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Safety Gap Setting

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radioman

Wed Nov 10 2010, 03:05PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Hello all,

I have just about perfected my first coil and it is performing well, but I have a question about setting the safety gap. Everyone says to set the gap so that it just stops firing with the transformerâ€™s own voltage (no caps connected). I have carefully done this numerous times but when I power up my coil, the gap fires continually and I have to open it up to about 1/8 inch (each side) to get intermittent firing. My NST is a Franceformer 7500 volts and my caps are 14 Cornell 942Cs for a total of 0.01 mfd. Whats going on??

Thanks,

Radioman

HV Enthusiast

Wed Nov 10 2010, 03:16PM

Registered Member #15 Joined: Thu Feb 02 2006, 01:11PM
Location:
Posts: 3068

I usually set it as you have described, but then open it up just slightly more.

monokel

Wed Nov 10 2010, 03:39PM

Registered Member #2981 Joined: Thu Jul 08 2010, 01:47PM
Location: Germany
Posts: 35

Under cirtain circumstances the voltage which occures across the connectors of the NST if a load is connected to the NST can be higher that the off-load voltage. You can imagine that there is a voltage across the leakage inductance of the NST which adds to the off-load voltage (of course the phase difference must be considered when adding these voltages). E.g. when using a RSG, this phenomenon can occure.

In contrast to this, if the load connected to the NST would be a simple resistor, the voltage with load is always lower then the off-load voltage.

Herr Zapp

Wed Nov 10 2010, 04:16PM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Radioman -

Assuming that the input voltage to your NST remains at a constant value, what you are seeing is probably due to resonance of the NST secondary with the capacitance of your MMC plus stray capacitance of the wiring in the primary circuit. Keep all the wiring from your NST to the tank circuit as short and direct as possible.

This effect is sometimes seen in neon sign applications where the output of the NST is run through metallic conduit to the actual neon tube itself. Capacitance from the HV wiring to the conduit can cause resonance that drives the NST voltage significantly higher than the "rated" output voltage.

Herr Zapp

J. Aaron Holmes

Wed Nov 10 2010, 04:23PM

Registered Member #477 Joined: Tue Jun 20 2006, 11:51PM
Location: Seattle, WA
Posts: 546

First off, if your coil's primary spark gap is a static (not rotary) gap and is connected directly across the HV suppy, then I'm a firm believer that a safety gap is just a redundant annoyance and should be omitted. Additionally, if you're using a "Terry Filter", I would suggest getting rid of the MOV section. The resistors and the capacitors are where a Terry Filter adds value in such a Tesla coil.

If you're using a rotary gap, then having a safety gap makes a lot of sense.

IMO.

Best,
Aaron, N7OE

Herr Zapp

Thu Nov 11 2010, 06:56AM

Registered Member #480 Joined: Thu Jul 06 2006, 07:08PM
Location: North America
Posts: 644

Radioman -

Are you are using a 7.5kV, 30ma NST? If so, your .01uF tank capacitor value will be "resonant" with your NST's secondary inductance and potentially can "ring up" to extremely high voltages. See Richie Burnett's excellent analysis of AC resonant charging at:

This resonance can result in rapid destruction of your NST or your MMC through insulation breakdown. For a static-gap system, a tank cap velue of around 1.4 - 1.6X the "resonant" value is recommended. This will almost entirely eliminate resonant voltage rise.

You could try reducing your MMC to 9 capacitors. This will reduce your MMC voltage safety margin, but will also reduce the potential for uncontrolled resonant rise. A "safer" (but considerably more expensive) solution would be to increase the number of capacitors in your original string to 18 or 19, and then add a second identical parallel string. This would yield a value of ~ .0158uF.

Another option would be to build another completely new MMC, using smaller value C-D 942 caps, like .1uF instead of .15uF. Two strings of 13 caps each would yield an MMC value of around .0154uF with a DC voltage "rating"of 26kV. Save your .15uF caps for your next coil.

Herr Zapp

radioman

Thu Nov 11 2010, 04:11PM

Registered Member #3026 Joined: Fri Jul 23 2010, 02:46PM
Location: Los Angeles, California, U.S.A.
Posts: 54

Herr Zapp,

Thank you for your excellent, right-on-target advise. Let me see if I understand this in laymanâ€™s terms (I am not an engineer). The transformer secondary winding and my MMC form an LC â€œtankâ€ circuit (I never thought of this!). Mine happens to be in resonance and this is what is making my safety gap fire continually. You suggest I go to 9 capacitors for a total value of .0166 mfd. to get the circuit out of resonance. Am I correct in assuming that the safety gap will then fire only occasionally? (By the way, I have installed the Terry filter without the M.O.V.s.).

Thanks again,

Radioman

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