Re: The ignition coil driver circuit again!!!!

["Jim Lux" <jimlux-at-jpl.nasa.gov> (by way of Terry Fritz <twftesla-at-uswest-dot-net>)]

Exceeding the max Collector to Emitter voltage when cutoff (BVceo in the
data sheet).. 2n3055 is rated at 60V, and probably will do 70-80 volts
depending on temperature and which mfr lot you got.  What happens when you
exceed this is that the transistor starts to conduct (much like a zener),
except worse, because it can cause the base to become forward biased which
can turn the transistor partly on.   In any case, big voltage across
transistor + current flowing means lots of power dissipation, which will
melt it in short order.  This is especially true if part of the transistor
breaks down first (as it will), so the heat is concentrated on the die.

There ARE transistors designed to have a "good" breakdown characteristic
(i.e. they don't self destruct, given suitable external energy limiting). 
They are used in things like electronic ignitions driving, of all things,
ignition coils.  The typical ignition circuit sees 300-400V or so when the
transistor turns off (making the spark). Typically, the spark in the
cylinder actually limits the voltage, when it starts to conduct. However,
if the sparkplug misfires, or the lead falls off, or the transistor turns
off more quickly than planned (high di/dt), then the voltage can go higher.
 

You could just specifiy a 700-800Volt transistor (like used in TV
horizontal output apps). But,  HV transistors cost more to make than low
voltage, and automotive apps are very very cost sensitive (pennies count:
If you make 10 million ignitions a year (which they do), you can afford to
have an engineer spend a year ($200K) figuring out how to reduce the cost
by $0.02), so the solution is to specify a 400V transistor, with good
avalanche characteristics to handle the few times the voltage spikes
higher.  Most transistors can take some abuse before catastrophically
failing....

It is unlikely that you are worried about saving $0.75 on the parts cost of
a tesla coil, so the moral of the story is to go pay $2 or $3 for a surplus
transistor that can take the abuse rather It than saving a dollar by using
the 2N3055.  If you need one CHEAP, (like free), find a dead TV set and
scavenge the horizontal output transistor. 


----------
> From: Tesla list <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: The ignition coil driver circuit again!!!!
> Date: Sunday, September 03, 2000 3:16 PM
> 
> Original poster: "wsc1914" <wsc1914-at-hotmail-dot-com> 
> 
> What do you mean by breakdown?
> 
> ----- Original Message -----
> From: Tesla list <tesla-at-pupman-dot-com>
> To: <tesla-at-pupman-dot-com>
> Sent: Sunday, September 03, 2000 6:49 PM
> Subject: Re: The ignition coil driver circuit again!!!!
> 
> 
> > Original poster: "Jim Lux" <jimlux-at-jpl.nasa.gov>
> >
> > The problem is probably breakdown, not over current.  The 2N3055 can
take
> > 15 Amps all day long and, on a good heat sink, can dissipate 150W.
> >
> > ----------
> > > From: Tesla list <tesla-at-pupman-dot-com>
> > > To: tesla-at-pupman-dot-com
> > > Subject: Re: The ignition coil driver circuit again!!!!
> > > Date: Saturday, September 02, 2000 4:20 PM
> > >
> > > Original poster: "wsc1914" <wsc1914-at-hotmail-dot-com>
> > >
> > > Do you think it would take the strain off the2N3055 transistor if 2
of
> > them
> > > were put in parallel, then the current would the shared between them.
> > >
> > > Oliver.
> > >
> > > ----- Original Message -----
> > > From: Tesla list <tesla-at-pupman-dot-com>
> > > To: <tesla-at-pupman-dot-com>
> > > Sent: Saturday, September 02, 2000 10:22 PM
> msnip...
> 
>