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I am short an adjustable negative voltage regulator, so I am trying to link two lm317 regulated power supplies to form a dual power supply. Is this possible? They are both connected to a center tap transformer. One is using the center tap as ground, the other is using the center tap as the positive supply. Will a dual power supply set up like this work and hold the regulated voltages? Also, is there another way to make a dual power supply with only lm317s?

The schematic is something like this. Sorry for the fast patch together with paint, but I think this will get the gist across. The component values are not set in stone, so don't worry about the values. I am worried about the general layout and the order of things.

enter image description here

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    \$\begingroup\$ Why don't you use the LM337 for the negative part? \$\endgroup\$ – anrieff Aug 19 '18 at 7:10
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TL;DR: Might work, after tweaking. The proposed circuitry is bad, and the overall principle would be bad, too.

First of all: you've got a center-tapped transformer giving you dual-voltages. So, use a single transformer instead of two. You've got 6V of overhead on both of these secondary half-windings – more than enough to regulate down to 6V (the current capability of the transformer is much much higher than that of your LM317, so you don't need more than a half wave in energy per cycle – or your fuse is far far too large).

Yes, in principle, this can work. You must make 100% sure that the secondary sides of your transformer are floating.

But then you'd have two power supplies regulating the same potential – to the lower LM317, the upper essentially looks like a complex load, and thes will make everything potentially oscillate. So, you'd actually tie what you've labeled "ground" to actual ground.

Now, the LM317 is an old thing, and it's much worse at regulating falling load edges than rising ones – with the effect that your supply's source impedance is um not really defined, and if you draw significant current at changing amplitudes, you'll see nonlinear distortion of the supply voltage.

Getting the output voltage to stabilize reliably is probably far more time-consuming then waiting for digikey, mouser, element14, arrow, tme.eu, rs-components… to deliver a negative voltage regulator.

While you do that, pick something a bit better than the LM317. It's a historical component, mostly used for compatibility reasons and to demonstrate the principles of simplest semiconductor devices, far less often for practical usage.

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  • \$\begingroup\$ my mistake, it is the same transformer with the 4 diodes attached. it is meant to run small loads like arduinos and oscillaors that need a dual supply. with them running on the same resistor do you think it will work for this. thank you very much for your help. \$\endgroup\$ – Teage C. Aug 19 '18 at 7:35
  • \$\begingroup\$ Add the fact that it is the same transformer to your text above. By the way: that will NOT work. If you look at your schematic the center tap is on both sides of the bottom LM317 \$\endgroup\$ – Oldfart Aug 19 '18 at 7:43
  • \$\begingroup\$ transformer not resistor. there are a handfull of differences between my scratch paper, and my edit of a copy paste. r2 is supposed to be a 10 pot as a variable resistor. the idea would be to have two variable voltage supplies that i can run in series to turn into a dual. also product of spare parts. \$\endgroup\$ – Teage C. Aug 19 '18 at 7:44
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    \$\begingroup\$ "there are a handfull of differences between my scratch paper, and my edit of a copy paste." Then we can't answer your question. Please provide the correct schematic. There is a CircuitLab button on the editor toolbar and you can save an editable schematic inline with your question. \$\endgroup\$ – Transistor Aug 19 '18 at 8:38

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