Ignition Coil Drivers

[Music] [Music] in this video i'm going to show a few different ways to drive an ignition coil to produce high voltage arcs any engine that burns gasoline needs a spark to set off the mixture which is provided by spark plugs which are in turn energized by a high voltage transformer called an ignition coil the coil runs off low voltage dc input like a car battery and when contact is broken a high voltage pulse is created which results in a spark ignition coils come in many shapes and sizes but on the inside they're all essentially the same a primary coil is wrapped around ferromagnetic core which is usually ferrite or something similar the primary is covered in electrical insulation and the secondary winding is wrapped directly over it the whole assembly is encased in a metal can and usually flooded with oil to prevent arcing and to keep things cooler the high side of the primary connects to vcc which is usually a 12 volt battery the low side of both coils are tied together and the high side of the secondary coil has its own terminal that outputs the high voltage now you may be wondering what's the difference between this and any other transformer after all this isn't the first time i've used transformers to produce high voltage the fundamental difference between the ignition coil and other transformers is that it's a pulse transformer it isn't designed to operate with a continuous alternating current like a flyback transformer or a microwave transformer instead it uses a high inductance primary to create a relatively large magnetic field when current flows through it when the current is suddenly shut off the large magnetic field collapses and the sudden collapse creates a high voltage pulse this is perfect for low frequency applications like a gas engine where pulses are only occurring maybe 10 or 20 times per second the inductance also affects how quickly a coil will reach peak current as shown by this formula larger inductance causes current to ramp up much slower let's have a closer look at that in a simulation here the 15 micro henry inductor represents the flyback transformer primary and the 7.5 millihenry inductor represents the ignition coil primary both are supplied with 12 volts and limited to 3 amps through a 4 ohm resistor now watch the graph below as i start the simulation the yellow line represents current through the 15 micro henry inductor and the red line is current through the 7.5 mil henry inductor the smaller inductor reaches peak current in a matter of micro seconds whereas the larger inductor takes almost 10 milliseconds to peak this is because the larger inductor is storing more energy in the form of a magnetic field as described by this formula so for instance suppose both coils had four amps flowing through them the ignition coil in this example would store 250 times as much energy as the flyback transformer coil now like i showed before the time rate of change of current is equal to the voltage over the inductance but that formula can be rearranged to show that the voltage developed is equal to the inductance times the rate of change of current so if you had a current through a relatively large inductance which was shut off very quickly you'd develop a large voltage spike so let's see an example of that if i shorten my power supply leads together the output maxes out at 10 amps if i tap them together there's some tiny sparks from the supplies filter capacitor but very little stored energy otherwise because the leads have almost no inductance in them now suppose i put the same 10 amps through this coil my meter says it has a little over 10 mil henries so let's see the difference in the spark giving the coil 10 amps and suddenly breaking contact creates a big bright blue spark big enough to hurt my eyes a little bit there's less than 20 volts going into the coil but when i break contact the arc is thousands of volts let's look at another simulation to see what's happening here i have a large inductor connected to a switch and across that switch is some capacitance and a damping resistor which is a crude approximation for the air gap between the coil terminal and the power supply lead the top graph is the voltage across the cap and the bottom graph is the current across the inductor i give some time for the inductor to build up current and then suddenly shut off the switch watch what happens to the voltage on the top graph when i resume the simulation the inductive spike peaks at over 10 000 volts remember we only had 12 volts coming in now i'll expand this idea even further by adding a secondary coil to this arrangement and turning the whole thing into a transformer a step up ratio of about 20 is realistic for an ignition coil like this now i'll graph the voltage of the secondary coil and run the simulation again the voltage peaks at an amazing 200 000 volts with just 12 volts of input now realistically a car ignition coil will probably only handle 30 or 40 000 volts before the spark jumps between the windings but this just shows the immense power of an inductive spike okay that's enough theory let's see how to build some circuits that can actually do this let's start with this one which only requires a relay and a capacitor this is about as simple as you can get the primary coil and a small capacitor across the relay terminals forms an lc tank and as it oscillates current goes in and out of the relay coil which turns the relay switch on and off let's see it in action [Music] this is an amazingly simple and effective circuit but the downside is that it relies on moving mechanical parts which are bound to break after a while let's look at another approach using a mosfet transistor in this circuit a 555 timer creates a square wave output of around 130 hertz which is amplified by a small transistor and then used to drive a mosfet which turns current to the primary coil on and off with a waveform that looks something like this a small capacitor is put in parallel with the