On Sun, Nov 16, 2003 at 12:59:18PM -0800, Syd Levine (AnaLog)
enlightened us thusly
> Declan,
>
> Hmmm, sounds like the original design is marginal. There is 5 volts
> applied to the mosfets to "pump em up", but the 4049 is also powered
> by the same 7805. That means the very highest effective trigger would
> be 10 volts, not 12. Right now it is only about 7 volts, and that
> will not dependably switch the mosfets. There are problems on the
> adjacent board where it is only delivering a 2 volt pulse (I suspect
> it should be 5 or more).
>
> I have just about decided to trash part of the circuit and put in a
> much simpler and dependable circuit. The entire wire wrap section
> should go; it simply is not dependable enuff for oilfield service.
>
> Thanks for the insight.
Is it short of a second pulse to pump it up to 15V? What a lousy design
anyhow - wire wrap out in rough terrain always causes problems.
You also now know that the reason your fet blew was probably that
saturation wasn't reached in switching. The gate behaviour of a mosfet
at marginal levels is (wait for it) _very_temprature_dependent!
>
>
> > 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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