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Greetings and a small starter project (ignition coil / Jacobs Ladder)

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magnet18

Sat May 14 2011, 01:39PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
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Posts: 624

Steve McConner wrote ...

The advice to use low voltage, high current FETs is bad.

They worked for flybacks, I figured they'd work for ignition coils, guess not.
*shrug*

Fulmen

Sat May 14 2011, 03:31PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Well, just because something shouldn't work doesn't mean it won't work

As for simply switching the power: That won't give me the on-shot function I'm after. I want it to run a single spark up the ladder then shut down as it blows out at the top (that's a cool door bell). I know there is an optimum size for such a capacitor, but as it limits the primary voltage I didn't think it would cause any damage even if the secondary doesn't produce any sparks. Wouldn't the energy simply be dissipated in the primary as heat?

Of course, if you try to increase spark gap too much without any capacitor something will fry, as the spark gap becomes your only limiter. I believe that's what killed my FET (didn't have the C3 at the time). But with a suitable cap I believe primary voltage can be limited regardless of the secondary output. Or am I missing something here (quite likely)?

magnet18

Sat May 14 2011, 04:57PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
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Posts: 624

Maybe use a rotating gap sort of thing so it initiates and then the gap moves as it travels up the wires, and then a timer to shut off the circuit after a few seconds (time how long it takes for the spark to climb and maybe add a second) then rotate the gap back into position?

Fulmen

Mon May 16 2011, 06:32PM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Well, I'm not convinced my idea of adjusting the capacitor won't work (please try to educate me if I'm wrong), so I guess I'll continue that line of thought for now. An all-electric solution seems simpler if I can get it to work.
But a hidden, adjustable sparkgap sounds like a good backup plan. I will off course need a shut-off timer to prevent the circuit from draining the battery, shouldn't be a major obstacle.

Steve: Thanx for the praise on the multivibrator, I figured I'd start with something that I could actually understand and yet learn from. A 555 somehow strikes me as cheating (besides, those are just black boxes doing stuff beyond my current comprehension), but that might just be my dad speaking. He's an old school engineer who started out with tubes, guess I've picked up a few quirks from him

As for FET's, do you have any pointers? I found some Fairchild FQP12N60C (600V/12A), odes these sound like a good choice for general HV-abuse?

Adam Munich

Tue May 17 2011, 12:52AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

If you want a very impressive one shot ladder, just take an MOT and hook that to the electrodes. Then rig up a little solenoid "ignitor" at the bottom of the ladder. Simply pulse the solenoid and you'll have a very large, loud and very bright arc climbing the ladder. I can't think of a more robust or impressive way to accomplish the task.

Fulmen

Tue May 17 2011, 08:15AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

Nah, that's just throwing power after a problem. Besides, such a devise will be a lot more lethal than I'd like.

magnet18

Tue May 17 2011, 11:11AM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
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Posts: 624

You have an arc of plasma traveling up 2 wires, and you think it's going to be safe?
If you want you can always use an isolation transformer.

Fulmen

Tue May 17 2011, 11:44AM

Registered Member #3883 Joined: Fri May 13 2011, 06:30PM
Location: Norway
Posts: 87

You don't think there is a significant difference between 1KW and 5W when it comes to risks?
Anyway, I'm going to make a coil driver for my first project. Period. I have a MOT, but I'm keeping that for a later project (1KW tesla sounds fun).

Adam Munich

Tue May 17 2011, 04:22PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

MOTs don't supply enough voltage to make a reliable spark gap tesla coil...

Also if you have 30kV, 5 watts means there is only .1mA --way too little to make a nice jacob's ladder. I'd say 20W would be the lower limit, and also you won't get that "beeerrrawww" sound with an iggy coil since there is no 60Hz hum: instead you'll likely get a 1-2khz squeal.

magnet18

Tue May 17 2011, 07:45PM

Registered Member #3766 Joined: Sun Mar 20 2011, 05:39AM
Location:

Posts: 624

In case anyone's wondering, 4khz is about what nails on a chalkboard sounds like (IIRC... capacitor values gone wrong on plasma speaker...)
Would be great for halloween.

For a large tesla coil you're probably going to want something more like a neon sign transformer, or if you go really big, an x-ray transformer.

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