Bench Top PSU

[benjamin77]

I am in the process of building a bench top power supply. I have robbed a previously working PSU from an old PC. It wouldseem it needs to be under load before it works.

Any suggestions on how I can do this ?

[Sean Williams]

If it is an ATX power supply you also need to link the power on line to ground.

Otherwise, a few K across the 5V rail ought to do it.

Sean

[evingar]

Most PC power supplies I have come across will give you voltage out under a no load condition. - Although the rails may not be 100% to spec.

If you aren't getting anything out at all, I would suggest that you need to do as Sean suggests.

I'm not 100% sure that a PC PSU is the best thing to use as a bench PSU. The 2 major problems are that its not usually possible to adjust the rails and the output may be quite noisy - especially if the PSU is a "bargain basement" type. I guess it all depends what you want to use it for

I have an ancient Weir 413D type linear supply that I picked out of a skip decades ago. It had a shorted power transistor. Now it is repaired, it does admirably well for most of my low current development and test work. I also don't have to contend with a noisy fan !

[benjamin77]

I just wanted to use it as a low voltage supply for battery transistor radios via a selection of binding posts. Is this not to be suggested ?

[cmjones01]

Originally Posted by benjamin77

I just wanted to use it as a low voltage supply for battery transistor radios via a selection of binding posts. Is this not to be suggested ?

It should work, as long as the power-on wire is grounded, and as long as you want 3.3V, 5V, or 12V-ish. The power-on wire is usually the green one:

http://pinouts.ru/Power/atxpower_pinout.shtml

As others have said, it might be electrically noisy, though shouldn't be too bad at low loads. I have often found that the the +/-12V rails aren't really regulated, and tend to be rather low if there's little load on the 3.3V or 5V rails and rather high if there's a heavy load. I know this is the opposite way round to the usual power supply behaviour, but the voltage feedback seems to be taken from the 3.3 or 5V rails. If they're working harder, there's more oomph (to use the technical term) going into the transformer, and so the +/-12V unregulated outputs will tend to get stronger.

Load it, measure it, see what you get.

Chris

[jimmc101]

The main problem with a computer PSU is that it is a switchmode unit, if you use it to power a radio you may find that all you can receive is interference from the power supply itself!
The other problem is that it can supply several amps,enough to melt tracks in the event of a fault in the radio.

Have you considered building your own power supply?
Using a linear regulator chip such as the LM317T keeps it simple, there dozens of examples on the web, this seems to be a good example:

http://home.cogeco.ca/~rpaisley4/PSupply.html

"A Test Bench Power Supply", 1.25 to 12.5v @ 1A using about a dozen components.

You don't get interference from a linear power supply.


Jim

[kalee20]

Originally Posted by jimmc101

You don't get interference from a linear power supply.

You do! Try using an LM317 to stabilise the LT line of a directly heated valve radio! Nothing but mush! Extra LC filtering is the order of the day!

However, it's nothing like as bad as a switch-mode unit, agreed.

[Skywave]

Kalee20: re-your Post immediately above.

I believe you, of course, but did you fit the 0.1uF (or thereabouts) caps. on the I/P and O/P pins, each to the regulator common gnd. connection and the wires of these caps. as short as possible?
I ask simply because there may be something I can learn here. I've used LM317 for all sorts of odd tasks, but never experienced your reported problem. OTOH, I do know that the localised decoupling (as per the above) is important.

Al. / Skywave.

[kalee20]

Well Al Skywave, it was a lash-up at least 15 years ago! I seem to remember I didn't have an output capacitor (the LM317 doesn't have a common ground like the 78xx series 3-terminal regulators. National Semi recommend an input capacitor but an output capacitor is referred to only as 'improving the transient response'). After finding the mush, I added local capacitors which helped a lot, but not as clean as a battery. Of course, the LT line is sensitive to microvolts of interference as this is effectively injected into the first stage cathode.

My own lesson learned was, whether recommended or not - and some regulators stress as a selling point that they're not necessary - suitable value input and output capacitors are good practice, and attention paid to layout with good current routing is not wasted.

As benjamin77 wants to use his power supply to power transistor radios - presumeably for fault finding - it does seem that he needs a PSU which is 'blameless' as far as interference is concerned. Switch-mode units will work, but I wouldn't have much confidence with a cheap computer unit unless outboard external filtering is added (with possibly some extra screening against radiated interference) - and even a linear needs attention to detail.

Another point, which hasn't been made yet - is that a variable current limit for bench power supplies is a great life saver. Output fuses could be added to the binding posts described, in case of connecting a radio with a short-circuited output stage, but its's nothing like as convenient. And it's really demoralising to see a thin copper foil lift itself off the board with a wisp of smoke before melting away!

[benjamin77]

So basically i had better bin that idear and start again. What I was hoping to do have one half of the bench supply set up with a variac and the other with vaiable voltage up to about 30V. Am I trying to complicate things by having to different suplies in casing ?