# power transformer question

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#### Bill Fitzpatrick

I have a power transformer with a 120vct (unloaded) seconday. I need to reduce the voltage at this seconday to about 100vct. The transformer has a extra high current winding of about 16v. I've gotten the impression that I can use this extra winding to BUCK the primary. If this is true, how do I wire it in?

#### paulb

First make sure there is no electrical connection between the two output windings (use an ohmmeter). If there is, you cannot use this technique (although you might be able to get the right voltage by experimenting with different windings).
Wire the 120V and 16V windings in series. They will produce 136 or 104V, depending on the phasing; swap the connections on one of the windings if necessary.
The maximum current will be the minimum of the two windings that are in series.

#### Bill Fitzpatrick

paulb said:
First make sure there is no electrical connection between the two output windings (use an ohmmeter). If there is, you cannot use this technique (although you might be able to get the right voltage by experimenting with different windings).
Wire the 120V and 16V windings in series. They will produce 136 or 104V, depending on the phasing; swap the connections on one of the windings if necessary.
The maximum current will be the minimum of the two windings that are in series.

Thanks, but you missed the point as we discovered in private email between us.

The idea is to wire the extra seconday into the primary in a bucking fashion so as to reduce the output of the 120vct seconday.

Any other takers to the question "is this OK?"?

#### GRollins

Bill,
If by bucking, you mean in parallel but out of phase, I don't think it will work. The higher voltage (and presumably also higher current) winding will try to drive the lower voltage winding. If I'm understanding your original question correctly, the voltage might cancel, but the current from the higher voltage winding would probably see the lower voltage winding as something approximating a dead short; you'd be limited (again) to the lower of the two current ratings. In order to pull it off, I'd think you'd need the EMF from the two windings to be somewhere close to the same so that one wouldn't overpower the other.
What's wrong with Paul's solution? It should do the trick unless you're limited by the current.

Grey

#### grataku

What the heck is "bucking" anyways?
By that you mean to put the primary and the low voltage secondary in series so that the ratio V/wound is lower on the primary and therefore the other secondary voltage is lowered, right? I never done it before but try it what's the big deal. Put a small fuse on the primary just in case you f-up. If you have it use a variac in any case a transformer won't just die instantly. You definitely want to have the coils in phase. If you don't the transformer may let you know by making strange noises.

[Edited by grataku on 06-07-2001 at 12:52 PM]

#### paulb

Thanks, Grey, I'm not the only one that misread Bill's posting. He needs a centre-tapped secondary to get a +/- supply.
Bill, I still think your concept of bucking the primary will work. Fusing is (always) a good idea. Please let us know if you try it.

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