John E. wrote:
Terry Given sez:


BTW in that position its probably a 47V zener, clamping the peak drain
voltage.


I'd been turning over in my mind that this is indeed a zener, not simply a
"plain" rectifier. It is indeed a 47 volt zener.

Why was this diode chosen in the design? I'm familiar with the standard diode
being used to short-circuit the back-EMF from the solenoid, but I can't
figure out the purpose of a zener used in this location.

Vdd
/\
|
|
SS
SS Solenoid
SS
|
+-----+
| |
| |
BUZ72 | /---/ ZY47
FET |--+ /\ Diode
-------| |
|--+ |
| |
\ |
0.27R / |
\ |
| |
| |
/// ///

I think that should show proper in Courier or Monaco... or Paris (c:

I must add that Vdd is *reported* to be 42vdc. I was handed this board with
scribbled specs. May be higher or lower or in a parallel universe.

Thanks,

If Vdd was 42V, then a 47V zener sticks 5V reverse voltage across the
coil, so the current will decay 5/42 times faster than it built up.

Whereas if you just use a conventional freewheeling diode, Anode to
Drain, Cathode to Vdd, there is 0.7V(ish) reverse voltage across the
coil when the FET turns off, so the coil current decays 5/0.7V times
slower than the 47V zener.

Of course the actual zener voltage wont be 47V, it'll be higher,
depending on the actual current.

One can achieve the same objective at lower loss with a 4.3V zener in
zeries with a freewheeling diode, but thats 2 parts.

So it is possible that the zener was used to get a suitable rate of
decay (although ramp down is more accurate) of coil current.

Or perhaps the designer was a bit stupid, used no freewheeling diode,
then discovered the FET broke, so added the zener. You might be
surprised how many **** designs make it to market.

Cheers
Terry