Low Wattage Bleeder Resistors

[muttled]

I have an 18vac toroid that I'm hooking into a power supply circuit but I don't have any 2 watt resistors to bleed the caps. Can I use 4x of my 1/4w resistors at maybe 33k instead of 1x 2w resistor at 2.2k?

 

[audiostrat]

This will change the discharge time, so if you do it, just have it on your mind and be careful: the time constant if you use two 33k in parallel for each cap will go up by a factor of 7-8 - if you needed a minute to charge them off, now you need half a quarter in theory.

Wattage-wise you should be fine, a 33k across a 25V voltage dissipates around 18mW.

 

[Mark Johnson]

In your shoes I would probably use four (8.2K, 0.25W) resistors in parallel. These will give you a margin of safety large enough to accept 110% mains input and, at the same time, (Vno_load = 115% of Vmax_load), while keeping each resistor's dissipation below 125 milliwatts. That's half of their 250 mW rating.

Math: { (18 x 1.1 x 1.15 x sqrt(2))^2 } / 8200 = dissipation per resistor, in watts

 

[Michael Bean]

If paralleled resistors are tightly bundled together, their cumulative power rating must be decreased. Four 1/4 watt resisitors bundled together shouldn't be counted on to dissapate any more than about 1/2 watt total to be safe.

Mike

 

[muttled]

If paralleled resistors are tightly bundled together, their cumulative power rating must be decreased. Four 1/4 watt resisitors bundled together shouldn't be counted on to dissapate any more than about 1/2 watt total to be safe.

Mike

I was thinking run 2 on the top of the board and 2 on the bottom to space them out a little bit.

Also, I was thinking of 4, 1/4w resistors per rail, but I'm reading here that 2 would be "safe" but with a higher discharge time?

 

[Michael Bean]

Any number of resistors is safe as long as you don't exceed thier individual ratings. Each resistor will dissapate power based on Ohms' law, so with more resistors across the cap, more power will be dissapated, giving a shorter discharge time.

Mike