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Old 28th July 2007, 01:18 AM   #1
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Question Regulated Lab DC power supply

I have a bunch of old multi-section electrolytics (NOS "bombs") that need reforming. As I do not currently have any sort of regulated power supply in my collection of equipment, I have decided that it might be worth building one; as it would be a useful tool in the future. I saw this also as an opportunity to apply what I have learned thus far about tube design and design my own Power Supply specifically for this purpose. I was assuming any unit between 500 and 600 VDC would be useful for this kind of application. I drew a schematic of my "design", it is HERE.

Before I go any further I would value some input on what I have thus far; as you can see I have not thought of what the values of the filter caps or chokes should be. Off the top of my head I was thinking something like 20uF 600V electrolytic(s) and 5H chokes.

If this will work, regardless of whether there are better designs more suited to this kind of thing, I'd like to build it as a practice project; something to get my feet wet.
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Old 28th July 2007, 03:30 AM   #2
Colt45 is offline Colt45  Serbia
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1. Adjustable != regulated.

2. The CLCLCLC thing is overkill for an audio amp, nevermind reforming caps.


3. The rheostat is going to have to be goddamn HUGE. super super super inefficient.
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Old 28th July 2007, 03:30 AM   #3
jayme is offline jayme  United States
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Use PSUD2 to model it. You can find it here:

http://www.duncanamps.com/psud2/index.html

And, BTW, that design is not "regulated". It is rectified, and then smoothed. Regulation requires some sort of active element, if I am not mistaken.

Model the behavior using a 10mA load draw:
1) At startup until stabilization
2) with a 1mA current change after, say, 10 seconds

With 3 choke stages, you will need to be careful about ringing.

I built a PSU similar to your specs, but it turns out my wall AC isn't so great. I think the voltage varies with current draw, which plays havoc with my B+ output voltage. Anyway, I am in the process of adding a shunt regulator (aka a couple 03D gas tubes) to lock down the voltage.
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Old 28th July 2007, 07:48 AM   #4
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Right, thanks for the input.

The Duncan amps tool is great; thanks jayme.

And you are right, Colt45, it IS way overkill; I knew that it was over kill in the beginning, but thanks to the Duncan amps tool and its simulations I now see just HOW MUCH overkill it is. I think two caps and one choke is overkill enough for this application. Also, you are right, It is not REGULATED DC I want, I only want to control the output.

The reason the rheostat is there, and the reason this is designed the way it is is because of the way and old-timer explained to me how he reforms electrolytics. Paraphrased- by using a rheostat and mAmmeter in conjunction with a high voltage supply; he explained I should connect the cap to the voltage supply and slowly adjust the rheostat, stepping up the voltage watching the current as I go, being careful not to let it leak too much. Other websites I have researched suggest connecting the positive lead of the cap IN SERIES with a resistor at full voltage for five minutes; but wouldn't it be better to bring it up to full voltage gradually? Using a potential divider circuit such as this? The leakage current through a one-meg rheostat wouldn't be more than 500 uA, right?
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Old 28th July 2007, 05:21 PM   #5
kevinkr is offline kevinkr  United States
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Think about using a variac on the primary side of the HV transformer with a separate filament transformer for any tubes that might need it. (Should you decide to use it for more than reforming caps.)

You may find a 100Va variac on eBay for a lot less than the huge rheostat you would need otherwise. In addition if you use it to power a project voltage regulation under varying loads will be far better.

IMO Sometimes you can pick up surplus power supplies for much less than it would cost to build from scratch. I picked up a 2KV Isco (494) electrophoresis supply on eBay a couple of weeks ago for $40 + $30 for shipping.. Constant current, constant power, and constant voltage modes of operation. Old Heathkit IP-17 another good all around choice, not to mention many old tube Hickok and Lambda supplies which strongly outperform the Heathkits in ripple levels, output impedance, regulation, and stability..
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Old 28th July 2007, 05:50 PM   #6
7N7 is offline 7N7  United Kingdom
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600V is a bit steep perhaps, but I have built 450V regulated supplies using 317s.

As for an adjustable supply, well I think that the variac is the answer.

Also, save space and money and use silicon rectification; no need then for a heater/filament supply and no varying "tube drop".

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Old 28th July 2007, 06:13 PM   #7
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Throw the electrolytics in the trash. They are NOS food. Yuck.
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