Syd, You say it worked fine with the 820. 5V isn't enough. The things
should be switched hard on (12V) and hard off. There are logic level
fets which will work at 5V but the IRF820 isn't one of them. Beware also
savage capacitance inside the fets, notably Drain/gate but also
gate/anything. This is good for providing unwanted switch on and
component failure on switch off.
The gate can be pumped up like a capacitor. Is there a 5V regulator,
e.g. 7805 on the switch on?
As has been mentioned, if your fet blew drain/gate, your firing circuit
is toast.
If you are trying to make money & fix it, eliminate uncertainties.
On Sun, Nov 16, 2003 at 11:24:20AM -0800, Syd Levine (AnaLog)
enlightened us thusly
>
> The trigger pulse is generated on a different board, then is
> conditioned with a 4049 cmos chip. It seems weak, but there are so
> many other problems, I am having trouble figuring out what it is
> supposed to look like. The 4049 is powered by a separate 5 volt
> regulator, but the input to it is only a 2 volt pulse, which seems too
> weak.
> >
> > How is the trigger pulse generated and what does it look like right
> > now? If the original MOSFET was shorted, it probably also shorted
> > either drain or source to the gate. This might have killed the
> > driver circuit either by overloading it (short to ground) or by
> > bypassing the high voltage pulse directly into the driver....
> >
> >
> > SLA> the 840 with regard to the trigger pulse? Most of these type
> > tools
> use an
> > SLA> SCR; this is my first experience with a mosfet fired
> > transmitter.
--
With best Regards,
Declan Moriarty.
--
Author: Declan Moriarty
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