John E. wrote:
John Popelish sez:
Higher zener voltage means faster current ramp down. But
you will probably have to go quite a bit higher to see much
difference. The resistive drop of the coil is already
starting the ramp down with a 42 volt reverse voltage. But
that drop falls as the current falls, so the zener is really
there to speed the tail of the process, unless its initial
voltage is on the order of the supply voltage. So you might
consider one as high as 22 to 39 volts. But then I would
look for a 1 watt unit, to handle the power pulse that will
end up more there than in the coil resistance. But you
should definitely see some decrease in the power down time,
to about 37% if what you will get from a 4.7 volt zener if
you switch to a 33 volt one. So you can see that the turn
off time is not dominated by the zener till its voltage gets
near the supply voltage. But increasing the zener voltage
drop helps.
Seems we're creeping back up toward the original 47v zener (although it was
connected across the FET, not the coil). Any advantage to simply using
another 47v part along with the rect. in the configuration you recommend? Is
this a case of "bigger (v) is better"?
The advantage in moving the zener is the lower energy
absorbed per discharge (for the reason I explained earlier).
At 47 inverse volts across the coil, you are getting pretty
close to the 100 volt mark, which will stress the fet a bit
more. Are you confident in its ability to handle that
voltage? And there is a point of diminishing returns. The
37% discharge time I gave above referred to the time for the
current to reach zero. But that is not really the time for
the magnetic field to reach zero, because the iron parts of
the solenoid will circulate eddy currents that support the
field for a bit. Then there is the inertial time constant
of the mechanism that delay s movement, after the magnetic
field stops holding it against the return spring.
If you used a 1000 volt zener, the coil current would hit
zero in a really short amount of time, but the valve would
close in just about the same time as if you used a 500 volt
zener.
My gut feeling is that, unless this solenoid and valve
mechanism were designed with fastest possible reaction time
in mind, going much above 22 volts on the zener will not pay
off in much decreased valve action.
But a handful of 1 watt zeners in the range of 4.7 volts to
47 volts cost only a few bucks, if you want to take the
experimental route. Can you rig up some mechanical pickup
on the valve, so you can, measure the response time effect
of various zeners? That would make it pretty obvious where
the diminishing returns come into play.
A better way to speed the release might be to put a parallel
resistor and capacitor in series with the coil, so that the
coil voltage actually decreases a little after the cap
charges to the IR drop of steady state operation. That way,
you have the large pick up force to get the valve open, but
a reduced holding force to keep it open, so there is less
magnetic field to quench when you want it to close. This is
called a pick and hold strategy, and there are special
driver chips that perform this function with two switches,
one on each side of the coil.
At energize, both switches turn on, applying full voltage
(often a voltage the coil would not tolerate, continuously)
to the coil to ramp the magnetic field up as fast as
possible. The current is sensed, and when the required pick
current is reached, one of the switches pulse width
modulates the current down to the hold value. When turn off
time arrives, both switches open, and the coil dumps its
energy back into the supply through a diode across each of
the switches. So the supply voltage acts like your zener
voltage. Very fast and energy efficient (there is minimal
heat in the coil, and no intentional power wasted anywhere
else in the circuit) but probably not practical as a
retrofit in this case.
http://www.ortodoxism.ro/datasheets/...onics/1331.pdf
But something to keep in mind if a board layout comes along.