From: "Bill Bowden" <**firstname.lastname@example.org*>
Subject: Re: MOSFET overvoltage protection
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Date: Tue, 08 Oct 2002 23:24:37 GMT
NNTP-Posting-Date: Tue, 08 Oct 2002 23:24:37 GMT
Organization: AT&T Worldnet
Tony Williams wrote in message
> In article ,
> Bill Bowden <**email@example.com*> wrote:
> > Ok, I added a 12 volt zener across the gate (NTE5127A) and a 200
> > volt, 1 watt zener across the drain/source (NTE5105A) but I'm not
> > sure of the power rating for the drain zener.
> > Do you have an idea how much power the zener will need to dissipate?
> The zener should not conduct significantly during
> normal operation, because most of the stored energy
> should be going into the spark. However, the zener
> might have to cope with the unusual circumstance of
> (say) a spark plug not firing, so would have to be
> able to safely absorb regular 50mJ energy dumps.
> Perhaps you could resolve a slight nag about what
> you have done? On the contact breaker there is a
> capacitor, about 0.1uF to 0.2uF afair. This cap is
> an essential part of the Kettering ignition, it
> forms a resonant circuit with the coil, so that
> (when the c-b opens) the voltage across the coil
> rises in a (sort of) quarter-sine shape, up to
> a defined peak of 200 to 300 volts. This has two
> effects, it restricts the dV/dT that the c-b has
> to withstand, and it lengthens the spark time.
> This cap is critical. Is it still connected to the
> coil, or did it disappear in the modifications?
> Note that "200 to 300 volts". Your 200V TVS is
> probably too low. What you should have is (say)
> a 500V MOSFET, protected by about a 400V TVS.
> To get 400V you can put lower-voltage devices in
> series, enough so that their total power capability
> is 10W.
> Tony Williams.
This thing is to be used to replace an old Model-A type "buzz box"
which are hard to find. The breaker points on the old car supply
power to the box and you get several sparks. When running at
a good RPM, one spark is enough, but when starting we need
several sparks. I don't know if there is a capacitor used inside the
buzz box. In any event, the spark jumps a much wider gap without the
capacitor (0.4uF which may be too big). It's slightly erratic at a
1/2 inch gap using the capacitor and will not jump 3/4 inch. Without
the capacitor, the spark is steady at 1/2 inch and erratic at 3/4.
So what's better, longer duration at lower voltage, or shorter duration
at higher voltage? The MOSFET is a IRF740, 400 volt, 10 amp, 0.55 ohm.