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Old Mar 11th 2010, 04:33 AM   #1
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Induction coil help

hi guys
recently i acquired a 25mm spark gap induction coil. the inout voltage is 6V. now how can i calculate the output voltage? or please tell me what is the output voltage!! i want to use this induction coil in place of a nst to build a tesla coil. is it possible?
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Old Mar 12th 2010, 07:56 PM   #2
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Originally Posted by mahdir24 View Post
hi guys
recently i acquired a 25mm spark gap induction coil. the inout voltage is 6V. now how can i calculate the output voltage? or please tell me what is the output voltage!! i want to use this induction coil in place of a nst to build a tesla coil. is it possible?

Well using a spark gap coil, you are going to have a spike in charge release due to some very high breakdown voltage that ionizes the air enough to allow a current to flow for some brief period of time until the pressure on the charge (the voltage) decreases to the point where the air between the conductors is no longer ionized.

So you need to find what the ionizing voltage is for some gap between the cross sectional areas of the two conductors that are 25 mm apart. That voltage is the voltage where the air ionizes and the spark gap coil discharges from one conductor into the other and a spike in current and voltage occurs.

You may be able to replace the part you described with this device. It depends on how it operates; if the device requires a low voltage input and builds a high voltage with time from the constant current flow until the breakdown voltage is reached it will discharge slowly in frequency; or has a switching mechanism that breaks the path of the current flow probably quite quickly. Although I find it unlikely that it discharges at a fast rate as this would require great shots of power from the source on a regular basis, and unless the battery is a big source, it will get very warm, very quickly. Although I must be honest and say I cannot remember exactly how a spark gap coil functions.

Many Smiles,
Craig

Last edited by clombard1973; Mar 12th 2010 at 08:40 PM.
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Old Mar 13th 2010, 12:10 PM   #3
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Hi, thnx for the reply.
Can you tell me wat the ionisatoin voltage is for air, [the spark should have been like 0.2mm diameter (estimation)], i googled it with almost no informative results... also, do you think i would be better off using a nst instead of an inducution coil?
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Old Mar 13th 2010, 03:04 PM   #4
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Hello again,
Air breaks down at about 30 kV/cm. A large Tesla coil of more modern design often operates at very high peak power levels, up to many megawatts (millions of watts). It should therefore be adjusted and operated carefully, not only for efficiency and economy, but also for safety. If, due to improper tuning, the maximum voltage point occurs below the terminal, along the secondary coil, a discharge (spark) may break out and damage or destroy the coil wire, supports, or nearby objects.



Tesla experimented with these, and many other, circuit configurations (see above). The Tesla coil primary winding, spark gap and tank capacitor are connected in series. In this circuit, the AC supply transformer charges the tank capacitor until its voltage is sufficient to break down the spark gap. The gap suddenly fires, allowing the charged tank capacitor to discharge into the primary winding. In the typical circuit (above), the spark gap's short circuiting action prevents high frequency oscillations from 'backing up' into the supply transformer.

Experienced Tesla coil builders almost exclusively use this above circuit, often augmenting it with low pass filters (resistor and capacitor (RC) networks) between the supply transformer and spark gap to help protect the supply transformer. This is especially important when using transformers with fragile high voltage windings, such as Neon-sign transformers (NSTs). NSTs are designed to have high leakage inductance to limit their short circuit current to a safe level.

Due to that fact alone, that NST's are designed to limit their short circuit current to a safe level, I think is reason enough to use a NST in place of your spark gap induction coil.

For a variety of technical reasons, toroids provide one of the most effective shapes for the top terminals of Tesla coils. The toroid really behaves as one "plate" of a capacitor with the other plate being Earth ground. This allows "streamers" to visibly discharge and appear to go in every direction, but for every charged particle that is released or taken on by the toroid, another is pulled from Earth or sent into the Earth. So the secondary portion of the circuit is considered to be an LC circuit and has a resonant frequency of 1/sqrt(LC), if I remember correctly, the primary portion of the circuit should be tuned to match the secondary resonant frequency such that for every discharge from the toroid, the inductor in line with it should be receiving a peak amount of energy from the primary coil such that a current flowing through the secondary inductor that doesn't "want" to change the rate of current flow instantly, will induce a large enough voltage to force charge through and out of the toroid into the air.

There is a back and forth happening between the secondary and the toroid, and you want to be sure that the toroid has all the charge within it when the next input of energy comes into the secondary. This allows the secondary to take on all that energy without "fighting" against an induced voltage from one it already has that is working against an induced one. Now a NST will give you the high voltage that you want, but you still need a spark gap that will ionize the air allowing for a rush of current flow that induces a large voltage in the primary and the secondary allowing the secondary to discharge through the toroid into the air. I believe a typical spacing is about 25 mm for a spark gap (it has been a while so don't hold me to that). I don't know how your spark gap coil operates. Were you going to use it in place of the secondary portion of the transformer from a NST with this device? If so, how is it powered? Is it simply an inductor that has its leads close together such that if a large voltage is induced within the inductor, if large enough, allows a discharge of current through the gap?

