Building a bench power supply

Hi All

I am building a dual supply for general hobby work - 5v fixed and 0 to

15v variable. Suit microchips, TTL stuff, and small motors etc etc

Basically 2 separate supplies in the one box, with the monitoring meter switchable between V and I, and between supply A and Supply B

No sweat with that.

What I am not sure about is whether to have the supplies fully floating or earthed or floating with a removable link to earth or a removable link for the negs.

ie both neg output terminals earthed? Both just joined - ie common negs

-

The variable supply will be based on a switch mode power pack I happen to have - output fed to an adj voltage regulator - hassles?

The other on a xformer power pack feeding a 7805 etc

Housing is a plastic box - can fit with a 3 pin lead/plug if I need the earth

Will appreciate views, ideas, and advice

David

"quietguy" wrote in message news: snipped-for-privacy@REMOVE-TO-REPLYconfidential-counselling.com...

Do what is done on most better bench supplies. Have the units seperate, and arrange the outputs so you can change the interconnection terminals. You will want a -ve rail sometime. Add a ground connection somewhere though. If you are going to drive motors, this implies a reasonable current. Think carefully about the amount of heatsinking involved. Also, consider whether the regulators can survive having their outputs driven 'above' the incoming voltage. This is what will happen if the two supplies happen to get joined in parallel, and the variable unit is set above the voltage on the fixed system.

Best Wishes

Generally I avoid building power supplies. Not because I'm scared of them, but usually I can find what I want at a decent price where someone else has done the hard yards, and my bech time can then be spent on more interesting/challenging projects for which there is no COTS solution.

Having said that, there is certainly a place for building to get exactly what you want, and a satisfaction when (if) it all powers up and performs to your expectation.

Starting with the variable part, unless the max load current is low the use of a linear following the SMPS module is going to create a heat dissipation issue. I'd investigate whether the SMPS output voltage can be directly controlled over a suitable range.

For the 5V module, again knowing your max load current info would have helped to make more meaningful comment. I'd certainly consider running the 78X05 off the SMPS module if the current draw is modest. It would save using a second stepdown device, although it has common ground implications.

The most flexible grounding solution is to have both the outputs floating and the provision for ground links to each supply's negative terminal. This is widespread among COTS power supplies.

Be aware that there are safety impications wher the outputs are fully floating - a fault in the unit can result in line voltage on the output terminals.

Thanks for the replies guys - very helpful

I agree that a 'bought one' would be nice, but I just don't have that sort of cash to spend. ie $200 - $300+ for a 3A dual supply unit

Will give some thought to including current limiting - hadn't considered it so thanks

Not sure whether the SMPS can be modified (be me) re o/p - they are tricky things that I have had litte (read none) experience with, but...

The possibility of a fault raising the o/p to mains level is scary - especially in that much of the stuff I will be playing with will also be connected to my Windows computer, my CRO input, and my fingers as well. Mmmm

- don't like the smell of burning flesh, but then I may not be in a state to smell it or anything else!

However, just had a thought about earthing the o/p of the supplies - is this likely to be a hassle in that the plug packs then have 240v between prim and (if earthed) secondary? I suppose not but as a very old time TV tech (B&W w/valves days) I have a grave mistrust of anything that doesn't have an earthed metal chassis

Cheers

David

This thread has some info about modifying a pc power supply to make a variable power supply:

Using the information I gleaned there I disabled overvoltage protection on a salvaged pc power supply. Then to make the voltage adjustable I looked for the inverting and noninverting input pins on the pwm chip (a tl494). A potentiometer (or a transistor controlled by a pot) from one of these pins (I forget which one) to ground causes the output voltage to vary.

Current limiting is another matter. You want it inside a voltage regulation loop. So the simple potentiometer scheme outlined above for setting voltage has to be abandoned. Use feedback from a voltage divider on the output into the base of a transistor controlling one of the input pins on the tl494, and this will set the voltage. Then you can put the current limiting part of the circuit (a sense resistor and transistor) inside the voltage feedback loop.

Float them, that way you can set the variable to 5+ and hook it's positive to the neg of the other and hava a +/-5V supply for playing with op-amps etc...

Bye. Jasen