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Old 13th April 2004, 04:59 PM   #1
jleaman is offline jleaman  Belgium
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Default Digital Variable Power Supply

Does any one have any links to some digital power supplies or schematics. I want to build a variable power-supply for my wok bench.. i want to have 50 0 50 and variable. maybe about 3 -6 amps with a screen with a read out of some sort.. id buy one if there is no schematics at all..

j'
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Old 13th April 2004, 05:36 PM   #2
gmarsh is offline gmarsh  Canada
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The best thing to do is buy yourself a Xantrex "60-5" (60V/5A) lab supply or two.

Building one of these things in a linear form would be the easiest and by far the cheapest thing to do, but it would require an obscene amount of heatsinking. Building a switching power supply that's stable at both 0A and 5A loading over an enormous voltage range is best left to the professionals...
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Old 13th April 2004, 05:39 PM   #3
jleaman is offline jleaman  Belgium
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it doesn't have to be switching it can have a nice big toroidial : O) wich.was my idea any way.
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Old 13th April 2004, 06:16 PM   #4
UrSv is offline UrSv  Sweden
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And you pre-regulate the input anyways so that dissipation never goes above 30-40 W or so...
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Old 13th April 2004, 06:55 PM   #5
jleaman is offline jleaman  Belgium
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ok.. so any one got any links for me then. ?
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Old 13th April 2004, 07:17 PM   #6
gmarsh is offline gmarsh  Canada
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Forgive me for the "sorry, should buy one" reply. My post here came shortly after I responded to a post on another forum from a guy who decided he was going to build his own computer motherboard

I lack schematics for a high voltage linear power supply, but here's a basic concept of one... First, you'll need an unregulated 50 volt supply - use a big toroid and a bank of caps. If you want to pull high currents at low voltages out of the supply, see if you can get a transformer with multiple taps - this will keep the power dissipation in your pass transistors reasonable.

Look up the datasheet and some app notes on the TI/Unitrode UC3834 chip; I've used this before in a couple of designs. These chips only do 40V unfortunately; you'll need to use a zener diode or something similar to drop the unregulated voltage from your transformer down to something that the chip can handle.

Get a few big PNP transistors, rated for at least 60 volts and lots of power dissipation, to use as your pass transistors. You'll want a big heatsink to bolt these to. Depending on the hFE of the transistors involved, you might want to use an extra one of these to form a darlington configuration, since the 3834 can only sink 250mA if memory serves me correct... eg for 10 amps output, the PNPs need a hFE greater than 40.

You'll also want to put a zener diode in series with the base of your PNP transistor, to subtract a few volts and keep the output driver of the 3834 within a safe voltage range. Watch the power dissipation in this zener, though...

Then design an appropriate control loop (I think it can't be too hard to steal one out of an appnote or something) and you should be good to go.

I'll draw up a rough schematic or something if you're interested.
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Old 13th April 2004, 07:17 PM   #7
UrSv is offline UrSv  Sweden
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Search for "bench power supply" and a few nice threads will show up.
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Old 13th April 2004, 07:46 PM   #8
tool49 is offline tool49  Canada
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One simple way to do it would be to buy a variac from a cheap source such as All Electronics ( www.allelectronics.com ) or Marlin Jones ( www.mpja.com ), and to build yourself a nice simple powersupply using the variac, a transformer, two bridges and a few caps. Using the variac to vary the input voltage would allow you get any voltage you want and you would always get a rated capacity of approximately 5A with a 500W variac. For safety and regulation purposes, you should add a transformer between the bridges and the variac. Then, the ratio between the ouput of the circuit and the input would be the one between the primary voltage and the secondary voltage of the transformer. (A good transformer would be a 500-1000VA 35V-0-35V)

The scripted schmatic would be as follow:
wall->variac->transformer->bridge->caps->load

For the readout part, you could always use some cheap RadioShack DMM.

Hope this helps!
Sébastien
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Old 13th April 2004, 08:00 PM   #9
Coulomb is offline Coulomb  England
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I built the exact same thing from a Kit from Kits R Us, works like a charm.

Regards

Anthony
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Old 13th April 2004, 08:40 PM   #10
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Default Re: Digital Variable Power Supply

Quote:
Originally posted by JasonL
Does any one have any links to some digital power supplies or schematics. I want to build a variable power-supply for my wok bench.. i want to have 50 0 50 and variable. maybe about 3 -6 amps with a screen with a read out of some sort.. id buy one if there is no schematics at all..

j'
This one is -50 - 0 - +50 -- the HP6129
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