A DIY Homemade
Ignition Coil Driver - A High Voltage Power Supply
One of the simplest ways to make a battery powered High Voltage power supply is to use a common car ignition coil. Ignition coils are a type of induction transformer based on the Tesla Coil invented by Nikola Tesla in 1891. The voltage rise is not given by the turns ratio like in a standard transformer, but is proportional to the rate of change of current in the primary circuit. This means to get a high output voltage you must be able to stop the power flowing into the coil as quickly as possible. In old cars this was simply done mechanically. For use as a HV power supply this needs to happen rapidly over and over. To do this a spacial square wave power supply is uses which switches power on and off to the coil hundreds or thousands of times per second.
Standard ignition coils can be obtained from most car parts stores for around £25. It is not essential to use two 12V batteries like shown in the circuits shown below, but it will allow you to obtain bigger sparks. We have some compact induction coils available for sale for under £20. Click the link to check stock.
This driver circuit is based on the commonly used 2n3055 transistor due to it high power switching capability. While these are cheap and high temperature tolerant, they are susceptible to voltage spikes caused by the inductive nature of the load (ignition coil). Pretty much any power transistor, IGBT or MOSFET can be used in this circuit as long as it is rated for at last 5A and 100V. Ones with higher voltage ratings will be less likely to be damaged by spikes. Further protection methods are outlined lower down this page and in the comments. If you use a MOSFET or IGBT instead of a bipolar transistor like the 2n3055, you should also add a pulldown resistor of about 10k between the base/gate pin and GND.
|T1||BFY51 Small Transistor|
|T2||2n3055 Power Transistors or HV MOSFET or IGBT|
|R1||100 Ohm Resistor|
|RC1||0.1µF Capacitor + 10K Resistor|
RC1 is used to help suppress high voltage spikes that can destroy the power transistors.
T2 represents two power transistors connected in parallel and mounted on a heatsink.
This next circuit is designed for a higher powered output. Two ignition Coils are connected in parallel but with opposite polarity. This means that the output voltages of each coil are out of phase or opposite to each other (when one is positive, the other is negative). Using this configuration the output is taken from the two coils output terminals, whereas the circuit above uses the output terminal and ground.
These circuits will work great for driving ignition coils for high voltage but they can be susceptible to damage from inductive spikes. When an ignition coil is being driven unloaded (open circuit on the output) there will be significantly increased back emf and risk of damaging the driver circuit. We sell an ignition coil driver module which has built in protection against most spikes that would damage a driver. It also includes an early warning indicator which will show you how severe the back emf is from your load.
Protecting Your Ignition Coil Driver
If you build an ignition coil driver to make high voltage sparks and arcs, you will need some sort of EMI protection for your circuit. Without it, it is very likely you will destroy the transistors or driver ICs.
Snubbers are a tricky subject, but in general they are used to reduce electromagnetic interference (EMI) or voltage spikes. There are many ways to reduce EMI and it can often be useful to use various snubbers in different parts of the circuit. These diagrams represent a few possible ways you can snub EMI in an ignition coil driver. These are known as dissipative snubbers because the excess energy is disspated as heat or light.
The top digram uses a series connected capacitor and resistor. The values used will depend on your drive frequency. (See RC1 at top of this page). Generally speaking, a bigger capacitance and smaller resistance will snub more, but also absorb more drive power thefore reducing efficiency. A compromise must be found that best suits your setup.
The next diagram uses a device known as a MOV (Metal Oxide Varistor). These are semiconductor devices which will only begin conducting when the voltage between its terminals exceeds its rated value. It will stop conducting when the voltage goes low again. In the example shown above, the MOV will short out any spikes coming from the load, but it is also shorting the driver circuits output for the same brief instant. The MOV chosen must be able to dissipate the power ans have a voltage rating that will cause it to activate before the voltage gets too high for the drive circuit.
You can also place a small neon indicator bulb (Ne1)in series with a 1k resistor and place this between the low voltage wires to your ignition coil. This bulb will begin to glow when the back EMF reaches about 100V or more. If you see it glowing, you need a better snubber like RC1 (top diagram) or a MOV (varistor) rated to clamp the voltage below the maximum your components will tolerate.