Low-Voltage Tesla Coil Uses a Relay Instead of a Spark Gap

[Teodor] writes in with a unique Tesla coil he designed and built. Unlike most Tesla coils, [Teodor]’s design is able to run with a fairly low input voltage because it doesn’t use a static spark gap like most Tesla coils. Instead, his coil uses a relay in place of a spark gap.

[Teodor] built his coil using leftover components from his old school, making good use of some parts that might have otherwise been thrown away. The most critical component of his circuit, the relay, is just a standard normally-closed relay that is rated at 20A. [Teodor] wired the relay so that it energizes its own coil whenever it is shut. This causes the relay to briefly open every time the coil is energized, creating a resonant circuit. The resonant circuit charges a tank capacitor and places it in series with the primary coil inductor every time the relay closes, forming the tank circuit of his design.

With [Teodor]’s design, the resonant frequency of the secondary is nearly identical to that of the primary. This creates a significant voltage boost, helping produce very high voltages from such a low input voltage. The only downside to this design that [Teodor] recently discovered is that the relay contacts get red-hot after a few minutes of operation. Not optimal, but it still works! Check out [Teodor]’s writeup for more details and instructions on how to build your own.

24 thoughts on “Low-Voltage Tesla Coil Uses a Relay Instead of a Spark Gap

      1. No i get all my high power relays in tungsten, generally the spring breaks or the the metal breaks
        relays are not designed to operate that fast XP
        Lasted a good few days before having to replace the relay

    1. What frequency where you using? The coil of a modern day V8 can operate at frequencies in the order of 0-16+ KC all day long without ill effect. I’m not sure if an auto coil is going the draw more current in this application than it would proving an spark ignition source.

      1. Called the vibrator. While would fail in time, they did operate a long time before failing. In the event the Tesla coil would operate satisfactory with a current draw similar to the current draw in the high voltage circuits of the car radio the vibrator should last as long as they did in the radios? Land mobile radio receivers use vibrator power supplies, perhaps if NOS could be found they would work out better? Motor starters have tough contact surfaces, and the coil is probably a continuous duty cycle, could a heavy but, but noisy vibrator could be made out of one if the Would the coil over heat when used at frequencies over 60 cycles? No doubt a more robust return spring would have to be used

  1. Virtually all valve (tube) car radios used a buzzing relay, inductor and capacitor arrangement to step up the voltage. It was the main method of dc-dc voltage boosting for decades. For long term reliability the relay contacts need to be large enough and of a suitable material so they don’t get too hot and weld together or erode. Makes perfect sense to use this well developed method for a Tesla coil.

    1. The contacts of this relay seem to be made of a metal with a high enough melting point that the contacts don’t weld even after a 5 minute run, and they are red hot (actually,light yellow…) They are probably made out of tungsten…

      1. I’m using the AC mains, so a FET would be a suitable choice if I’d rectify the mains voltage, but that requires a diode bridge and a filter capacitor, so the voltage of this circuit would be 360VDC, so with the resonant charging through the choke that would result in about 1kV of charging voltage on the capacitors, so the DC rating of the MMC would need to be around several thousand volts, that requires more capacitors, so the design would be more expensive…
        And also, I forgot to mention that you would need an oscillator that drives the FET, so you will need a low voltage power source. That makes the design more complicated and more expensive, almost quadroupling the components list and the overall cost. My main purpose designing this was to design a simple, cheap, and easy to make Tesla coil weekend project, without the need of high voltage components and transformers… Also, the losses in a FET would be too large at such high peak currents where the losses of a FET are R*I^2. IGBT’s were already experimented with and gave excellent results, but that’s OLTC topology and is rather expensive…

      1. maybe a TRIAC, but we need to find a TRIAC that has high enough surge capability to withstand the high peak Tesla coil currents, and if we find something like that, I bet that It’ll come at prices well over 20USD, so a relay was a better choice because It doesn’t require external electronics to drive it and also I had it lying around…

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