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Registered Member #4361
Joined: Fri Jan 20 2012, 12:46AM
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Posts: 11
Sulaiman: Good idea; i can tailor the amount of HV by uf of the capacitor. I can fire it with a HV transistor (FET or bipolar). This will be used in a semi-commercial application, so this is easier than creating a high-current 12 VDC supply for the coil.
Registered Member #3883
Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87
Do you need absolute control over spark timing or can you accept a small (a few ms) delay? If so, simply charge a capacitor with a low-power supply, then discharge that into the coil and interrupt.
Registered Member #4361
Joined: Fri Jan 20 2012, 12:46AM
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Posts: 11
Fulmen: Delay is not an issue; a few seconds delay would be fine. I simply need the HV pulse to initiate a reaction. The low-power, moderate-voltage charge to a capacitor is a good and inexpensive idea, suitable for production.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
For commercial use I'd definitely go with a thyristor as the switch e.g. BT151X-500R, around $0.50 in quantity as for a given voltage, current etc. they are cheaper and more robust, maybe triggered by an opto-triac. You may even be able to use just an opto-triac as even smd ones have >= 1 Amp rating and can be very economical.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Going from 12 volt directly to 15 kV is pushing the turns ratio a bit ... generally the higher powered trigger transformers (which should be the easiest to source and a lot more compact than ignition coils) seem to take a couple of hundred volts on the primary.
Information Unlimited has some transformers which should fulfil your needs ... they are not very well respected here, but at least they print prices to give you some idea.
Registered Member #4361
Joined: Fri Jan 20 2012, 12:46AM
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Posts: 11
Thanks Brain, but automotive ignition coils routinely got to 30 kV and more. Space isn't a problem, and a "couple hundred volts" may be less convenient. I'm getting 2 cm arcs; physics tables say this is >25 kV. This was from a motorcycle coil.
Registered Member #543
Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992
There may be some confusion here over the operation of ignition coils in the age of mechanical commutation.
The primary back EMF developed by the collapse of the magnetic field - typically about 300V - is stored in the capacitor, and then dumped back into the primary.
Such coils frequently had a 1:100 step-up ratio, producing ~30 kV across the secondary.
There is a decent enough explanation here:
The ignition coil is not a transformer in the way of mains transformers, for example, but is a special case of a choke and depends for its action on the dV/dT calculus. It will not work if you put 300V 50Hz across the primary, as a true transformer would.
Registered Member #2901
Joined: Thu Jun 03 2010, 01:25PM
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Traditional ignition coils get those voltages using flyback ... not pure transformer action (there is still transformer action, but that just determines the stresses the switch sees).
You seemed to want to go with Sulaiman's suggestion, which is completely different (you can't use thyristors for the flyback). What Sulaiman is suggesting is more like CDI.
Registered Member #4361
Joined: Fri Jan 20 2012, 12:46AM
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Posts: 11
Mary, I guess it's actually dB/dT - rate of mag field change. But a good reminder that it's not turns ratio; thanks. In fact, one can get a healthy spark by simply interrupting current flow though an inductor, with no secondary.
Brain - I'm a little rusty on thyristors, ever since big FETs became available. So my first choice is the FET.
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Need single pulse from ignition coil
