Flyback transformer

555capacitorflybackmosfettransformer

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

this
(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.

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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,

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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?

Best Answer

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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

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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).

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