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dragosmze 6th October 2012 10:36 AM

Using two positive voltage regulators for a dual polarity power supply
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is there a problem in using the circuit (see the image)?

PI1/PI2 came from separate windings of a transformer (after rectification and filtering).

What I want to do is to get more dual polarity outputs, each to supply a separate circuit. I know there is possible to have a output circuit to get the value of amps globally needed .. but there should be no problem for me to use more lm 350 like in the image (I have enough in "stock" :scratch:).

What I need to change there for this to work? I think there should be some diodes on the output of each lm 350 to prevent it to interfere with the others.


Preamp 6th October 2012 11:47 AM

your circuit should work just fine. The separate transformer windings and rectifications are important, though.

Where do you think the diodes should go? As long as the outputs are not wired in parallel (don't do it), they're not supposed to interfere, AFAIK.

dragosmze 6th October 2012 06:24 PM

Well, I have tested this with 3 outputs (so #6 chips) conected as in the image ... (with #2 more...) but something it's not just right; the variable resistor it's not working for the bottom half of them, instead to change the voltage for each of the chips, I can change all from the first one (not sure how to explain) .. but seem the problem it's where these have the + output conected with the - from the top half. I think the circuit closes by another path :rolleyes:
I have try disconnecting the last two from the bottom (by opening JP53 and JP63) and used the #3 connector to power a few headlight bulbs :D. I got power over both porarities and I could change the voltage level by changing the value of the variable resistor of the first botom chip (U4). I was expecting to get only the positive one working ...

Frank Berry 6th October 2012 06:32 PM

You will need three separate transformer windings. One winding for each output.

dragosmze 6th October 2012 07:12 PM

Think I got what's going on :D ... GNDO1, GNDO2 etc. are connected together because of GND1 ... practically the bottom #3 chips are working in paralel if I got it right :Ouch:

Preamp 6th October 2012 07:50 PM

Dang, you're right :eek:! So the whole thing's not going to work unless you use negative regulators instead or you have to supply each reg with its own winding.
Sorry for not finding this fault earlier :o, I would have made the same mistake...

dragosmze 8th October 2012 05:44 PM

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Maybe you can recommend me a transistor to be used like in the image (if possible in a TO-220 package).
I'll be using a lm317 with the input at 31V and want to get 28V regulated :D at about 6A (but I think something like 8A would be better).
Also, do not know how to calculate R7 (I suppose R1/R6 just need to be a small value but as equal as possible across all Tr - if there is a need to use more than one).

Preamp 8th October 2012 05:59 PM

Maybe use an MJE15031? 150V, 8A, TO220, should be easily obtained.
I'm not sure about R7, too, but I'm better not guessing :o...
You could place a diode (like 1N4007) in the ground line of the 7812 to compensate for the additional voltage drop of the power transistor.

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