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Author Topic: High voltage (~200VDC) bench supply with shut-off bleeder  (Read 188 times)

WA2ISE

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High voltage (~200VDC) bench supply with shut-off bleeder
« on: April 03, 2021, 03:27:35 PM »

As many of us, I built a high voltage (160V or so, 300ma) bench power supply.  One hazard is when it is lightly or no loaded, its filter caps will keep a high voltage charge for a good long time.  I figured a good way to reduce this hazard is to have a bleeder resistor cut in when turning the supply off.  I just needed a double pole double throw toggle switch (to replace the single throw single pole I used before).  One pole does the powerline, and the other pole will connect the bleeder resistor to the high voltage output when the switch is thrown to "power off".  I used a power resistor of about 2500 ohms 10W as the bleeder.  This discharges the caps in about 10 seconds.  Yes, the caps are fairly big.  120VAC secondary to a cap, then a choke, and another cap. 

Having the bleeder not connected while power is on cuts current waste, until I need it when I turn it off. 

                      +-------------------- transformer primary
powerline---o   o   o nc
         ON                     OFF   switch terminals
              nc o  o   o----resistor------B+ supply output
                      |
                  ground      nc= no connection
« Last Edit: April 03, 2021, 03:33:19 PM by WA2ISE »
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W7CXC

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #1 on: April 03, 2021, 03:58:52 PM »

There are no bleeders across the output cap in normal operation?  I thought that was kind of standard practice but have been wrong before for sure. Glad to see home brew is still out there. 73's


73's
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K3UIM

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #2 on: April 03, 2021, 04:02:26 PM »

It seems that was always the case "in my time" too.
Charlie
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Charlie. K3UIM
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Where I am: You will be!
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AC2EU

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #3 on: April 03, 2021, 04:44:41 PM »

Put me in the "I don't get it" column!

Why not use a reasonable value like 47K /2 watt resistor? it only draws 4.3 ma  and will have a 100 uf cap  discharged to 23 volts in 10 seconds. No remembering to the throw the switch required!
 
« Last Edit: April 03, 2021, 04:49:26 PM by AC2EU »
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AA4PB

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #4 on: April 03, 2021, 05:40:06 PM »

It is normal procedure to have a bleeder resistor across the output at all times. Not only will it discharge the caps when the supply is turned off, but it will stabilize the output voltage somewhat by keeping a minimum load on the output and preventing the caps from charging to peak voltage when there is no or light load.
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Bob  AA4PB
Garrisonville, VA

W7CXC

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #5 on: April 03, 2021, 06:42:33 PM »

PB + 1    and in the quiescent state of even a regulated supply it adds stability to the supply by giving the regulation circuits some minimum static current/voltage change to work against.   73's
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KM1H

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #6 on: April 04, 2021, 08:27:48 AM »

?? 200V is high voltage ??  ::)

I couldnt resist ;D

Carl
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K3UIM

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #7 on: April 04, 2021, 09:11:41 AM »

Carl,
In a DC receiver, "You betcha, Red Ryder!".
<Hidden Text> Giggle, gasp, snort<End Text>
Charlie
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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!

WA2ISE

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #8 on: April 04, 2021, 01:43:21 PM »

?? 200V is high voltage ??  ::)


I call anything that can shock you as high voltage.  Especially if newbies are used to the world of 13.8VDC. 

Anyway, I placed a 47K 5W resistor across the switch contacts that switch in the 2500 ohm power resistor when the supply is turned off.  So, when the supply is on, it sees 47+2.5=49.5K bleeder.  The 2.5K resistor just discharges the caps faster upon turn off.   

In the world of vacuum tube audio preamps, people sometimes use the power supply bleeder as a voltage divider, to produce about 50V.  And that is used to elevate the tube heaters AC supply winding DC bias voltage above the cathode voltages.  Idea is that you get less hum leakage into the audio on the cathodes.  So the bleeder also serves a 2nd purpose. 
« Last Edit: April 04, 2021, 01:52:44 PM by WA2ISE »
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AC2EU

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #9 on: April 04, 2021, 01:55:10 PM »

OK that will work.
However, rather than building in a switch, why not just make a "lightning stick"?

Take a 50 ohm 10 watt cement resistor, mainly because it's easy to grip, and attach a ground lead to it.
Use the other end like a probe to fully discharge any residual voltage left in the circuit. At 50 ohms, that happens pretty quick

KM1H

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #10 on: April 05, 2021, 05:09:12 PM »

Quote
I call anything that can shock you as high voltage.  Especially if newbies are used to the world of 13.8VDC.

Good point altho I regularly got bit at receiver level voltages to 400V or so when starting out. As long as I wasnt in the bathtub or standing barefoot on a wet basement floor it was considered a right of passage. Ive also survived 2900VDC without apparent damage, damage, damage.....drool, loss of bladder control :o ::)
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K3UIM

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Re: High voltage (~200VDC) bench supply with shut-off bleeder
« Reply #11 on: April 05, 2021, 05:30:09 PM »

Carl,
Woo_haw!!
Charlie
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Charlie. K3UIM
Where you are: I was!
Where I am: You will be!
So be nice to us old fogies!!
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