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Dornier 09-24-2012 03:52 PM

Help making an AC transformer for my sprinkler
I believe the transformer on my Hunter sprinkler controller is shot. I need to test it.

I have never tried to make one before, and I thought it would be fun.

It's output is stepped to 26VAC and 750mA.


notmrjohn 09-24-2012 04:34 PM

The voltage induced across the secondary coil (out put) may be calculated from Faradays law of Induction which states that: where Vs is the instantaneous voltage, Ns is the number of turns in the secondary coil and Φ is the magnetic flux through one turn of the coil.
That's all you need to know, have at it.

There are other considerations to take into account; Leakage flux, leakage inductance, effect of frequency, winding resistance, hysteresis losses,eddy currents, magnetostriction,mechanical losses, various and sundry stray losses, and other problems such as electrocution.

You'll need several hundred feet of wire and some flat iron rings, the size of wire, number of turns, and number of rings can be determined from Mr Faraday's formula.

AltaSparky 09-24-2012 05:14 PM

You want to make a transformer? The poster above gave a lot of good topics to understand before you undertake them. I would study them as well as transformer construction. Or you could just buy a transformer, would likely be cheaper.

Dave632 09-24-2012 05:53 PM

The ratio of input to output voltage is equal to the ratio of the number of turns of the input winding to the number of turns of the output winding.

You want roughly a 5:1 stepdown ratio (120V / 26V), so you would have 5 times as many turns in the primary (input) winding as in the secondary (output) winding. For example, 120 turns on the primary and 26 turns on the secondary. Just how many turns to use depends on core size and type, current required, etc.

Radio amateurs have been building their own transformers for years. Find a copy of The Radio Amateur's Handbook for more detailed information. Be sure to read the chapter on Safety first, though.

I'm sure it would be cheaper to just buy a replacement, but it wouldn't be nearly as much fun!

notmrjohn 09-24-2012 07:02 PM

Thanks sparky, with a name like that you must know what your talking about.
And Dave does too, " 120 turns on the primary and 26 turns on the secondary". will give you approximately 26V from 120V, But what alloy and distance between windings and numbers of coils will give the 750mA? What is resistance of your coils?

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