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Back EMF protection for ignition coil Answered

Hi, i made an ignition coil driver circuit (check image) witch is nothing special cause they are everywhere on the internet but what's really bothering me is the back EMF that would fry the 555IC so if i put a rectifying diode (IN4007) at the VCC pin and the reset pin (like in the circuit image) would it solve the problem? Btw this is my first post so sry for any unclear information, thx.


Jack A Lopez

26 days ago

I seem to recall having almost exactly the same problem, with a 555 based circuit driving a car ignition coil. Basically it would work for a few seconds, and then the 555 IC would be fried.

After some trial and error, I discovered a circuit arangement that seemed to protect the 555.

I do not recall the exact reasoning that went into this design, because I put it together like 20 years ago, but I still had the gizmo itself, so I was able to look at it today, and map out the circuit by inspecting the board.

Some pictures of the gizmo, and a hand drawn circuit diagram are attached.

I am not sure, but I think what finally fixed it, is this arrangement with two diodes (both 1N4007, I think) connected anode-to-anode, with the cathode from one diode going to the spark coil primary, and the cathode from the other going to supply power to the 555 circuit.

Also there is some low-pass mojo, consisting of a 47 ohm resistor, and a 100 microfarad electrolytic capacitor, sort of smoothing out the voltage that feeds the 555 timer IC, and the two BJT transistors, a 2N3906 PNP and a TIP122 NPN.

Actually the TIP122 is two transistors, connected as a Darlington pair, and I think there is a protection diode in that package too, wired in the usual way, antiparallel with the collector emitter junction.

Strangely, this circuit does NOT have a diode placed in parallel with the coil, as a free-wheeling diode, and I think that might be intentional. I mean, I think I tried that, but then I left it out, because leaving it in place seemed to adversely affect the spark output.

In other words, I think I got more power to the high voltage output (in the form of bigger and brighter sparks) by NOT having a free-wheeling diode connected in parallel with the primary coil.


27 days ago

It might be easier to use a beefy diode over the coil contacts so the generated voltage when the magnetic field collapses won't go back into the circuit.