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I connected an ignition coil to 240v in an attempt to make a Hv generator. The ignition coil was connected I series with a capacitor and light dimmer. When turned on the ignition coil did nothing at all. I think this means it had an open secondary coil... this is supposed to damage the coil. Somebody tested the coil, and said it was fine. Should I be worried?

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    \$\begingroup\$ Automotive ignition coils are not designed to work on household AC power. If anything, I'd expect you damaged the primary coil rather than the secondary. \$\endgroup\$ – Peter Bennett Jul 3 at 23:19
  • \$\begingroup\$ Somebody tested it. Do you think they could have been wrong? \$\endgroup\$ – Steve Jul 3 at 23:29
  • \$\begingroup\$ what is the 240V that you are connecting to? \$\endgroup\$ – jsotola Jul 4 at 0:17
  • \$\begingroup\$ Ac mains, in series with light dimmer and capacitor \$\endgroup\$ – Steve Jul 4 at 0:26
  • \$\begingroup\$ @PeterBennett The light dimmer "chops up" the AC waveform into a pseudo-square-wave, which is roughly what the transformer expects (points opening and closing repeatedly). I have used this setup to drive ignition coils many times. \$\endgroup\$ – DerStrom8 Jul 4 at 3:41
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I have used a dimmer switch in series with a capacitor on 120V mains many times to drive an older style ignition coil. There were plenty of instances where I had the spark gap too wide, but did not damage the coil. Generally you should only be concerned if you run it for an extended period of time. It can cause breakdown within the coil which will lead to permanent damage. However, if you only had it running for a relatively short time, and especially if the test technician said it was fine, then no, I don't think you have anything to be worried about.

I recommend looking into your circuit to determine why you didn't get a spark. It could be as simple as your spark gap was too wide (try at around 5-10 millimeters, for most coils). It also could be that your capacitor has too low of a capacitance (try 1-2uF), or that you need to adjust your dimmer switch a bit until you find that sweet spot. As I mentioned in the comments, the dimmer switch "chops up" the mains sine wave so that it has relatively sharp edges:

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The application and sudden removal of current from the primary winding in the ignition coil causes the voltage to spike substantially, and this in turn induces an even higher voltage on the secondary coil. However, if the "pulse" applied to the primary coil is of the wrong shape (dimmer knob is not adjusted to the right position), the voltage may not be removed quickly enough to generate a spark at the output.

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  • \$\begingroup\$ Thanks for the reply! The coil was probably only running for a minute total (not in one go), is this a short amount of time? Also, I think the capacitor was wayyyy too low, around 5nf. With such a low capacitance, could the cool be damaged? \$\endgroup\$ – Steve Jul 4 at 4:05
  • \$\begingroup\$ Again, if the technician said the coil tested fine, I'm sure it is ok. The purpose of the capacitor is to limit the current. Lower capacitance means lower current. At 50Hz, a 5nF capacitor has a reactance of around 636.6 kilohms (1/[2*pifC]). That means your current is limited to roughly 240V/636.6k = 377uA. This is nowhere near enough for the coil to operate. Usually ignition coils expect several amps, though you should be ale to get a (weak) spark with only tens of milliamps. Switch to a 1uF capacitor (make sure it's rated for AC) and try again. \$\endgroup\$ – DerStrom8 Jul 4 at 4:17
  • \$\begingroup\$ Ok sweet. So could I use it in my car? \$\endgroup\$ – Steve Jul 4 at 5:31
  • \$\begingroup\$ Because it’s in my car \$\endgroup\$ – Steve Jul 4 at 9:11
  • \$\begingroup\$ Where would you get 240V mains in your car? It's more efficient to just use the car's ignition system by itself. Depending on the vehicle you may be able to find aftermarket ignition systems too for better performance, but unless you race for a living it's probably not worth it. Maybe you should state your reasoning for wanting this in your car? \$\endgroup\$ – DerStrom8 Jul 4 at 12:47

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