OT Bleeder Resistor

[Bob Miller-6]

I sent this once before but I never saw it show up, sorry if it's a dupe.


I have been looking at the June 2006 QST article "Homebrew Solid-state 600 W
HF Amplifier". I'm gathering the parts for the power supply but have a
question concerning the bleeder resistors. The author, K0GKD, used four
22ohm 75W resistors he had in his "junk box" in a series/parallel
configuration.

 In addition to providing a discharge path to ground for the caps, the
bleeder resistor provides a measure of voltage regulation as well and this
is where my question comes from. What would be the optimum value for the
resistors? I've searched the net and various handbooks but haven't found an
answer.

I have (4) 35000uF 80V caps available for the supply.

Thanks for you input.

Bob

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[Mike Scott-7]

Bob,

I am not an expert in bleeder resistors but here is a way to think of what
you need from first principles.

The purpose of the bleeder resistor in power supplies is to discharge the
filter capacitors to a safe level in a reasonable amount of time after
turning the supply off. The decaying voltage of the supply after hitting the
off switch will be equal to:
        V(t) = Vo * e(-t/RC),
where Vo is the power supply voltage when operating, t is time in seconds
since hitting the off switch, R is the bleeder resistor in Ohms and C is the
filter capacitor in Farads.

You didn't tell us what the supply voltage is and how you are going to
connect the capacitors (all series, 320 v supply? all parallel, 80 volt
supply?)

Any way they are configured, if each 35,000 uF capacitor is charged to the
maximum of 80 volts a 390 ohm resistor across the terminals of each
capacitor will discharge each one to less than one volt (arbitrary safe
level) in 60 seconds. Each resistor will draw 205 ma current during normal
operation and each resistor will need to safely dissipate a little over 16
watts.

If you are willing to live with 2 minutes (120 seconds) until voltage drops
to under a volt then the resistance across each capacitor can be 780 ohms
with dissipation of just over 8 watts.

At five minutes R = 1950 ohms, dissipation of each is 3.3 watts.

As you noted, the bleeders will also provide a continuous load to the power
supply helping with regulation.

The four 22 ohm resistors in your article would work but you would waste a
lot of electrons to heat. If you did try to put one 22 ohm resistor across
one 80 volt cap charged to 80 volts you will draw over 3.6 amps in the
bleeder and the resistor would need to dissipate just over 290 watts (each
resistor) before your amplifier gets any power. There is something missing
in the configuration in the article before it makes sense.


Mike Scott
AE6WA
Tarzana, CA (near LA)
Elecraft KX1 4-Watts

-----Original Message-----
From: [hidden email]
[mailto:[hidden email]] On Behalf Of Bob Miller
Sent: Sunday, June 11, 2006 11:14 AM
To: [hidden email]
Subject: [Elecraft] OT Bleeder Resistor

I sent this once before but I never saw it show up, sorry if it's a dupe.


I have been looking at the June 2006 QST article "Homebrew Solid-state 600 W
HF Amplifier". I'm gathering the parts for the power supply but have a
question concerning the bleeder resistors. The author, K0GKD, used four
22ohm 75W resistors he had in his "junk box" in a series/parallel
configuration.

 In addition to providing a discharge path to ground for the caps, the
bleeder resistor provides a measure of voltage regulation as well and this
is where my question comes from. What would be the optimum value for the
resistors? I've searched the net and various handbooks but haven't found an
answer.

I have (4) 35000uF 80V caps available for the supply.

Thanks for you input.

Bob

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