John E. wrote:
John Popelish sez:


I would think that a partially shorted zener would keep the
solenoid energized, giving a "gummed up" symptom. If the
board allows space for the modification, I would replace the
47 volt zener with a series combination of a 4.7 or 5.1 volt
zener in series with a 1N400X or similar small rectifier
diode, connected directly across the coil, instead of across
the fet.


What I know of the design goal of this circuit is that it must activate the
solenoid quickly from off to on and quickly from on to off with as little
"ramping" as possible. With the given circuit, what does this knowledge say
about the selection of possible replacement component(s)?


V = L*dI/dt, so dt = L*dI/V

L & dI are constant, you are increasing V to get a nice low dt.

the BUZ72 is a 100V part, so you have PLENTY of headroom there.

the existing circuit turns the solenoid off about 8x slower than it
turns it on.



The rectifier cathode connects toward the positive supply
end of the solenoid, but the zener cathode points toward the
fet drain.


Anode-to-anode, with the rectifier "on top", the pair being connected across
the solenoid?

it doesnt matter if the rectifier is on the "top" or "bottom", only that
its cathode faces towards the supply, so it prevents the zener from
working when the FET is on, and allows the zener to work when the FET
drain voltage rises above the supply.

so a K-K connection with the recitifer at the bottom and the zener at
the top, or A-A with the zener at the bottom and the rectifier at the top.



Can you find a place to put those two components?


Yes, pretty easily. It's not too heavily populated. Lots of "vertical
implementation" possible (c:

Thanks for your suggestions, John.

Cheers
Terry