primary coil which is extremely important because it determines the voltage that's developed when current is shut off we need to make sure the voltage spike isn't larger than the maximum drain's source voltage of the mosfet which in this case is 200 volts now there is a diode in place to protect the mosfet in the case of over voltage but we don't want to rely on that all the time so we have to carefully select the capacitor size i know the energy of my inductor and when current is shut off that energy is all going into the parallel capacitor by rearranging the formula for capacitor energy and solving for capacitance i can find out the capacitor value i'll need to keep the voltage spike from exceeding 200 volts now the current across the mosfet resembles the sawtooth pattern but that's not the case for the lc tank formed by the capacitor in the primary coil when the mosfet is shut off the lc tank current oscillates back and forth at its resonant frequency the oscillation of the lc tank can actually cause the voltage to go below ground which would cause current to flow through the protection diode or body diode of the mosfet stopping the oscillation and potentially damaging components the blocking diode at the mosfet input prevents this from happening okay let's see the circuit in action here's the gate voltage of the mosfet and here's the high voltage arcs [Music] these arcs aren't quite as powerful as the relay driver because this circuit is limiting the primary coil voltage to 200 volts so to make longer arcs i'll need a mosfet that tolerates a higher voltage on the positive side this circuit could run all day without any problems here's the voltage across the lc tank with my scope probe in 10x mode the circuit is ringing at about 150 volts with a nice smooth damping curve but when the arc forms the damping happens much faster because it's effectively causing a short circuit that the ringing energy is dissipating through so that worked pretty well but the output is a series of damped resonant oscillations after seeing this the first thing i wondered was whether or not i could just drive the coil at its resonant frequency and get a higher power output which would look like this it seemed like the obvious solution would be to connect the coil to a half bridge driver and dial in the frequency until i hit resonance but that didn't seem to work because i never got more than about three times vcc with this weird chopped sine wave waveform maybe the key here was to inductively couple the ignition coil with an isolation transformer i know for a fact that if the secondary side of a transformer is connected to a capacitor it will get a tremendous voltage boost when the primary hits resonant frequency as we can see here my probe is in 10x mode here when the secondary hits resonance there's a huge increase in voltage across the resonant cap and the primary draw is exponentially more current so maybe if i connect the ignition coil primary in series with the isolation transformer secondary and drive the whole thing at resonance i'll get continuous high voltage out of the ignition coil secondary and here's what it looked like all assembled [Music] the output results were pretty interesting to look at but didn't produce the high voltage resonance i was looking for i suspect it has something to do with the fact that the inductance of the isolation transformer secondary is so different from the ignition coil primary the former is about 200 micro henrys while the ladder is about 7.5 millihenries so it's pretty lopsided to solve this i rewound the secondary of the isolation transformer to have almost the exact same inductance as the primary of the ignition coil and tested the circuit again matching the inductances seemed to work because this time i got almost 300 volts across the ignition coil primary at resonance despite getting the ignition coil to resonate successfully the output was pretty disappointing probably two or three thousand volts at most also the fact that i had to use a whole separate step up transformer to make it work kind of defeats the purpose so i don't recommend this approach at all i wasn't quite satisfied yet so i revisited my mosfet driver circuit with a new type of component that could handle more voltage and igbt an igpt is an insulated gate bipolar transistor and is effectively three children hiding in a trench coat pretending to be an adult because it's just a small n-channel mosfet driving an npn and pnp transistor these are really useful for high voltage high current low frequency applications they're particularly common for electric cars i chose an stgb20h60 which can handle 600 volts at up to 20 amps and here's a look at the improved driver circuit i didn't have 600 volt diodes so to get the blocking and over voltage protection i connected three in series of the same type used before which were 200 volt diodes i reduced the parallel capacitor down to 288 nano farads gave the timer a 5 volt regulator so that i could supply higher voltages to the circuit and put a potentiometer in to adjust the duty cycle of the driver the potentiometer allowed me to adjust the duty cycle from practically zero percent to ninety percent and the output frequency was about 300 hertz in the new circuit the new circuit was dramatically more powerful [Music] the arcs were so powerful they'd jump the insulation between the terminals and would even occasionally jump a one-inch gap it's difficult to get a direct measurement but this was probably in excess of 40 000 volts judging by the gaps that the sparks would jump ark burned a path through the insulation on the coil and ultimately ruined it but it was a super cheap unit so it was worth sacrificing to get some cool footage that's all for this video now i'm gonna go turn off the fire alarms in my house

Channel: Hyperspace Pirate

Views: 486,754

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Id: 1EabqJJnWhc

Length: 12min 12sec (732 seconds)

Published: Sat Oct 09 2021