John E. wrote:
Terry Given sez:

So it is possible that the zener was used to get a suitable rate of
decay (although ramp down is more accurate) of coil current.

I'm told that the solenoid this circuit operates is for a vacuum valve that
must operate quickly and repeatedly. It was thought by the person who handed
me this pcb that the solenoid was operated with 2 voltage rails, switching
between opposite opening voltage and closing voltage. But according to
measurements by him (and the fact that there's only 1 FET), the purpose of
the zener here seems to make sense.

But how can a 4.7v zener and one diode drop serve similar purpose as a 47v
part?

In the present circuit, when the fet turns off, the coil
generates a voltage in the direction that tries to keep the
current going. That means that the end that had been pulled
negative to ground suddenly goes more positive than the 42
volt rail. At 47 volts the zener comes on, and provides a
path for the decaying coil current. So, during that energy
dump process, there is about 47-42=5 volts reverse voltage
across the coil, driving the current toward zero. But the
energy in the zener is being fed from both the coil (the 5
volt part of the 470 and by the supply the 42 volt part of
the 47), since the coil current is also passing through the
supply.

The only advantage I can see to this wasteful and stressful
(to the zener) method of driving the coil current to zero,
is that the supply current ramps down to zero, smoothly,
rather than switching off as the fet does. But I doubt that
is a consideration in this circuit.

If you put a rectifier and zener directly across the coil,
the rectifier keeps the zener out of the circuit when the
fet is on, but connects it as a voltage clamp when the fet
switches off. Now, the only energy going into the zener is
that being dumped out of the solenoid, as its current ramps
down to zero. The supply stops contributing the moment the
fet switches off. You can adjust the ramp down time by
swapping zeners with different break down voltages. But I
would start with a 4.7 or 5.1 volt unit to get things back
about the way they were to start. But a 6.8 or 7.5 volt
unit may make the solenoid work better with an insignificant
additional voltage stress for the fet.

The supply should also have some bypass capacitance
connected very close to the fet source and the positive
supply connection of the solenoid, to make sure the fast
interruption of the current (that didn't happen with the old
zener) doesn't bounce the supply rails around enough to
unset either the fet gate drive or some other load connect
to the 42 volt or ground rails. A microfarad or 10 would do
it. I 1 microfarad, 50 or 63 volt stacked film type would
do it well.
see:
http://www.panasonic.com/industrial/...abd0000ce8.pdf