Palindrome wrote:
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.

its pretty hard finding a FET without a body diode, so negative
transients are invariably taken care of regardless of the type of clamp
circuit.

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,

by "series element" you must be referring to the FET. Yep, the zener
will protect the FET against voltage spikes on the 42V bus. Of course
FETs nowadays are rated for avalanche energy.....


caused by other inductances
elsewhere reacting to the sudden change in current.

Que?

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.

seeing as Im being a pedantic sod, I'll point out that tantalums are not
electrolytics (and vice versa).

I once had a serious brain fart in this regard, making a small motor
controller at Uni. It ran from a 3-phase supply, and seeing as
full-wave-rectified 3-phase AC has ~15% ripple, I figured I didnt need a
DC bus cap.

Which worked fine, until the first time I turned the H-bridge off with
current flowing in the motor 30 minutes, 4 FETs and a complete set of
gate drive circuits later, I added a large cap. oops.




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.


assuming the thing ever worked properly, which it sounds like it did.

conceivably a shorted solenoid could have stored enough energy to end up
snotting the zener, but as you say, a supply overvoltage would
definitely kill it. And it doesnt even have to be that much, just
continuous.

Cheers
Terry