The metal oxide varistor is necessary to protect the power MOSFET from excessively high voltage pulses caused by inadvertent poor adjustment of the timing resistor, and also from leakage inductance in the coil. Sometimes, this circuit works slightly better if a capacitor is also placed across the MOSFET.
Values around .05 to .1 uF are suggested. The capacitor should be rated for at least 400 volts DC, and preferably have an AC rating of at least 200 volts or a DC rating of at least 600 volts for good reliability in this circuit. Capacitors near or over .2 uF often result in lower output voltages.
It is recommended to only place the capacitor across the mosfet, not across the coil primary. I have known parasitic oscillations to occur from placing the capacitor across the coil primary.
The MOSFET should be mounted on a heat sink.
Operating and adjusting this circuit
Before actually connecting the ignition coil, it is recommended to verify that the rest of the circuit works. One way to do this is by placing a resistor (anything from 10 to 500 ohms) from the mosfet drain to B+, where the coil primary would go. There should be a square wave across this resistor, easily detectable with an oscilloscope or a piezoelectric tweeter.
When the circuit is verified to work, adjust the variable resistor for highest frequency, remove power, then put the ignition coil in place.
Before applying power, connect an insulated wire with stripped ends to either of the coil’s primary leads. (I am assuming the usual ignition coil, with two primary terminals and one high voltage terminal.) Also have a short bare wire coming out of the high voltage terminal at the top of the coil.
With power applied, some high voltage should be present. If you bring the end of the wire that is connected to the primary close to the high voltage wire, you should see sparks. You may have to get the wires close together.
UPDATE 10/24/2000 – By some reports from people trying this, at maximum frequency the high voltage output is too weak to make visible sparks. If this happens, try reducing the frequency in minor increments until you get noticeable high voltage (or until something is obviously wrong).
Once the circuit is known to be working and generating high voltage, slowly adjust the variable resistor for a lower frequency. This permits current to build up in the coil to a higher value before being switched off. The result is higher voltage, indicated by sparks being able to jump through a longer gap. It is OK to keep reducing the frequency until the voltage stops increasing. Do not touch any part of the 555 circuit including any leads of the timing resistors or timing capacitor; doing so can inject interference into the circuit.
It is not recommended to reduce the switching frequency below the point at which the voltage stops increasing. Doing so will increase power consumption, increase MOSFET heating, and lead to excessive heating of the metal oxide varistor.
If the metal oxide varistor heats up significantly, increase the switching frequency (lower timing resistance) until the output voltage decreases slightly. If the MOV still gets hot, try placing a capacitor across the mosfet as suggested above. If this does not work, be sure that the MOV is a V130 (or 150) LA20 or equivalent, about 20 mm. (.8 inch) in diameter, or of a larger size. A larger capacitor will eliminate MOV heating, but do not use one larger than necessary, since this will reduce the output voltage.
The peak output voltage will be approximately 15,000 to 20,000 volts, but this will vary with the ignition coil type and the voltage at which the MOV starts conducting.
If you need more voltage, it is permissible to use a higher voltage MOV, two MOV’s in series, or to add zener diodes in series with the MOV, or to use a zener diode bank instead of an MOV. However, whatever you use must conduct heavily (several amps) at a voltage lower than 400 volts in order to protect the mosfet. If you do this, you will have to readjust the switching frequency in order to get the higher voltage.
It is not recommended to short the high voltage terminal of the ignition coil to either primary terminal or to ground unless a 2000 volt diode is in series with the high voltage terminal. Otherwise, you can draw “forward” pulses, which would lead to more heating of the coil and the MOSFET. Two 1,000 volt diodes in series will work. If you need to determine the polarity of the high voltage pulses, use a neon lamp; only the negative electrode will glow. Leave a spark gap also when you do this, otherwise you will put “forward” pulses through the neon lamp, possibly overloading it. If you want to change the polarity of the high voltage output, reverse the coil’s two primary terminals.
If you are going to short the high voltage output, it is also recommended to add some resistance. Either around a 10K, 10 watt resistor in series with the high voltage output, or a few ohms (10 or 20 watts) in series with the primary, or around an ohm (10 watts) in series with the source terminal of the MOSFET (makes the MOSFET turn on less when current is high). A primary or source resistor simply limits current flowing through the coil, one in series with the high voltage output speeds up removal of stored energy from the coil.
The lower the resistance of any load connected to the high voltage terminal, the more slowly the magnetic field in the coil will decrease. If magnetic field remains the next time the MOSFET is switched on, the primary current will not gradually build up from zero, but start at a value corresponding to the remaining magnetic field. In such an event, the current may build to excessive amounts and overheat the coil and/or the MOSFET.
No resistor is necessary if the output is loaded down to a couple of kilovolts peak voltage. If the MOSFET is an IRF740, it should be OK to short the high voltage output for a few cycles, such as in charging a capacitor from zero to above a kilovolt or two. Once the capacitor gets past this voltage, the coil will work more normally.
If you want to charge a capacitor with the high voltage output, you will need a diode (or diode bank) that can withstand the peak-to-peak voltage, which is the sum of the main high voltage pulse voltage and the forward pulse voltage. In other words, slightly more than the voltage of the main high voltage pulses.
This circuit works at supply voltages anywhere from about 8 volts to 15 volts. However, you will need to readjust the switching frequency (by adjusting the timing resistor) if you change the supply voltage.
If you want more output power/current, one way is to use a different 555 (or other) oscillator circuit that will turn the MOSFET off for a shorter time than it turns the MOSFET on. In this case, it becomes more important to not short or excessively load the high voltage output, since the magnetic field has less time to collapse.
Another option to get more current/power is a higher supply voltage for the coil. The coil should work OK at voltages up to 24, maybe 30 volts. Note that the 555 does not want voltages that high, so you will need two different supply voltages. You will also need to make the 555 switching frequency faster since the current through the coil primary will build up faster at higher voltages. In addition, the core losses in the coil will increase, which will make the coil hotter. You may not be able to operate this circuit continuously at voltages higher than about 15 to 20 volts.
1) This circuit is intended to generate high voltages, which can be dangerous. The voltages/currents put out by this circuit may be able to electrocute someone. Sparks may be able to start fires. Use all due cautions. It is recommended to remove power before making adjustments.
2) Wattages for any power resistors mentioned above are believed to be usually adequate. It is up to you to determine if any power resistors will overheat.
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