jasen wrote:
On 2007-03-04, Terry Given 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.

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.


42V turning on 5v turning off, I get 5/42 fraction as fast. (about 1/8 the
speed)

read harder.

5/42 = 0.118. 0.118 times faster is, indeed, slower. admittedly I didnt
have to make it a reading comprehension test, but its more amusing this way.



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.


huh I'm getting 42/0.7 (which is over 50 times slower)

are you assuming a 5V vcc? OP claims 42V.

no, the original voltage across the coil during turn-off is Vz - Vcc =
47 - 42 = 5V. When a freewheeling diode is used, the voltage across the
coil is 0.7V.

so the current ramps down 5V/0.7V ~ 7x slower with a freewheeling diode.

note the not-so-confusing sentence. I should have written:

"so the current ramps down 0.7V/5V times faster with a freewheeling diode"

but I'm being nice



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