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lamazoid

Wed Mar 21 2007, 08:56PM

Registered Member #573 Joined: Fri Mar 09 2007, 11:26AM
Location: Russia
Posts: 14

Hi everyone!

I want to write a program for calculating pulse transformers like ignition coil, those used in stun guns etc.. But unfortunately i can find no theory for them, exept common description like this one:

Can anyone help with this?

Steve Ward

Wed Mar 21 2007, 11:42PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

This seems like a tricky one.

I believe you can model them similarly to a tesla coil, except the primary is "untuned". The basic operation is storage of energy in the primary inductance of the transformer. Then by interrupting the primary current, energy is forced out of the secondary winding. By conservation of energy you know what the output energy is, and if you know the L and C paramters of the winding, you can figure out what the peak voltage and current will be. For better predictions you can factor in winding resistance and possibly losses in the magnetic core material, but i dont know too much specifically about that. You also have to be sure you arent saturating your core, otherwise the calculations go right out the window.

The biggest problem will be determining the capacitance of the secondary winding. This factor is as important as any, so without knowing it, you wont have much idea as to the output voltage/current (though you can still know the potential energy there). I dont know of way to calculate this capacitance, it seems to be something that you measure experimentally (by knowing the inductance - which is easy to measure - and applying a step or pulse to the coil, you can find the resonant frequency and work out the capacitance).

Sam

Thu Mar 22 2007, 02:26AM

Registered Member #227 Joined: Mon Feb 20 2006, 10:47PM
Location: Cambridge Ontario, Canada!!
Posts: 127

if You are just looking for testing, You could use a micrometer to adjust a spark gap, then You could estimate the output voltage, as for current, maybe a current clamp?

lamazoid

Thu Mar 22 2007, 12:15PM

Registered Member #573 Joined: Fri Mar 09 2007, 11:26AM
Location: Russia
Posts: 14

Steve, could you give some exact formulas? E.g. for capacitance-inductance-voltage relations?

ragnar

Thu Mar 22 2007, 01:51PM

Registered Member #63 Joined: Thu Feb 09 2006, 06:18AM
Location:
Posts: 1425

Lamazoid, please could you spare us the hassle and try a Google search or your local library? We'll be happy to help if you get stumped.

lamazoid

Thu Mar 22 2007, 05:57PM

Registered Member #573 Joined: Fri Mar 09 2007, 11:26AM
Location: Russia
Posts: 14

I did a huge search before posting here. And all i found relates to left or say, low voltage side. This is well described on coilgun sites like this:

For example, i have modelled a typical stun gun circuit (1kv; 0.33uF; 0.1 Ohm; 15uH) This results into pulse width of about 8us and peak current of 140 A.

So, for the secondary winding we have pulse width, inductance, resistance (dc) and propably capacitance too. And we need to find optimum ratio between them. That's what i am asking about. Because *sorry* i was unable to find any information!

Maybe you will experience more luck, or you can just tell?

Steve Conner

Thu Mar 22 2007, 06:12PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

It's all just standard transformer theory. Or Tesla coil theory: a Tesla coil is just a pulse transformer with an air core, and a pulse transformer is just a Tesla coil with a ferromagnetic core.

The equations you would use are the same differential equations that a circuit simulator like SPICE uses.

You may not be able to rearrange these equations to get an analytic solution for the parameter you want to optimize. SPICE doesn't even try: it solves them by numerical integration.

My own interest was in finding analytic solutions to optimize solid-state Tesla coils, but I never got anywhere. I found one interesting transfer function that wasn't generally known, but I failed to explain it to anyone else. Terry Fritz made much better progress with a program that numerically solves the Tesla coil circuit over and over again, walking the solution space to find the optimum. I think: I never quite understood what he did.

I personally gave up on my analytic stuff and used parametric runs in PSpice to optimize my Tesla coil designs.

Dr. Shark

Fri Mar 23 2007, 11:32AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I think the main problem with ignition coils and the like is that in general you will at best have a vague idea about the capacitance and inductance of the secondary side given just geometric parameters. It's a lot easier to get accurate estimates of your circuit parameters with a nicely wound Tesla coil.

If you can measure these parameters, the maths is relatively straightforward. Look at weakly coupled oscillators, a concept that frequently comes up in physics, to see how the energy sloshes back and forth between primary and secondary side.

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