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4hv.org :: Forums :: Tesla Coils
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Old - Vintage Tesla coil circuit question

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AlgoryThm
Thu Aug 23 2012, 12:55PM Print
AlgoryThm Registered Member #4134 Joined: Tue Oct 11 2011, 12:34AM
Location:
Posts: 5
Hello! Well, I have some time that I've been working on tesla coils and I think I understand the basic principles of LC tank resonance etc. I recently found a Bob Beck's tesla coil circuit, quite unusual as it seems...And this is because he uses a Ford T Model Ignition Coil to give power to the primary, all well so far. But the strange thing is he uses two HV capacitors IN SERIES with EACH primary coil lead WITHOUT rectification! I was aware of the ignition coil circuit where a diode is connected in series with the spark gap and capacitors but it's the first time I see something like this. Could somebody please explain to me how it works and if it works? Because I have no clue how resonance is established here, because the current is pulsed DC and not AC...

This is the schematic:
1345726412 4134 FT0 Beck Circuit


Thank you in advance!
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Sigurthr
Thu Aug 23 2012, 01:23PM
Sigurthr Registered Member #4463 Joined: Wed Apr 18 2012, 08:08AM
Location: MI's Upper Peninsula
Posts: 597
In that schematic there is no indication that the output from the ignition coil is DC. Pulsed DC is being applied to the ignition coil's primary, but this should create an AC signal on the secondary; this is called "flyback" mode is and is how flyback transformers work.

About the two series capacitors; electrically two series capacitors is the the same as one capacitor with half the capacitance and twice the voltage rating. It doesn't matter if the cap is on the "top" or "bottom" side of the primary circuit, and it doesn't matter how many times the capacitor is physically divided up.

If you're wondering why the cap is in series and not in parallel, this is probably because the SGTC arrangement of a series cap + parallel spark gap causes the main gap to also function as a parallel gap. In this set up the spark gap essentially severs the high frequency side of the primary circuit every time it fires, keeping the HF out of the power supply circuit.

The circuit appears to function normally, but do not assume it is in resonance. A lot of these low power "tesla coils" are not actually resonant at all. One give-away is the size of the finished unit, small coils with disproportionate winding ratios and dimensions tend to indicate nonresonant operation. This is one reason they perform poorly and why many people who think of tesla coils think of tiny sparks coming out of a SMALL coil. (Another reason is the vast array of medical quack devices sold as tesla coils which did as I described.)
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Steve Conner
Thu Aug 23 2012, 01:28PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
It works fine if the ignition coil is powerful enough (or the tank cap small enough) that it can charge the capacitor in a single pulse. A rectifier would let the coil charge a bigger cap over multiple pulses.
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AlgoryThm
Thu Aug 23 2012, 02:33PM
AlgoryThm Registered Member #4134 Joined: Tue Oct 11 2011, 12:34AM
Location:
Posts: 5
Well, a huge thank you to both of you my friends for the rapid replies, you helped me a lot! Now I can say I understand what all this is about. I didn't know though that an ignition coil's secondary gives out real AC, I somehow always thought it was pulsed DC...I suppose I have still a lot of things to discover! Anyway, thanks once again! smile
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Steve Conner
Thu Aug 23 2012, 03:14PM
Steve Conner Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
A transformer winding can give some weird voltage waveforms, but the average value of them must always work out to zero. So yes, in some sense the output of any transformer must be "real AC".
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