You said something about it being a 6 Volt coil. Is it perhaps a 4 port device with a primary that takes in 6 volts at some frequency with a secondary with many more windings such that a very large voltage is induced within the secondary whose leads are close enough together to cause a "spark" from one to the other while it discharges. If so, it sounds very much like a NST anyway. The advantage to the NST is the fact that it limits its short circuit current to safe levels. Can your induction coil do the same? If not, and you have access to a NST, I would use that instead. Of course there are ways to limit the short circuit current with your device that don't affect the high voltage.

When the gap is ionized, it essentially creates a short circuit across the secondary which is outputting a high voltage, so if shorted it will draw as much current as that large voltage divided by the small secondary winding resistance will give; it can't supply all that current on its own so it "steals" some from the capacitor that is discharging through the gap and a large current will flow, very a short period of time, but fairly frequently, through the secondary (depending on the polarity of the voltage across the secondary at that time as it may instead take current from the other side of the gap and use it to push up through the secondary). Hence the importance of limiting the short circuit current through the secondary.

If you can make your device output high voltages over a range of frequencies, since you don't know what will be the final frequency you want until you know the resonant frequency of the secondary portion of the circuit and then you want to input a signal at that frequency and adjust the element values in the primary portion of the circuit until that portion of the circuit also is resonant at that frequency; and it can handle large discharging currents, unless you limit it in some way, then your device should work just as well as a NST with just some spark gap set up within the primary circuit portion.

Although if it takes in a 6 volt alternating signal at some frequency and can output a very large voltage at that frequency, the available current from that secondary, which is the source of energy for the main portion of the circuit, will be small. Nonetheless, on every half cycle more and more charge gets stored on the plates of the capacitor until eventually it is so great that the pressure forces charge to jump the gap ionizing the air causing a large discharge of current that induces large voltages of varying frequencies in the secondary connected to the toroid, from the primary, causing it to charge more so; until the secondary and toroid get to the point where the toroid discharges into the "air".

A NST usually takes in about 120 Vrms, a much larger input than 6 Volts. So it will induce a larger voltage in the main portion of the circuit allowing the capacitor to charge up far more quickly and then discharge through the gap also far more frequently. This means that the toroid will also discharge "streamers" far more often. It may be that even at 6 volts, if the frequency is great enough, that you can make it transfer energy into the toroid at the same rate as the higher voltage input NST running at a lower frequency. The problem is finding a high frequency 6 volt signal that can remain at 6 volts and still output enough current so that the power at the secondary is about the same as that going into the primary.

It not that a 6 volt high frequency supply cannot output enough current, it's just that at high frequencies, inductors "appear" to have a large resistance associated with them, called their reactance. It doesn't dissipate any power, but it does reduce the input current. That's why a NST with a high input voltage also sees a large "resistance" at high frequencies, but is made to operate at some particular frequency (typically just the wall outlet frequency of 60 Hz) such that the input power is almost entirely transferred to the secondary winding.

If your device is made to output a large voltage from a small input voltage then it needs to run at a higher frequency than the NST to output a high enough voltage; but at the high frequency the current may be limited to a point where the input power is not sufficient to transfer the power needed by the secondary to build charge upon the capacitor at a rate that is fast enough to make the toroid discharge at a rate that makes it look "cool". In other words, it may only give off a "streamer" every second or two and not a continuously appearing discharging of "streamers" that are usually seen from a Tesla coil. What may even be worse, is if the device is designed to operate at a high frequency so that the secondary voltage can be boosted to large values, and the manufacturerís know it will only source a small secondary current; that's all well and good. Now, since the device is made such that the secondary coil is only good for small current outputs, when the cap reaches the point where it ionizes the air and a large current flows then without some kind of short circuit limiting device for the secondary for your device, it will almost certainly ruin the secondary windings.

On the other hand, since the frequency is so much greater for a 6 volt input to create a large voltage at the secondary, even though the current is limited further by this higher frequency it does push less charge, but more often, into the capacitor, so the device may well operate just as well as an NST (assuming it can handle the short circuit current flow, or has a limiting device connected to it). You just have to run it at its rated frequency.

I don't know if any of this has helped, but here it is anyway. I guess if I knew a little more about the device you are considering using in place of a NST, then I could better answer your question.

Many Smiles,
Craig
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