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G 29th July 2004 01:13 AM

Best place to place bleeder resistor in a power supply?
 
2 Attachment(s)
The below image is a PSU I am modeling for my preamp. I would like to place a 220K bleeder resistor in the PSU but I'm unsure where to put it. Does it matter?

Eli Duttman 29th July 2004 01:34 AM

Gavin,

The purpose of bleeder resistors in PSUs employing cap. I/P filters is to discharge the caps. when the unit is powered down.

R3 will discharge C4, at a minimum. Put the 220 K part across C3, as that will surely discharge C1, C2, and C3.

BTW, please explain the purpose of a current source connected from B+ to ground. I'm scratching my head.

fdegrove 29th July 2004 01:37 AM

Hi,

Quote:

Does it matter?
Basically that depends on how safe you want to play it...
As you have a choke and some resistors in series with the B+ line, I'd put a bleeder across all the caps.

By doing so you'd still discharge the B+ reasonably fast enough not to get killed by unexpected high voltage present on one of the caps.
As you want to use a 220K resistor and you have 4 caps, I'd wire a 1M resistor across each of the caps.

Cheers, ;)

G 29th July 2004 01:43 AM

Quote:

Originally posted by Eli Duttman
Gavin,

The purpose of bleeder resistors in PSUs employing cap. I/P filters is to discharge the caps. when the unit is powered down.

R3 will discharge C4, at a minimum. Put the 220 K part across C3, as that will surely discharge C1, C2, and C3.

BTW, please explain the purpose of a current source connected from B+ to ground. I'm scratching my head.

Duncan's PSU designer won't let you put a resistor in parrallel with the load. So I put in a current tap to simulate a 220K resistor in parallel. At least that's what I had in mind.

G 29th July 2004 01:44 AM

Quote:

Originally posted by fdegrove
Hi,



Basically that depends on how safe you want to play it...
As you have a choke and some resitors in series with the B+ line, I'd put a bleeder across all the caps.

By doing so you'd still discharge the B+ reasonably fast enough not to get killed by unexpected high voltage present on one of the caps.
As you want to use a 220K resistor and you have 4 caps, I'd wire a 1M resistor across each of the caps.

Cheers, ;)

I can do that.

:D

fdegrove 29th July 2004 02:03 AM

Hi,

Eli,

R3 being the load, i.e. the circuit itself, the value goes up as the unit is powered down.

In all fairness it will likely discharge the lot quite fast enough under normal circumstances but...
I've seen people pulling the tubes out of their units, powering down and, without bleeder resistors put where they belong, being quite amazed at seeing their soldering iron fried by touching a closeby cap with it....and that's the happy ending for you...;)

Cheers,;)

Sch3mat1c 29th July 2004 03:26 PM

Um...

What's the purpose of everything past C2, aside from the loads?

No wonder you want a bleeder resistor. For that much capacitance (suitable for filtering a good 500mA) you'll need almost as much as your 12k load resistor there, maybe 20k. Such will reach 99% of operating voltage in about a minute (five time constants), assuming your load turns off instantly (which it doesn't. Adding that in, it might take 20 to 30 seconds.)

Tim

pedroskova 29th July 2004 05:35 PM

If you are planning on biasing up your heaters with dc from your B+, the voltage divider that you use will act as your bleeder. Just be sure that it's at the end of your B+ supply.

G 29th July 2004 06:17 PM

Quote:

Originally posted by pedroskova
If you are planning on biasing up your heaters with dc from your B+, the voltage divider that you use will act as your bleeder. Just be sure that it's at the end of your B+ supply.
Excellent point! Thanks. One less thing to worry about.

cliffhiker 30th July 2004 07:43 PM

As long as the primary will sustain the voltage... here's a great way to bleed it quickly that I've used on low voltage supplies

http://www.reed-electronics.com/ednm...es/70501di.pdf

or

http://www.reed-electronics.com/ednm...bestofdi_power


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