a multimeter to double check while putting the negative probe in a chassis bolt hold and using the positive probe to check the caps.
 
1/4 watt is definitely not enough for consistent usage (you might get away with it once or twice).

Take the usual formula for calculating power:

P = V * (V/R), where V is the voltage dropped across the resistor, and R is the resistance. Since you are dropping all the voltage to ground, use the full B+. This power rating is NOT needed for a discharge resistor because the voltage isn't applied constantly. But it is useful for demonstration:

Example #1: B+ = 500Vdc, and your resistor is 10k: 500*(500/10,000) = 25W!

Example #2: B+ = 500Vdc, resistor is 100k: 500*(500/100,000) = 2.5W. Skimp (because voltage isn't applied constantly) and use a 2W resistor.

So the smaller the resistance, the more power it will dissipate.

Here are some other notes:

- you can splice the resistor into the wire and cover it with electrical tape.
- the higher the resistance, the slower the caps will drain.
- once you drain those caps, they will sometimes 'recharge'. So always check 'em. Sometime I leave the discharge resistor clipped in place.
- use flameproof resistors. Metal oxide or cement. If you happen to forget that your discharge resistor is clipped in place, then you will need the full power rating or be happy that you used a flameproof resistor. Trust me.
 
I use a 100k 5W. .25W is not big enough. Also solder the the resistor to the probe end and cover with heat shrink wrap. Much neater and studier than simply clipping it or wrapping with black tape.
 
if you will leave your amp on run then pull the plug from the wall or IEC socket, everything running through the caps will drain instantly.
 
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