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Old 3rd March 2008, 06:58 PM   #1
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Default Making your own winding for auxiliary supply

Hello all,

In the interest of saving $, I have a basic toroid (~200VA, +/- 25VAC secondaries) for a gain clone amplifier.

I would like to add a basic microcontroller with pushbuttons to my amp for things like volume control and other low voltage housekeeping, but I don't want to add an extra transformer just for this. I only need a ~6V supply that I would be regulating down to 3V logic level.

One option I have is to just buy a cheap AC-DC wall cube. Another option I have thought of is to wind a few turns of regular wire around the toroid core to make a third secondary - then I could rectify that voltage and make my DC supply from there. When I say "wind a few turns to make a third secondary", I mean just winding the wire over the existing toroid, NOT removing the transformer tape, or anything like that.

Has anyone tried this before? I am not sure how much current I would be able to get from the core in this manner because the windings of the third secondary would be further away from the core due to the thickness of the other windings, the transformer tape, etc., and hence bad magnetic coupling.
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Old 3rd March 2008, 08:23 PM   #2
mag is offline mag  Switzerland
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Hi if you need only some mA to power up logic stuff just wind
some wires around the toroid. No need to disassemble it, just wind them over the tape. You can even use a standard plastic insulated wire.

Start with 10 turns and mesure the AC voltage across the wire than adjust the number of turns to reach your desired voltage.

If you need 3V for logic stuff try to have something around 5-6V AC. For rectifying you can use a simple 1n400x diode and a 470-1000uF capacitor. It becomes 7-8V DC on the capacitor.
Then regulate that voltage using an LDO like LM317 or others LDO of your choice.

A small fuse on that winding will be a good idea, if you short it, even if it is not so tightly coupled with the primary you will have enough current to burn the wire....
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Old 4th March 2008, 07:36 AM   #3
Mooly is offline Mooly  United Kingdom
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How about a "Watt-less Dropper". Use the reactance of small caps in place of resistors and run direct of the secondary. A couple of caps and diodes is all that is needed plus a regulator (3v3 zener ?) and a resistor or two. All depends how much current you need as to whether this is viable.
Regards Karl
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