Terry Given wrote:
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
Tony Williams just pointed out my mistake.
I'm so used to dealing with SMPS inductors I forgot we were talking
about a solenoid.
In a SMPS inductor (or transformer) some external circuit is used to
limit the current - pulse width, peak current control etc, and in order
to minimise losses, Rdc is very small. In which case V = LdI/dt is the
"right" equation to use (as I*R is very small)
but a solenoid or relay isnt (generally) used that way - Rdc sets the
current, and is most assuredly not "very small", and of course I*R = Vcc
which is not "very small"
in which case its more about L/R time constants. I = Vcc/R, so when you
switch the solenoid off, the voltage across the internal inductance
rises to Vcc (because of I*R) + Vclamp.
So the difference between 42+5 and 42+0.7 is bugger all, and the
difference in decay time is, likewise, bugger all - well not bugger all,
but certainly not 7x.
Oops.
Cheers
Terry