What is the purpose of spark gaps in Tesla coils, spark gap transmitters, etc.?

From what I can tell they keep the high voltage from interfering with mains power, but that doesn't make sense to me?

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Re-design7 years ago
It's a way of letting the voltage build up to a point then at that point the arc will bridge that gap and and the magnetic field of the coil collapses then starts to build up again. It's this up and down of the field that generates the high voltages. This can be done with transistors etc. but the spark gap works great and is dirt simple. In a transmitter one side is connected to an antenna and one side is connected to a ground. This causes the static to be transmitted over most of the radio spectrum. Very illegal because it will interfere with all radio over a huge area and it makes the FCC really angry with you for doing it. Spark gaps are sometimes put on antennas so that is the static from a storm or an actual bolt hits the antenna it will jump the gap and go directly to ground and maybe not damage everything in the house.
(I know I'm replying to an old post) Unless there happens to be a transformer on the other end. That'll regulate the signals so you won't get angry people. Although the "interfere with radio signals part" gives me an idea...
A transformer on the other end won't do anything. If one side is antenna and one side is ground it covers most of the radio spectrum.
*startled by the facts*
They do that to you sometimes. But there is always next time.
Can you do it with transistors easily ? I always thought the switching frequency and power levels made it very difficult - an air-break switch like that is incredibly fast rise time.
That's the idea of an OLTC (off-line Tesla Coil), it charges a capacitor using the line voltage (hence no power transformer) and then uses an IGBT to discharge it through the primary coil. Other solid state coils do not use an LC circuit to create resonance, but rather a half or full bridge oscillator that can be driven by feedback or by a constant frequency source such as a 555 timer. Of course, high dV/dT can easily kill transistors if you drive them too hard.
Yes, but what you're looking for in an SSTC isn't high dV/dT, but dI/dT.

As for the "Other solid-state coils don't use an LC circuit to create resonance" comment, I would have to mention DRSSTCs, which use a tank capacitor to create a resonant primary circuit.
Purpose: make sure the capacitor on the power supply end charges up to a specified charge. If it's not there, the capacitor would be just draining it's power to whatever. And you can click the "best answer" button anytime now...
There was a better answer about 6 months ago.
I'll have to admit, yes, that one is better...
chriskarr7 years ago
The purpose of the spark gap is to allow the capacitor to charge until it reaches the voltage at which the spark gap can fire; the capacitor will charge its current, first, before charging its voltage. As the voltage raises, the capacitor's total energy increases. The spark gap allows said capacitor to discharge its entire energy in fractions of a millisecond, and the primary circuit 'rings down' until it has transferred as much energy to the secondary as possible.

The spark gap is, essentially, a voltage-triggered switch. There are other voltage-triggered switches in the world (Sidacs being one, used in the SISG, or Sidac/Igbt Spark Gap), though the spark gap is one of few which can tolerate these voltages without being destroyed and with a fairly good amount of power efficiency.
Wyle_E7 years ago
The gap and the capacitor form a relaxation oscillator, but the most important function of the spark gap is to add negative resistance to the radio-frequency resonant circuit formed by the capacitor and the primary coil.  That's what sustains the RF oscillation during each pulse from the relaxation oscillator.

"Negative resistance" sounds weird, as if you could get current without a source of EMF, but what it means is that as the current through the spark increases, the voltage across the spark _drops._
seandogue7 years ago
it's kinda like a pot or a scale. the distance a spark will jump in air is proportional to the voltage. If you know the gap distance, then you can calculate the output voltage If you charge a capacitor and set a given gap, the capacitor will charge up, exceed the voltage for the gap, and a spark will fire across the gap. the capacitor will then charge again, and fire again...and again and again... You can control the resultant high-voltage frequency by increasing or decreasing the spark gap. the output voltage and frequency in the secondary coil reflects that frequency and voltage.
By varying the distance between the spark gap electrodes, you can control the "bps" or number of times that the capacitor discharges per second. With a 60Hz power source, this would optimally be 120 times per second, allowing the capacitor to charge up fully each time before it discharges through the inductor. You cannot, however, vary the LC constant by experimenting with the spark gap - in other words, the frequency that the LC circuit oscillates at will be the same regardless of the bps.
I'm amazed at how many times I'll spend 10 or 15 min's. Thinking about and writing and answer then post it and have someone else post at exactly the same time! Could be something like "great minds".
"I'm amazed at how many times I'll spend 10 or 15 min's..."

Lol, that's just about how long it took me to word it too.
revelae (author) 7 years ago
thanks!