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PSU Question

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Posted on May 6, 2020 at 07:42:24
amandarae
Audiophile

Posts: 2591
Location: So.Cal
Joined: November 30, 2004

I am tracing a PSU and on the actual circuit, I found a resistor across C1 about 1.8 M Ohms (Brn-Grey-Green-Silver). Is that a "bleeder" resistor to drain the caps once power is off? How does it affect the operation when PSU is on?
Thanks!
Abe

 

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RE: PSU Question, posted on May 6, 2020 at 09:48:47
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 Remember choke I/P filter critical current. That bleeder you described is trivial and only serves to discharge. Frequently, a smaller valued bleeder is placed across the 1st filter cap., to ensure that the critical current is drawn. A reasonable approximation for the critical current (in mA.) is given by V/L. Things work out to require 1000 Ω/H. of inductance. Please observe that bigger chokes have smaller critical currents.


Eli D.

 

RE: PSU Question, posted on May 6, 2020 at 10:45:53
amandarae
Audiophile

Posts: 2591
Location: So.Cal
Joined: November 30, 2004
 Thanks for the response Eli!
The L for the V/L at 1000 ohms/H, that is the 1st L on the circuit (20H)? If so, the resistor can be 20K? or should I leave it alone with its current value as it does not do anything when the PSU is operating?

Abe

 

RE: PSU Question, posted on May 6, 2020 at 14:07:08
Eli Duttman
Audiophile

Posts: 10455
Location: Monroe Township, NJ
Joined: March 31, 2000
 By all means, leave things be, unless a particular problem is observed. Whatever current passes through the bleeder is wasted and converted into heat. The only reason for using a "modest" value in the bleeder position is guarantying critical current draw.

BTW, as the bleeder dissipates power according to the P = V2/R formula, a costly high wattage part is needed. Consider that 400 V. and 10 Kohms are creating 16 W. of heat and things get rapidly worse, as the rail voltage climbs.


Eli D.

 

RE: PSU Question, posted on May 6, 2020 at 17:35:57
amandarae
Audiophile

Posts: 2591
Location: So.Cal
Joined: November 30, 2004
 Thank you Eli!
Abe

 

RE: PSU Question, posted on May 6, 2020 at 19:13:48
Tre'
Industry Professional

Posts: 17294
Location: So. Cal.
Joined: February 9, 2002
 Does the paper correct itself and show 2.5 Hy for L1? If so is that enough inductance to be critical for whatever the circuit current draw might be?

I'm reading a 30ma load. That would require the L1 to be about 7.5Hy.

Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"

 

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