Russell McMahon wrote:

> If instead of a resistor we use a 36v zener then the voltage rises to 36v
> not only initially but throughout the time that current flows so dissipation
> will be faster. A zener will therefore be faster than a resistor for a given
> maximum allowable rise in voltage, as the resistor will rise to this value
> initially and then decay whereas the zener will hold the voltage until
> essentially all energy is gone.


Hi Russell,
Are you saying ALL the energy is dissipated by the
zener (excluding the diode for simplicity)??

I have believed that only the *peak* of the back emf
is conducted, which seems backed up by my experience
seeing a zener of 45v dissipate a lot less heat
than say a 20v zener on a 12v coil which is switched
at a set frequency. Obviously the time of conduction
is a lot less with the 45v zener but if you are
correct we are talking about a standard unit of
energy dumped each time the coil switches at a set
freq so there would be no difference in dissipation
with differing zener voltages...

Also on the cro I have seen the zener clip the peak
and still leave the lower exponential decay of the
wave (with the zener not conducting), judging from
the area under the curve NOT conducted by the zener
it would be the bulk of the energy, explaining why
a 45v zener will run stone cold and a 20v one cooks.
Or am I missing something here as usual at 2:30am?? ;o)
-Roman

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