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Old 5th February 2003, 07:49 PM   #1
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Lightbulb My bench power supply..

After some experimenting and studying some PS schematics ,ive
decided to build my own, want something so reliable to test my

the following is my own design (thanks to many pals in here,
they really helped me alot!)..
its a 1.5A regulated providing +-1.25 upto 35V using the LM317,
LM337 reuglators (can be substituted with more current handlers
like the LM350, LM138..etc)

i need to hear all of ur comments, suggestions on the main schematic and how to improve the output or to provide good

waiting ur valuable goods!
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Old 5th February 2003, 07:52 PM   #2
HDTVman is offline HDTVman  United States
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D2 is reversed. C9 & 10 don't need to be that large. Some regulators have problems with large caps on the output.
What ever makes the tunes flow
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Old 5th February 2003, 07:58 PM   #3
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You won't be able to output very high currents at low voltages.
For instance, if you tried to draw 1.5A at 1.25V, the 317 would
have to dissipate around 50W. This is far above their rating. I
don't remember exactly what the power rating is, but it is at
most 20W, probably less. Besides, the 317 does not have very
good thermal conductivity from junction to case, so in practice
it is very difficult to get even near their rated maximum power
without gigantic heat sinks.
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Old 5th February 2003, 08:30 PM   #4
halojoy is offline halojoy  Sweden
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Arrow My Bench Supply - I will draw a schematic - some time

I have a bench supply I made it myself.
I took what I had at home, and got it together.
My own circuit design.
It uses a LM385 as reference (set at +5.00V).
A pot can adjust Ref output 0.0-5.0 V
This is buffered and mirrored to a negative 0.0-5.0 V reference.

It uses one OP741 and one LM324, quad OP.
One 7808 and and a dicrete OP to mirror +8.0 V to -8.0 V
It uses BD241 BD242 as output transistors,
controlled by 2 OP amps in LM324.
With some homemade heatsinks of 2 mm Aluminium plates.
I have +5.0 +8.0 0V -8.0 voltages
and a tracking dual adjustable +- 0.0-20.0 V outputs.
I have Corse/Fine pot for the adjustable +-0-20V,
and also a pot to trim the negative +-5% to match positive exactly.
Output current is like 2x 0.20-0.30A
I have used 15-0-15V 0.5A trafo.

I am proud to be able to put out voltages down to
plus/minus Zero 0.00V.
To sum:
+8.0 +5.0 0.0 -8.0 Volt
Adjustable dual Track: +- 0.0 to 20 Volt, 0.25A
I will draw a schematic - some time.
And put it into my FTP-Server, along with my collection

/halo - do it yourself - get it as you like to have it
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Old 5th February 2003, 09:23 PM   #5
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D2 should be reversed due to the negative regulator ithink?

ive putted C9,C10 in the schematic caz i found many others have
em too but to be honest i dont know exactly what they do in there neither how to derive thier proper values..
can anyone explain?
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Old 5th February 2003, 09:30 PM   #6
HDTVman is offline HDTVman  United States
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If you look at the National web site, the application note recommends 1uf to 25uf for the output filter cap value. I would go with that.

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Old 5th February 2003, 11:23 PM   #7
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Default if it's truly a bench supply

then you want to be able to get down to "0" volts which means you need to bias the adjust pins of the regulators -- you need to pull the LM317 1.25 volts below ground -- since these devices are so cheap (about $0.40 in the US) just use one with a minimal value Radjust to derive the bias.

Nat Semi has an apnote on this.
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Old 6th February 2003, 12:53 AM   #8
sam9 is offline sam9  United States
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Default Sam9

A- If you find you come up short on current at voltages, you can use parrallel the regulators (2 or each). I tried this once when I needed more current that the LM317 could provide. Therewas probably sub-optimal about my approach, but it worked. They ran cooler, too.

B- On the other hand, if you want a firm upper limit of 1.5A you can put a series pass limiter after the the regulator(s). If you need a reference on that, G.R. Slone shows a basic one in his intoductory book on electronics.

If all you need to do is confirm amplifier function and set the initial bias, you don't need a "full power" PS. In fact, it may keep you from damaging the amp during trouble shooting (especially if current limited).
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Old 6th February 2003, 02:19 AM   #9
Sud is offline Sud
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I'm building something very similar at the moment

You won't get 1.5A out at +-35V because the capacitors will discharge below the dropout voltage of the regulators. You should be able to get 1.5A at 26V out, judging by Duncan's PSU designer. (available at )

Things to remember:
* regulators have a drop-out voltage, which is a miminum voltage difference between their Vin and Vout terminals for them to work. Typical values are about 3 volts, meaning that the output voltage needs to be about 3 volts less than the minimum voltage across the input caps.
* the lower the output voltage, the more power the regulator is dissipating at a given output current, as Christer pointed out
* the higher the output current, the lower the minimum voltage across the input caps.

Should be a nice power supply. Post pics when you've finished!
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Old 6th February 2003, 02:48 AM   #10
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I wanted to make a split rail adjustable powersupply, except the only transformers I have are a 32volt 6amp one, and a 24volt 10 amp one.... and the first will exceed the max input of adjustable regulators..... I only just thought of using 1 transformer per rail, but always limiting the 32volt one to the same as the 24volt one.... But.... That would be one PRETTY big powersupply.... and if I wanted to get full power into say a +-28 volts, thats a PRETTY BIG drop for the regulator on the 32volt transformer, and thats ALOT of heatsinking needed.. lol
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