How to safely switch a high voltage transformer?

Question

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?

Answer 1

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.

Answer 2

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.