Recently, I posted a question about a flyback transformer schematic I was having.

(source: eleccircuit.com)

If you didn't check the link, I was having problems with the frequency of the 555 timer as well as problems with the transformer core. The frequency issue is resolved (albeit a little clunkily - 10 100nF caps in series was a bit of a pain in the ass). However, the flyback still doesn't seem to be generating an arc.


This is the transformer core I used. It was salvaged from a cannibalized flourescent bulb driver. It seems to be the "real deal" - ferrite core, air gap etc. I have no idea what the turn ratio is though. I would assume the contacts to which I've soldered the red wires are for the primary and black the secondary. As one can see in this picture,


the (assumed) primary is rather thin (1 wire), while secondary seems to be quite a bit thicker, maybe 3 or 4 wires.

When I connect it all up and run it, it runs. Like I said before, it doesn't generate an arc. But the MOSFET heats up and there's an unmistakeable whine from the transformer, so there must be a time-varying current through it. I'm driving it with 12VDC (from a couple of batteries). I've tried using either side as the primary coil, with no luck (although the whine was of a different pitch).

Does anyone know what the issue could be?

  • \$\begingroup\$ If you're trying to step voltage upward, you drive the coil with the few turns of heavy wire, and take the output from the coil with many turns of fine wire. But the transformer you're using is probably not designed to handle the voltage you're trying to generate. \$\endgroup\$
    – Dave Tweed
    Jun 12, 2013 at 21:22
  • \$\begingroup\$ You are trying to produce 10kV from a transformer you know hardly anything about and wire up back to front. You need to do some research. For a start the secondary winding will need to be in layers with 2kV insulation between layers... where do i end? \$\endgroup\$
    – Andy aka
    Jun 12, 2013 at 21:31
  • \$\begingroup\$ Well I tried. I know hardly anything about it, true... But neither does google, it would appear. Tried every combination of the serial number or w/e that is I could think of. The only thing I can seem to find that references this type of transformer is some other site in which someone apparently built a flyback using almost the exact same type. I think. \$\endgroup\$ Jun 12, 2013 at 21:32
  • \$\begingroup\$ I should add, I tried it both ways around, with no luck. The transformer frequency was different though. \$\endgroup\$ Jun 12, 2013 at 21:35
  • \$\begingroup\$ What about building your own transformer from scratch? \$\endgroup\$
    – Dor
    Jun 12, 2013 at 23:26

1 Answer 1


enter image description here

WARNING: This circuit can produce LETHAL voltages and is provided for information only.

You cannot just use any old transformer. A salvaged fluorescent tube transformer is very unlikely to work.

If you don't (or can't) wind your own transformer then at least go for a transformer you know is designed for the job.

(1) Car ignition coil

(2) A line (flyback) transformer from an old CRT display

Both are designed and made to generate very high voltages

enter image description here

Things you should notice in this circuit.

(1) The 555 is decoupled from the 12V supply through a 56 ohm resistor (R3) and 100uF + 0.1uF (C1,C2). Try to get these capacitors connected as close to the pins of the 555 as possible (pins 1 and 8).

(2) The circuit does not require the 2N2222 transistor to drive the Mosfet. (Neither does your circuit). The 555 is quite capable of driving the gate of a Mosfet which is essentially a small capacitor that needs a sufficiently large voltage change (0 -12V is fine) and not a lot of current to turn it on and off.

(3) It uses diodes (D3,D4 - rated at 600V,3A)) to protect the Mosfet from voltage spikes and reverse voltage.

(4) It uses a different Mosfet - IRF640. The main differences are a higher maximum voltage and a lower turn on resistance. The lower turn on resistance means LESS POWER (I^R) will be lost in the Mosfet and it will run cooler (although you still mount it with a heat sink).

enter image description here

  • \$\begingroup\$ Since I've already built the circuit, I'm going to try using it first. I'll try to dismantle the core. I'm aware I should put few primary turns on, but how many secondary turns should I do? \$\endgroup\$ Jun 13, 2013 at 13:43
  • 1
    \$\begingroup\$ This schematic appears to be a derivative work from a copyrighted original found here (web page) \$\endgroup\$
    – Dave Tweed
    Jun 1, 2016 at 10:13
  • \$\begingroup\$ @DaveTweed possibly , but then again its hardly an original design or concept so just because someone slapped a copyright notice on 'their design' doesn't make it so - a 555 allowing control of M/S ratio followed by a MOSFET to driver to an ignition coil is hardly original. I notice their first copyright notice is 2007 (not exactly when the 555 came out) - I was building stuff like this back in the 70's along with a lot of others. \$\endgroup\$ Jun 1, 2016 at 12:08
  • \$\begingroup\$ @DaveTweed just comparing another 555 circuit your design. tinyurl.com/y5knbe52 When would one use the circuit on the left vs yours on the right? \$\endgroup\$ Aug 18, 2020 at 21:43

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