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Old 29th July 2012, 01:58 PM   #1
wicked1 is offline wicked1  United States
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Default Bipolar Supply, Unbalanced Load

Does running a bipolar power supply off a center tap transformer cause noise if the load is unbalanced?

I'm building a bipolar power supply for a synthesizer project, and have some noise..
99% of the thing runs on positive 12v. (Only the filter needs negative 12)
My transformer has 2 secondaries, and I had them wired up as a center tap. That's when I could hear buzzing. I rewired the power supply with a single secondary, as just a positive supply, and it works perfectly w/ no noise.

Is that because the load is so unbalanced?

Would wiring the thing up as basically two power supplies (using the transformer secondaries separately), one positive and one negative, and then connect the grounds after the regulators be a better idea?

The supply is simple.. Transformer to rectifier, 2200uf cap, 7812/7912, 100uf cap.
(Not going for hi-fi, or anything here)

Thanks for any advice.
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Old 29th July 2012, 03:56 PM   #2
DF96 is offline DF96  England
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You probably had the grounds in the wrong place. Almost everyone seems to connect the CT in a way which guarantees it injects buzz into the signal gound. Separating the windings accidentally forced you to get the grounding right.
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Old 30th July 2012, 01:45 PM   #3
wicked1 is offline wicked1  United States
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And by that you mean the I had the two secondaries wired incorrectly, right? Or do you mean another aspect of my grounding?

It's an Antek transformer. I went by their schematics, but that is definitely an area I'm not 100% clear on, as their schematics were different from others I've seen online (Antek schematics have no phase dot, either). But aren't the only options in phase or out of phase? They're definitely in phase, as I get 24v between the two secondaries... If I reverse the bottom secondary, I get ~0v. The way I have it is the two primaries are in parallel, and the secondaries in series, connected at the middle leads.. (bottom of first coil + top of second, blue+green wire connected, if you know about antek's lead coloring)
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Old 30th July 2012, 03:17 PM   #4
DF96 is offline DF96  England
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The usual mistake is to connect the CT to the star ground (or whatever ground arrangement is used), then the caps to the ground. This pushes charging pulses right through the ground connection.

Correct version is to connect CT to caps, then the other (smooth) end of the PSU to ground. Remember, with grounds you must not assume that a conductive connection implies equipotential, as there can be large currents flowing in some places.
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Old 30th July 2012, 03:48 PM   #5
wicked1 is offline wicked1  United States
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Ok, this is what I did..... Center tap as you said, is where the caps connect, and is ground for the regulators. Basically like a star ground for all the power supply components. Then the output from that (+12,0,-12) all goes through the power cable (about 1' or 30cm long) into the synth. The connection from the star ground in the power supply goes through the cable and is the ground (0v) for the synth.

Is that correct? If not, where do I get ground? I thought the center tap was basically making ground.

Thank you for your help. I do have it working the other way, using the secondaries separately, a bridge rectifier on each one.
But, I would like to understand what I am doing wrong w/ the center tap version.
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Old 30th July 2012, 04:18 PM   #6
DF96 is offline DF96  England
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The CT provides current in dirty big short pulses. You need to keep those current pulses well away from your ground, yet there must be a connection from CT to ground somehow. The solution is as I said: CT to caps, caps to regs ground, regs ground to star ground. That ensures that the star ground only sees pure DC.

Always ground a PSU from the clean end, not the dirty end, and don't mix up the ends into one ground. A PSU should really use a ground bus, even if one end (the correct end!) then goes to a star.
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Old 30th July 2012, 04:46 PM   #7
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Use two rectifiers instead of center tap.
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Old 30th July 2012, 04:53 PM   #8
wicked1 is offline wicked1  United States
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I am.. and that works perfectly. I'm just trying to figure out why my CT version did not work.
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Old 30th July 2012, 06:11 PM   #9
jitter is offline jitter  Netherlands
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Usually when people ask about grounding, I refer them to this excellent article.
Chapter 3 is relevant to this thread, but it's worth reading the whole article.

If you don't have the time to read it, here are two relevant images from that article. Also from it: "If signal reference current is routed through the power supply buss, this noise voltage will be impressed onto the signal by Common Impedance Coupling."
This may have been the reason why you had the noise.
Attached Images
File Type: gif Image1.gif (8.6 KB, 193 views)
File Type: gif Image2.gif (5.8 KB, 190 views)

Last edited by jitter; 30th July 2012 at 06:25 PM.
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Old 30th July 2012, 06:19 PM   #10
DF96 is offline DF96  England
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The second image (fig 3.2-4) shows you how not to do it. It is not a clean power supply - it uses the classic dirty CT noise injection which I have been telling the OP to avoid.

The first image shows why it is wrong. You ground a PSU at the clean/quiet end, not the dirty/noisy end or somewhere between.
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