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Voltage Regulator(s) for Bench Supply from Laptop Wall Adapter
Voltage Regulator(s) for Bench Supply from Laptop Wall Adapter
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Old 29th May 2009, 08:50 PM   #1
miallen is offline miallen  United States
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Join Date: May 2009
Default Voltage Regulator(s) for Bench Supply from Laptop Wall Adapter


I would like to create a bench power supply for experimenting. 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.

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?

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Old 30th May 2009, 03:28 PM   #2
Andreq is offline Andreq  Canada
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Join Date: Nov 2005
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You could probably use 1 simple LT317 (LM317). It can ouput 1.5A for the positive rail. And a lt337 (LM337) for the negative rail.

See :



Click the image to open in full size.
Here is a "fixed" voltage regulator, but you'd just have to replace the 7815/7819 by 317/337 (and adding the necessary resistor/potentiometer)

I think the only problem you'll have is the that you need an +20VDC & -20VCD supply.

The easiest solution would probably to get a center tapped 20-0-20 transformer (and don't be fooled like I did... 24VAC != 24-0-24, but 12-0-12!!!)
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Old 30th May 2009, 06:27 PM   #3
miallen is offline miallen  United States
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Join Date: May 2009
Yeah, I've since given up on the laptop adapter idea. The most practical way to get + and - voltages is to use the center tap of the transformer as ground and obviously the laptop adapter isn't going to allow me to do that (I suppose if I ripped it open it might but even if it had a center tap the secondary will be too small).

So I'm just going to do this the usual way and use a transformer. I have a book with an LM317 / LM337 circuit that is exactly what I want minus the rotory switch (_Practical Electronics for Inventors_ p 606).

At this point I'm trying to salvage parts from various places. For example I have a bunch of old computer power supplies. But despite the number of parts in them, they just don't seem to contain the parts I really need since they're switching and not linear. About all I can get out of it is the huge aluminium heat sink and the enclosure.

One thing I've thought about doing is calling up appliance repair shops. I would imagine they have boxes full of old transformers and diodes and such.

Is there any such thing as electronics fairs or swap meets? I'm in northern New Jersey, USA.

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Old 30th May 2009, 07:26 PM   #4
MJL21193 is offline MJL21193  Canada
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The circuit below would make a great lab supply (I know, it's what I use). It is adjustible with a dual pot and has bypass transistors (the MJW's) to handle more current.

Cheap parts can be found in old receivers and amps that you'll occasionally see at places like Goodwill, Salvation Army thrift shop etc. Easily get most of the parts for this power supply from one $5-$10 receiver.
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