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Old 24th August 2006, 11:17 AM   #1
mr.duck is offline mr.duck  United Kingdom
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Default +/- with pos and neg regulators vs 2 pos regulators

Hopfully the title explains it. To create a +/- PSU, you need a positive regulator and a negative one. But I saw a schematic that uses 2 positive regulators and have been wondering if there are any problems in doing it that way. Here it is...

Click the image to open in full size.

The one problem that I can see is that you have to use two bridge rectifiers. But thats no problem for me I was going to use two anyway. Is there anything else wrong with doing it like that?

Also, is there anything special you have to do with the grounding?

Cheers.
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Old 24th August 2006, 11:44 AM   #2
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It works fine. Be sure that your transformer has two independent windings as shown - a center tapped transformer would be shorted by the two bridges.

A potential disadvantage is if your transformer has marginal voltage output you lose an extra diode drop. Not a big deal if your transformer is sized properly.

Take the ground symbol as your star ground - use separate leads from each supply to the star ground.
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Old 31st August 2006, 12:39 PM   #3
mr.duck is offline mr.duck  United Kingdom
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ok, thanks. There is one thing troubling me when I look at the schematic. It looks like the positive output of the bottom regulator is shorted out to ground. Maybe I dont know how to read the schematic properly. Is the ground symbol ment to be connected to earth or just read it as 0v? It seems to me that the ground symbol is +24v relative to earth. Is that right?
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Old 31st August 2006, 12:47 PM   #4
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No both regulators are 'floating' and you connect them in series with the midpoint forming your centre/0V point. This is where you connect your chassis and mains earth to.

Using separate windings and bridges is actually the best way to achieve clean ground and nicest transformer loading.
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Old 31st August 2006, 12:59 PM   #5
mr.duck is offline mr.duck  United Kingdom
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ah makes sence, thanks. So if I decided to disconect the mains earth from the centre, and connected it to the +24v line (just because I can), then that would not cause a short? And that would now give me a 0v, -24v, -48v PSU?
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Old 31st August 2006, 02:47 PM   #6
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No you would still have a +24 0 -24 system. The mains earth can be connected where you like. It just references that point in your circuit to the soil on the floor, for safety reasons. 0V is usually used because it's where the rest of your circuitry is referenced to.

Electrical ground is simply where you refer all other voltages to.
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Old 28th September 2006, 12:48 PM   #7
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Default Re: +/- with pos and neg regulators vs 2 pos regulators

Quote:
Originally posted by mr.duck
Hopfully the title explains it. To create a +/- PSU, you need a positive regulator and a negative one. But I saw a schematic that uses 2 positive regulators and have been wondering if there are any problems in doing it that way. Here it is...

Click the image to open in full size.

The one problem that I can see is that you have to use two bridge rectifiers. But thats no problem for me I was going to use two anyway. Is there anything else wrong with doing it like that?

Also, is there anything special you have to do with the grounding?

Cheers.
This should work with an LM317 as well?
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Old 28th September 2006, 12:53 PM   #8
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Old 12th October 2006, 04:27 AM   #9
CBS240 is offline CBS240  United States
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What you have here is essentially 2 DC "batteries" connected in series. I made a regulator scheme similar to this but the reg output devices were N-channel mosfets. It worked great until I accidentally shorted one and after about a second of intense transformer buzzing, it exploded. .....Piece of the plastic case nearly embedded into my forehead. Lesson learned here was a fuse can be a good safety device....if you actually use one.
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