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Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
You're right, the bulb doesn't need ignited every half cycle.
However bear in mind that some short arc lamps are designed for DC. You can tell because they have one electrode (the positive one) much bigger than the other. If you power a DC lamp with AC, the small electrode will overheat, as it is positive half the time.
Likewise if you power an AC lamp with DC, whichever electrode is positive will overheat, as it is positive all the time, but was only designed to handle it half the time.
Given that short arc lamps tend to explode if they're at all unhappy, these little things can be important.
Ok guys, I've put this one together on 'paper' to get a better look. I'm am completely unsure if my HV induction 1:1 transformer has the right polarity. No one uses the dot convention on their schematics anymore, so I set it how I think it should be.
This should be nothing more than a rectified buzzbox with cap on the output, and a simple ignition coil pulse firing circuit which is "superimposed" on the bulb.
Oh, and yes. Yes, I am aware that my bulb does not have a curly little filament. ;)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I would add a spark gap and tank capacitor to the ignition coil/ferrite transformer section. Essentially converting it into a Tesla coil primary. That's more or less the mysterious circuit you saw on Youtube.
Connecting the ignition coil straight to the ferrite transformer won't work, there will be a massive impedance mismatch. The reason is that the ferrite transformer has to have only a few turns of thick wire, in order to carry the lamp current, so its magnetizing inductance will be so small that it just shorts out the ignition coil secondary.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
The circuit you posted, except the flaw Steve pointed out, will fire the ignition coil only once when you press the button - this won't be enough to charge the tank cap. Use a TRIAC "light dimmer" and put a few uF cap in series with it, this way you'll get AC pulses on the primary of the iggy coil and continuous output, rather than just a single pulse.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
I would add a spark gap and tank capacitor to the ignition coil/ferrite transformer section. Essentially converting it into a Tesla coil primary. That's more or less the mysterious circuit you saw on Youtube.
I used an un-suppressed automotive spark plug (eg. Ford Model 'T') as a spark gap in my circuit. I 'rolled my own' HV capacitors, and I also added a resistor to the output of the flyback I used, to 'encourage' the current to stay in the tank circuit, I used about 12" of suppressed automotive ignition lead (the stuff with a core that looks like carbon fibre) as an HV resistor (resistance ~330kOhms, if I remember correctly).
I'm not saying that's the way to go, just saying that's how I did it.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Dr. DC: There's no high voltage rectifier, so if the ignition coil can't charge the tank cap in a single pulse, it can't charge it at all. In that case you need a smaller tank cap, not a continuous pulse train.
If you wanted to charge a big tank cap, the ignition coil could be substituted for a DC output flyback. Ash's idea of a high voltage resistor is a good one, to prevent the RF going back into it and blowing up the diodes.
See Newton Brawn's many posts on TIG welder arc starters, as it's essentially the same thing.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yeah for some reason I automatically assumed a diode on the output of the ignition coil... I think the tank cap will need to store a relatively high energy, which an iggy coil won't be able to charge in one pulse anyway. So rectify it or use a single transistor flyback driver with a rectified transformer.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
In view of the above comments by Dr DC and SC, now that the circuit has been modified, will it still be ok to use a ferrite cored transformer to superimpose the HV, or would an air cored transformer be required?
EDIT: or would this depend on the resonant frequency of the tank circuit?
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Anything that increases the magnetizing inductance and coupling coefficient of the transformer is good. So ferrite is good. However given the large high frequency output of a Tesla coil primary, an air cored transformer may be good enough.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Steve Conner wrote ...
Anything that increases the magnetizing inductance and coupling coefficient of the transformer is good. So ferrite is good. However given the large high frequency output of a Tesla coil primary, an air cored transformer may be good enough.
...And the unsmoothed 100Hz rectified DC @~100A, 25V wouldn't saturate the core, or anything?
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Lets light an 1800W xenon short arc bulb
However given the large high frequency output of a Tesla coil primary, an air cored transformer may be good enough.