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.

The zener does a better, but more expensive, job of protecting the
series switching element. It limits both positive and negative
transients. A diode across the switched inductor does stop most (but not
all) of the switching transient - but doesn't protect the series element
from transients on the supply rails, caused by other inductances
elsewhere reacting to the sudden change in current. It is usual to
combine these sorts of design with reasonably fast (eg
tantalum)electrolytics placed locally - to act as energy "tanks" to
supply and sink transient power.


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.

As I and others have written - the diode didn't burn up because of
transient energy. There is a supply problem, somewhere.

--
Sue