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Voltage Regulator for Making Bench Supply from Laptop Wall Transformer

Hi,

I would like to create a bench power supply for experimenting with audio circuits. I'm not an EE and I'm just doing this for fun so I'd like to do this as cheaply as possible. I'm looking for advice about creating such a supply from existing or inexpensive components.

My requirements are dual +-9V at 500mA with low ripple (preferrably in the micro volt range).

I happen to have a 5 position double pole rotary switch so it would be great if I could switch between +-5, +-9, +-12, ... etc.

I also have a few laptop power adapters which output between 16-20V DC at ~3A.

It would be nice if the resulting bench supply could put out whatever the laptop adapter could supply so that I could connect multiple loads.

So given the above, it seems I just need a voltage regulator IC. I've looked around Digi-Key, Mouser and so on, but again, I'm not an EE and I'm a little lost. I would appreciate it if someone could suggest a chip. I would imagine I just want some linear dual output thing that's

3 bucks. Then with a few capacitors and resistors I can get the ripple down to where I want?

Also if someone knows of a blog post along these lines that would be great. I searched high and low for something like this but didn't find anything that used a laptop adapter (maybe I'll take some pics and try to put up page myself).

Any advice would be appreciated, Mike

Reply to
Michael B Allen
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So, your requirement is a tracking regulator with +/- outputs; that's relatively common, there are single chip solutions. Output current limit is typically 200 mA; that's enough for a lot of the kind of analog electronics that uses split-rail power supplies.

It might not, though, stay at 16V if you only drew 30 mA from it. And it's power-efficient enough to not need big heatsinks and fans. If you were to update an adjustable supply to match this, it WOULD need a big heatsink, because most adjustable regulators are LINEAR devices that take power from a high voltage filter capacitor, and the adjustment range (+/- 12V) you require means that the capacitor supplies maybe

15V to start with.

Power efficiency in supplying +5V from a 15V source is 33%. Raise the voltage to 20V, the efficiency goes to 25%.

So, using the laptop adapter as a source gives you efficient generation of the filtered source, BUT you lose efficiency anyhow in stepping down with the regulator. So, the expense and complexity of the laptop adapter is wasted.

"Laboratory power supply" or "bench power supply" should get you plenty of suggested circuits; any good book on electronics will have a chapter on the subject. _The Art of Electronics_, Horowitz and Hill, 2ed, calls it "chapter 6".

Reply to
whit3rd

Yeah, I've since discovered that if I want a split supply the normal thing to do is use the center tap of the transformer. Using the laptop adapter would require a voltage inverter so it does not really make the project any easier or more cost effective.

I found a circuit that does pretty much what I want in _Practical Electronics for Inventors_ p 606 which is a standard LM317 / LM337 circuit (although it's not clear what sort of ripple that circuit will have). All I need to do is factor in the switch. I will eventually get _The Art of Electronics_ but for now the book I have is surprisingly good.

Thanks, Mike

Reply to
Michael B Allen

Hi Mike

This article might help you:

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See also:

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You can use the following regulators:

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for the positive regulation. Use the equivalent 7905 for the negative regulators. Substitute the 05 for 12 and you get a 12V regualtor instead of a 5V regualtor. You can also get 7809 and 7909 9V positive and negative regs if you look hard enough.

Good luck

-- Bill Naylor

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Electronic Kits for Education and Fun

Yeah, I've since discovered that if I want a split supply the normal thing to do is use the center tap of the transformer. Using the laptop adapter would require a voltage inverter so it does not really make the project any easier or more cost effective.

I found a circuit that does pretty much what I want in _Practical Electronics for Inventors_ p 606 which is a standard LM317 / LM337 circuit (although it's not clear what sort of ripple that circuit will have). All I need to do is factor in the switch. I will eventually get _The Art of Electronics_ but for now the book I have is surprisingly good.

Thanks, Mike

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