# Transformer for Capacitor Discharge circuit

#### Xray

Joined Nov 21, 2004
58
Hi everyone!

I searched this forum but could not find what I'm looking for, so maybe someone here could help me. I'm trying to find a replacement high voltage transformer for an electric fencer. The original transformer is a basic iron-core transformer with a primary winding of heavy wire with a relatively small number of turns, and a secondary winding of about a zillion turns of hair-fine wire. The transformer arced internally and developed a permanant short, so I'd like to replace it rather than buying a brand new fencer. The original transformer is rated at 10KV output. It put out a narrow pulse about once per second. The circuit is a very basic SCR capacitor discharge circuit that drives the transformer. My question is, how can I calculate the output voltage pulse of any given transformer if the turns ratio is known, and also if the value of the capacitor and its discharge voltage are known? Or do you need to know other parameters such as the inductance of the primary and secondary windings, etc? Does it get more complicated than that?

Thanks!

#### SgtWookie

Joined Jul 17, 2007
22,210
Just going for the quick 'n' dirty solution, have you considered using a standard automotive ignition coil? If your circuit existing circuit could be adapted via a power resistor to output about a 5v pulse, you could just slap one right in.

What's the output voltage pulse of your system now?

#### Xray

Joined Nov 21, 2004
58
Just going for the quick 'n' dirty solution, have you considered using a standard automotive ignition coil? If your circuit existing circuit could be adapted via a power resistor to output about a 5v pulse, you could just slap one right in.

What's the output voltage pulse of your system now?
Yes, I have tried an automobile ignition coil. It seemed to work pretty well (judging by the length of the spark) but it's physically too large to fit in the fencer box. The original transformer is rather flat. But even if I could randomly (or should I say "accidentally") find a transformer (aka, "coil") that will work, I still would like to know how to calculate the output voltage pulse when certain input parameters are known. I know it's not just a simple matter of turns ratio, because the resulting output pulse is MUCH greater than multiplying the input voltage by the turns ratio. I just hope that the calculation doesn't involve anything as complex as calculating the magnetic field of the transformer using Maxwell's equations!

Oh, the fencer ouput is rated at 10 KV, which gives about a 1/2 inch spark at the fence.

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#### SgtWookie

Joined Jul 17, 2007
22,210
There's quite a bit involved in determining a good transformer design, and there are many trade-offs to be made; size, weight, cost, efficiency, etc.

Here's an intro to transformers and types of transformers:
http://en.wikipedia.org/wiki/Transformer

Here's an intro to transformer design:
http://en.wikipedia.org/wiki/Transformer_design

Your transformer is trading off a relatively high current pulse at a low voltage to get a low current pulse at a high voltage. It would be pretty tedious to try to re-wind your transformer by hand, but it could be done if you're determined.

Here's a link to an article on designing an audio output transformer:
http://www.geofex.com/Article_Folders/xformer_des/xformer.htm

Have you considered one of the little GM ignition coils? I don't know how much space you have available in your box, but the coils they started using in the late 80's or early 90's are much smaller than the old cylindrical types.

There's also the type that went into the distributor caps of the late 70's and early 80's GM V8 engine cars. They look like this:

The HV tap is machined into the middle of the other side of the transformer. You'd have to figure out a way to attach a HV wire to it.

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