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I am trying to make a small model internal combustion engine controlled electronically with an Arduino.

The idea is that fuel is injected and ignited with a small spark from a small high voltage transformer, such as the ones found in a plasma ball toy, or a cheap taser. The voltages on these are between 20 kV and 400 kV.

I am curious about how to effectively switch this on and off electronically. In particular, I noticed that the inductive coil feedback (am I saying that correctly?) results in a voltage spike on the low voltage side on connect/disconnect, so I actually manage to shock myself when, for example, I connect the input side by hand to a 5 V power supply.

Presumably this will be a problem if I try to switch it using a normal transistor-based solid-state system? Should I be using something like a reed relay? How should I prevent the "input-side" voltage spikes from damaging the other electronics connected to the power supply?

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  • \$\begingroup\$ What exactly are you planning to do with the transformer? Do you plan to drive it like a flyback transformer, or are you saying "transformer" when you actually mean some kind of dc-dc converter module (which I've seen people do on here a lot)? \$\endgroup\$
    – Hearth
    Commented Sep 2, 2022 at 14:10
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    \$\begingroup\$ @CaptainCodeman As we like to say here, no datasheet, no sale. It looks like that's a lot more than a transformer, since it calls it a "dc boost step up module". It's probably a complete flyback converter. I would recommend getting your parts from more reputable suppliers in the future--digikey, mouser, newark, arrow, that sort of thing. Definitely not amazon. \$\endgroup\$
    – Hearth
    Commented Sep 2, 2022 at 14:24
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    \$\begingroup\$ May be regular car ignition coil driver will help you. There are plenty of them around. \$\endgroup\$
    – Vladimir
    Commented Sep 2, 2022 at 14:36
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    \$\begingroup\$ OK guys, thanks for being nice and not bashing me for being a noob, I will try to find an ignition coil driver from a reputable source. Can you drive an ignition coil driver with an arduino? \$\endgroup\$ Commented Sep 2, 2022 at 14:51
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    \$\begingroup\$ Why reinvent the wheel when ICE ignition coils have been redesigned for many other reasons. Isolation is basic nanopdf.com/download/… but you do not need 1 million volts. or even 400kV (which I doubt is true under load) It's more like 35 kV when cold under high pressure \$\endgroup\$ Commented Sep 2, 2022 at 16:40

2 Answers 2

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Presumably this will be a problem if I try to switch it using a normal transistor-based solid-state system? I'm sure you could and that they used to, but relays can weld.

Thyristors, among the most rugged of semiconductor switches, are typically used. But they have their quirks...once conducting they latch on; something else has to bring the current to zero for them to be able to allow them to unlatch. Good thing sparks are typically AC and will eventually cross zero on their own.

How should I prevent the "input-side" voltage spikes from damaging the other electronics connected to the power supply?

Drive the thyristor with an optocouplers to protect everything else.

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  • \$\begingroup\$ Thank you, but the optocoupler doesn't prevent the high voltage on the input side; how do you deal with that ? \$\endgroup\$ Commented Sep 2, 2022 at 14:16
  • \$\begingroup\$ What are you referring to when you say "input side"? If you mean the transformer, remember that it is a transformer which already isolates...combined with the fact that transformers that step up in one direction step down in the other... \$\endgroup\$
    – DKNguyen
    Commented Sep 2, 2022 at 14:17
  • \$\begingroup\$ @DKNguyen See comments on the question; the thing they're using is a lot more than a transformer. \$\endgroup\$
    – Hearth
    Commented Sep 2, 2022 at 14:30
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    \$\begingroup\$ @Hearth Hmmm, yeah that is not a transformer. It listed as a "DC transformer" which is not a thing. \$\endgroup\$
    – DKNguyen
    Commented Sep 2, 2022 at 14:34
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This is probably some sort of flyback converter that may be driven by an oscillator and transistor, or even an electromechanical buzzer. Thus, it probably has considerable delay between applying input voltage and getting a high voltage output. If you got a shock when opening the input circuit, that can probably be limited by using a capacitor and a TVS device.

You could certainly try this, but you'll need to determine the time between applying input voltage and getting output voltage. This would require a scope, and you'd need to protect the input from the high voltage, perhaps by using a non-contact inductive sensor on the plug wires (like a timing light).

As you probably know, ignition systems for model engines can be purchased from suppliers who specialize in such products, but they tend to be costly.

Some model engine makers use old school points and condenser ignition with a standard automotive coil. Coils for lawn mowers, chain saws, and such might work, but they are usually magnetos that use a moving magnet to generate current which is interrupted by points, or a solid state circuit. A coil for a motorcycle might be a better choice.

You can replicate the action of such an ignition system by replacing the points with a high voltage transistor and switching the base or gate with the output of a proximity sensor that operates on a crankshaft position sensor. You will also need a capacitor to create a damped high voltage AC signal on the coil primary, and also a TVS to limit voltage. You can incorporate a smart advance/retard mechanism by using software to determine RPM and delaying spark accordingly.

Alternately, you could design a capacitive discharge ignition (CDI) system, where a capacitor is charged to a few hundred volts and then discharged into the coil primary with an SCR or transistor.

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  • \$\begingroup\$ Can you please tell me what I need to search for to get the model engine ignition system? Is it just "ignition system" ? \$\endgroup\$ Commented Sep 9, 2022 at 11:17
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    \$\begingroup\$ Sure. Just search "model engine ignition system" ;) \$\endgroup\$
    – PStechPaul
    Commented Sep 9, 2022 at 22:00

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