Auxiliary power supply idea

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This idea may be old hat, but I didn't find it in the archives.

I am building (slowly) a Tripath-based amp using a EB-0104A eval board, Jensen input transformers, etc. I am using a 1kVA, 55V toroidal transformer from Avel-Lindberg as the main pwoer transformer. I found some scrap rack-mount cases at work. Since the case is a little tight, and I didn't want to give up power supply capacitance (36000uF for each of the positive and negative supply rails), I had real trouble figuring where to put another transformer (even a small one) for the 5V supply.

Then it hit me! I could wind extra turns on the main toroid. With 120V RMS in, I get 0.69V RMS/turn. Ten turns gives me a nominal 8.6VDC with diode losses, and if I allow for a 25% output drop due to low line or heavy load, this gives me about 6.4VDC or so on the output. I plan to regulate this with an LT1763 low noise, low dropout regulator.

Also, I was thinking about winding the auxiliary winding with coaxial cable. That way I can tie the outer conductor to ground and have an electrostatic shield.

I hope to finish my amp sometime this summer (kids and work keep me quite busy).

Hope this helps someone.

It will work. I can only see two possible downsides:
1) If you start placing a serious load on the secondaries (total of all secondaries), you could cook the primary.
2) If you wind a lot of turns on the outside of the transformer for a high voltage secondary, you could interfere with the heat dissipation.
In your case, only wanting 5V, as long as you don't ask too much current, you should be in good shape. I'd recommend spacing the turns equally around the core so as to distribute the load (electrical and thermal) evenly.
Go for it.

Thanks for the input. My load current should be less than 100mA, and I had planned on distributing the turns around the circumference of the transformer.

I figured the extra windings were better than dropping voltage from the positive rail, where I'd be looking at about 6-8W dissipation. Now I should see less than a watt.

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