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Building a new +/- adjustable/selectable supply

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JimmyM:
A long while back I build an LM317 based adjustable bench supply. Nothing fancy. No current limiting or anything like that. Now I have a big Mastech HY5020 for the big stuff and still use the old LM317 based unit for small stuff. But I'm messing around with Op-Amps now and would like to build a +/- supply.
My thoughts are...
20V center tapped transformer, 1 Diode per transformer leg.
An LM317 & an LM337 (I wouldn't really need a lot of current for op-amp stuff)
Filter caps, etc.
Question: To reduce ripple on the regulator inputs, would it be better to use a C-L-C filter instead of just a Capacitor?
If I want just selectable outputs +/-5V, +/-9V, +/-12V, etc I can just use a 2 deck rotary switch to change the Adj-to-ground resistor value (trimmers on each of course to allow fine tuning and matching).

But what if I wanted to make a fully adjustable unit? Would I have to use a 2 deck pot? How could I ensure matching of the output voltages?
Thanks, guys. I know you folks have built something like this before.

ejeffrey:
You want a total of 4 diodes to make a bridge rectifier.

You probably want a 30 volt center-tapped transformer (i.e., 2x 15 volt AC windings). This will give you a rectified voltage of around 20 volts after the diode drop, which is plenty to get +/- 15 volt supplies.  If you only want +/-12, you can use a 24 VCT transformer.

You only need a single deck switch, all of the resistors can be connected to the adjust terminal, and then the switch connects one of them to ground and the others float.

LM317 has great ripple rejection at 100/120 Hz, so you don't really need more than that.  If you do anything I would use an CRC filter on the input side.  Even a couple of ohms provides a lot of attenuation of the higher harmonics that the regulator isn't so good rejecting.

Zero999:
http://www.silicontronics.com/index.php?action=ezportal;sa=page;p=7

JimmyM:
This is EXACTLY what I was looking for!
I even have a uA741 already. as well as a couple of bridge rectifiers.

Brilliant guys. Thanks a lot.

ivan747:

--- Quote from: JimmyM on July 05, 2011, 02:54:04 pm ---
If I want just selectable outputs +/-5V, +/-9V, +/-12V, etc I can just use a 2 deck rotary switch to change the Adj-to-ground resistor value (trimmers on each of course to allow fine tuning and matching).

But what if I wanted to make a fully adjustable unit? Would I have to use a 2 deck pot? How could I ensure matching of the output voltages?
Thanks, guys. I know you folks have built something like this before.

--- End quote ---

You can have he contact switch toggle between fixed voltages and one variable mode using a dual gang pot instead of a fixed resistor. I have though about modding a small supply with fixed voltages (3, 4.5, 6...) to do these on one of the switch's positions. The problem is that it won't fit.

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