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Old Fri 02 March 2007, 20:28
James Webster
Just call me:
James Webster's 300VA, 71V, $60 power supply

Total cost to build the 300VA power supply was:

AN-3225 transformer $33.50
two 5600uf 100v caps $10.00 ($5 each)
USPS priority shipping $11.35
subtotal $54.85

All of the above came from
They shipped the next day

Here is the hookup guide to the AN-3225 based power supply for USA:

120V power comes in through the IEC Jack.
Neutral conductor connects to both black transformer wires
Hot conductor connects to one leg of circuit breaker.
Circuit breaker connects to both red transformer wires

AN-3225 transformer has two 25V AC output windings (refer to picture for letter codes)
(A) green conductor goes to ~ symbol on rectifier
(B) blue is connected to (C) green. This is the ONLY way you can get 50V AC, no other pair will work!
(D) blue connector goes to remaining ~ symbol on rectifier.

Rectifier has a chopped corner to mark the + output. If you really look hard the - is marked on opposite side. You now have 50V DC.

Two 5600uf 100V (uf is pronounced MICRO FARAD) capacitors are soldered together in PARALLEL (that means positive to positive, negative to negative). The + conductor from the rectifier runs across the positive terminals of the capacitors, the - conductor from the rectifier runs across the negative terminals of the capacitors.

At the end of these two conductors you measure 71V DC.


1. If you mix up the B+C windings on the transformer, it will overheat and could start a fire, BE CAREFUL and get it right the 1st time.
2. The capacitors hold their charge even after the power is off. They can zap you even years later. A bleeder resistor could be added to drain them after power is cut.
3. The rectifier should be mounted to a metal plate to dissipate heat (it will last longer that way).
4. Be sure to use a fuse or circuit breaker.


I already had a rectifier KBPC2502($1.75) and four-way fuse block 43-214($3.49) from where shipping on a whole box of junk was $7.00 .
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Old Wed 07 March 2007, 03:27
Just call me:
For some strange reason, the capacitor guys go to a lot of trouble to mark their negative (-) pole while most othey guys mark the positive (+) pole. Don't assume that the marked poles are all the positive ones. A reversed capacitor can explode. At minimum, turn your back to this power supply before switching it on. Or, be a wimp like me, and put it on the driveway with a 50' extension lead from the workbench and close the door on the lead before switching it on "remotely". (a blown cap makes a papery oily mess). Of course, keep pets and kids away.

A temporary bleed resistor across the output is highly recommended if the Gecko's are not there. 22K 1/4Watt or 11K 1/2Watt, etc., etc. Even with a resistor, check the residual voltage before touching the outputs to anything.

Mike then picked up on the bleed resistor and other issues - that discussion has been moved here.
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Old Sat 29 December 2007, 12:44
Just call me: Ryan
I have the exact same setup and I have a question that I have seen talked about somewhere on the forum, but i can't find it now. As wired I get 71vdc from this setup (actually 73v at my house). If I were to dissconnect B-C, and then crimp A-B in a lug and C-D in a lug and then connect to the rectifier I should get 35v-ish with double the amps? Or would it be A-C and B-D connected? I am very cautious about working with 120volts and don't want to just be connecting wires together without knowing.

By the way, I am one the many (i'm sure) lurkers on this forum who visit every day to learn more, and just keep working away in the background trying to build a machine that suits me. It won't be done anytime soon but that doesn't stop me from checking in every day, usually 4-5 times to get inspiration from the people leading the way.

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Old Mon 31 December 2007, 20:35
Just call me: Doug #3
Conway (Arkansas)
United States of America

According to the toroid-transformer website, there are two secondary coils. At one end of each coil, the insulation is blue and at the other end, it is green. I'm certainly no electronics expert but I believe you should connect the blue wires together and then connect the green wires together. That will wire the secondary coils parallel and you should get half the voltage you would get if you wired them in series like James did above.

Hopefully, someone who is good at electronics will check my answer.
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Old Mon 31 December 2007, 21:26
Just call me: Ryan
Thanks for the reply. I did a few searchs on the forum and yesterday I tried wiring it like you said, blue-blue and green-green. Voila! 36v and no flames!
Control box is close to done, Gecko's in the mail and just deciding on which motors to purchase.
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Old Fri 17 February 2012, 23:55
Just call me: James
Detroit (Michigan)
United States of America
Mike asked what fuse should he use to protect the transformer, as he felt the 15A breaker in the above example was too large. Of course he is right, the breaker was just there as a safety net for testing (and not the most safe choice).

Generally, you divide the VoltAmps of the transformer by the line voltage to get the fuse rating.

So in this case we divide 300 (this is a 300VA transformer) by 115 (our shop line voltage) to get 2.6 amps. 300/115=2.6

The actual fuse we chose was 2.5 amp (a 2.6 amp fuse would be hard to find), slow-blow (don't want a fast blow fuse in this application).

2-1/2A 250V MDA

Last edited by WTI; Sat 18 February 2012 at 00:08..
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Old Sat 18 February 2012, 00:00
Just call me: James
Detroit (Michigan)
United States of America
Originally Posted by James Webster View Post

Rectifier has a chopped corner to mark the + output. If you really look hard the - is marked on opposite side. You now have 50V DC.

To be exact, you technically have 71V DC, but until you add the capacitors in the circuit to smooth out the wave, your digital volt meter will be fooled and read 50V DC.

I posted this so no one would panic that the Rectifier was defective when their volt meter only reads 50V while building the supply. Add your caps, and your meter will read correctly.
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