[CButterworth]

I will be building a dioe-bridge rectified PSU for a tube preamp.  My hope is to get around 300V from the PSU.  For the capacitors, I plan on using a number of AC rated motor runs caps ranging from 1uF to 100uF.

OK, so when I work with these, I'll want to install bleeder resistors, leaving them in place until I get all soldering finished, and my fingers away from contacts.

So, what rating resistors would work best?  I assume that I'll need at least 5W resistors.

Thanks,

Charlie

[randytsuch]

Easy to calculate.

For DC voltages,
just remember V=IR or I = V/R
and power P=IV = (V squared)/R

So, the power you need to dissipate is (V squared) divided by the resistor value you use.  I typically stick in a fairly large resistor in here, so it can be smaller.  The drawback is that you have to wait longer for it to discharge.

BTW, I just leave in my bleeder resistors in forever.  Better to be safe.

Randy

[randytsuch]

Oh, I forgot to mention one thing.

I would derate the power rating.  50% is a good number to shoot for.

So, if you pick a resistor value that would dissipate 8 watts, then I would use a 12 watt resistor.

[CButterworth]

Randy,

OK, so if I have a 450V rated capacitor and use a 100K resistor, I would need it to be (450 X 450) / 100,000 = 2.025W.  Make it 3W to be on the safe side?  Of course, when I model the PSU in PSUD, I only get 380V at the cap, but better to err on the full-power of the cap.

Does this sound reasonable?

Charlie

[randytsuch]

Randy,

OK, so if I have a 450V rated capacitor and use a 100K resistor, I would need it to be (450 X 450) / 100,000 = 2.025W.  Make it 3W to be on the safe side?  Of course, when I model the PSU in PSUD, I only get 380V at the cap, but better to err on the full-power of the cap.

Does this sound reasonable?

Charlie

Yeah, but you only really need to use the (380 squared) for you calculation.  But 3W is a standard size, so you would end up there anyway, with a 100K resistor.

Randy

[Davey]

Charlie,

100k is plenty low for a bleeder.  470k is a common value, but you could use something even higher than that.

Davey.

[CButterworth]

Randy and Davey,

Great!  I will use 470K resistors, 2W or 3W for permanent bleeders.  They'll be overkill in terms of their power handling, but will give extra piece of mind.

Charlie