Building a bench power supply

jefm

Hi gang,
For numerous reasons I'm building a bench power supply for my BASIC Stamp stuff, which is two of the pro development boards and a couple little regulators now and then. Input is 125VAC output is 12V 5A. I'll probably never use five amps but figure might as well build it big. Everything is actually rated higher then 5A but that's where the meter pegs and that's what the fuse will be for safety.

I'm new to making power supplies, besides the obvious safety stuff is there anything I should look out for? People will probably say "but Jefm why don't you just get a bench supply from the electronics shop". An upcoming project will need a custom power supply, I've never built one before so I made up another project for practice and that is useful. I already hooked up my transformer and got 12v out of 125v, that was fun. I'm a bit confused on how to hook up the five amp LM338K adjustable voltage regulator, so I might connect two 3A Fairchild linear regulators in parallel to get five (six) amps. Or I might go with a whole different voltage regulator. EDIT: wait, duh, do I even need a voltage regulator? The transformer puts out 12V, is it enough to stick a cap across the rectifier and call it good? The book seems content to regulate 12v to 12v so it must be there for a reason I suppose.

The schematic I'm using is from the Microcontroller App Cookbook Volume 2 page 29. Need to find some of those big 3000 uf capacitors, they are expensive. But that's this project, thanks in advance for any help.

Post Edited (Jef Miller) : 7/20/2008 1:39:32 AM GMT

Mike Green

You need a good heatsink for the regulators. If you're planning on feeding 5V logic mostly, that's a lot of heat (12V - 5V = 7V x 5A = 35W).
If you plan to feed regulators on your boards, that's a bit easier. Most 5V regulators are ok with inputs on the order of 7.5V. If you're using a Low-Drop-Out regulator, you could supply 6V.

sam_sam_sam

Here is the data sheet for this· regulator

http://cache.national.com/ds/LM/LM138.pdf

Here are some Thing·NOT TO DO·with these regulators

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I have used this· regulator in the past

MAKE SURE THAT YOU ONLY USE THE T03 case type

If you use the T220 you will have trouble with them

you can fry them·I have done it even with very good heat·sink

What ever you do DO NOT HOOK UP HEAVY DUTY MOTOR TO this·regulator

You will FRY it...>>>>>· You will be ··ing when this happens

I would run NO motors with this· regulators AT ALL

Also be very careful with the use·a Heavy Duty Relays can cause the same problem

YOU MUST USE A HEAT SINK WITH THIS REGULATOR

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These are very GOOD·regulators

http://cache.national.com/ds/LM/LM138.pdf

It was a lot of fun to built my own bench power supply·it last for many years until I·did· what i have warned you not to do some day i will fix it again

How I made mine own

·Have you seen the 10 amp power supply that Radio Shack use to have a few year ago I pull REGULATOR Board out·and used every thing else even the very nice Heat Sink I had Volt Meter and a Amp Meter on top of it

Like Mike said about Heat Sink is very true

If you're planning on feeding 5 Volt logic mostly

They do have a Five Volt Regulator I can remember what the part # is It also come in a T03 case as well it·will·handle ·also five amp as well

Now in the Nuts and Volts magazine they have a Switching Regulator

You can order it from there web site

for $30.00 as a kit very nice kit easy to put to getter

I hope this helps



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··Thanks for any··that you may have and all of your time finding them

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Sam

Post Edited (sam_sam_sam) : 7/20/2008 2:47:25 AM GMT