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?
Best Answer
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
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).