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TroelsM 29th March 2007 08:36 AM

DIY Benchtop PSU
Yep, this has been up before. Without any result.

Here's my thought: we all have a storage of powerful transformers, heat-sinks and some transistors.

I need a bench-top PSU and have almost all the above materials to make one, but I need the right idea/schematic.

The problem (at least for making amp's) is that you would like something in the neighborhood of +/- 60V/2-3A.... of course variable and protected against abuse. With a linear PSU that's over 400W of wasted energy in the worst case (lov voltage/ High current). Even with massive heat-sinks 400W is a lot.

In these eco-friendly times that seems wrong.


1) Several taps on the xformer will drop the waste in a "class-G" (or is it "H")- sort-of-way. That works, but not many xformers has that many taps.

2) SMPS. Should work. Could be tricky to get stable and noise-free over a 0-60V area.

3) SMPS + lin. reg. With a switch-mode preregulator one could ensure that there were only a 5-10W drop over the pass-transistor. That would lower dissipation a lot.

In lack of SMPS-knowledge I'm tempted to go the crude way, - meaning massive amounts of cobber, iron and alu.

Any ideas? Are there variable SMPS-schematics out there? Perhaps even simple ones?

Yes, this might never get to be a "project" as I have many other things to do, but the subject should interest a lot of people?

Regards TroelsM

AndrewT 29th March 2007 10:13 AM

the commercial ones use variable tappings (listening to the relays clicking as I ramp the voltage up or down). It works well.
You could be clever and use FETs for silent switching.

A secondary on a toroid could fairly easily be rewound with multiple taps. Even 25%, 50%, 75% 100% would be a big help.
Look, those 50% and 100% are already there. What about tapping into the midpoint of each dual secondary and soldering in 25% and 75% without having to rewind.

poynton 29th March 2007 10:33 AM

I am using a large low-voltage lighting toroid for my psu.
It has ten (10) windings at 10 amps of 12vac.
I bought it for the price of a pint from some shopfitters who were doing a makeover. All the old lighting was going in a skip!

Recycling and Reusing - very ecofriendly !


TroelsM 29th March 2007 10:34 AM

Hi Andrew

Yes, I know that relay-switching is widely used and yes it is effective. My problem is that I don't like messing around with the windings and that its hard to do on a EI-core.

Maybe one big xformer that converts from 230V to say 60V and then a (un-regulated for simplicity) switchmode with multiple secundary windings would do the trick? Then it would also be possible to have multiple isolated outputs from one PSU? But then again.. that would require a lot of fast diodes and low-esr-caps in the smoothing-stage.. bad idea. -and costly

I still like the idea of a switchmode stage that (crudely) tracks the output-voltage and makes sure that there is only a small voltage over the pass-transistor of the linear reg.

Regards TroelsM

pinkmouse 29th March 2007 10:53 AM

Why not just build two? One for power amps that can do, say, 40-60V, and a smaller for up to 40V. Yes, it's a tad more work, but your bench would look more impressive! ;)

TroelsM 29th March 2007 10:57 AM

Hi PMouse

I'm fighting the look you refer to as "impressive".... I would call it "space-wasting".

Up to 40V at say 2A is still 80-100W...

One PSU to rule them all... The search continues

Regards TroelsM

jackinnj 29th March 2007 01:12 PM

I know that the shipping to DK is expensive, but there have been a number of Eico 1078 variable A.C. power supplies on EBay -- quite inexpensive from what I can determine -- thus you can dial down the transformer on your power supply. Probably uses a Variac.

My big Lambda supply (600V 300mA) uses a Variac as well.

Note that there are some Heath supplies (IP-27) which use a transformer with many taps.

Surprisingly, the Heath and Eico supplies do show up on the European EBay sites.

Eva 29th March 2007 01:12 PM

I would go switchmode with gentle output filtering. There is no need for post regulation if it is done that way, and 300W may be easily packed in the size and weight of a standard PC power supply (0-60V 0-5A). Switchmode voltage regulation is quite straight to achieve, although adjustable current limiting is somewhat more complex.

TroelsM 29th March 2007 01:27 PM

Hi Eva

You´re well known as being the switch-mode-guru and of course you're right, that it's "quite straight to achieve", but I don't think many of us would be able to make it work. Sadly.

That's why I was thinking in a combined switch/lin-solution. I had a "feeling" the demands to the switch-part would be a lot easier if there were a lin-post-reg that could do the actual regulating and current-limiting.

Without a reliable current-limit i would be very scared of connecting my new amp to 60V/5A.. that's alot of juice if somethings not working....

Could you recommend any dedicated switch mode-chips to the pre-reg-job? Any hints?

Regards TroelsM

Circlotron 29th March 2007 01:47 PM

Big, safe and friendly.
I spent a full 10 years till the end of 2005 in a switchmode design lab, so my suggestion is use a variac in front of a conventional transformer + rectifier + filter. It'll work first time, every time, till the end of time.

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