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[TCML] help needed - running a Jacob's ladder from a MOSFET-driven ignition coil
Hello Folks,
I would need some help to design a MOSFET circuit to run an ignition coil
and was wondering if anyone n this list could give me advice. The goal is to
drive a Jacob's ladder from DC. I know there are many ways to drive a JL (I
used a NST, a dimmer circuit with a capacitor in series as described by Greg
Hunter http://myweb.cableone.net/grcarhunter/frames.htm) etc.But this time
I'd just like to try a MOSFET driver.
I am using a signal generator, as described on the RM electronics website:
http://www.rmcybernetics.com/projects/DIY_Devices/homemade_signal_generator2
.htm
This allows me to adjust both frequency and duty cycle independently
This signal generator then drives a pair of 2N3904 /2N3906, which in turn
drives a MOSFET, in this case a STP8NK100Z (6.5A, 1000V). The primary of my
ignition coil is in parallel with a (flyback diode and series resistor) to
reduce spikes on the MSOFET. The circuit looks very much like this one below
- just my signal generator is a bit more complex, and my MOSFET has higher
ratings
http://www.electronics-lab.com/projects/misc/001/index.html
I have a muffin fan to cool down the MOSFET, which is mounted on a heat sink
(finned, about 3 inches by 3 inches by 1 inch)
The problem I have is that my MOSFETs keep dying. I get a 1 inch arc at 20V
DC, some heating of the heat sink - then if I increase the voltage to 40V
the MOSFET dies.
I think I need a snubber circuit between Source and Drain, but I don't
really know how to design that. I could use another flyback diode+resistor
(DR filter), a MOV, and a RC filter. but I don't know how to select values
for either the DR or the RC filters. Could anyone help?
Many thanks
Thomas
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