A quick look at Farnell, 47uf @ 35v is only a few pence, but you can lose the zener for a resistor. In fact then you could lower the capacitor still further, does it matter how much the voltage rises to ? Higher voltage electrolytics tend to store more energy for the same size. As well as cost there are still pros and cons to both, not least the lifetime of electrolytics. Its up to the Mike to weigh them up, but maybe he prefers silicon ? :)
Colin =^.^=
Colin's RC suggestion is by far the best way to go with non-repetitive switching. The LC risetime nicely controls the dV/dT across the contacts as they open. C= 47 to 100uF and R= 500 to 1000 ohms, or thereabouts.
Maybe a little but I feel much comfortable using a cap since it doesn't dissipate energy, as compared to a zener used as a surge absorber. I'm guessing that the common intended use of zener is as a voltage stabilizer than a surge absorber. Overall I have a feeling that a cap would be much more reliable than a zener. We'll put a 3.3 Ohm in series with the cap to prevent current surge in case of reverse battery polarity. And of course a 2k2 in series with the cap to discharge it after it has absorbed the current surge during normal operation.
Beautiful aspect of dV/dT here. Its amazing how such a simple application can turn into a such rich discussion.
You still need the in4007 diode, so the 3.3R wont do much at all, capacitors may be more reliable in the short term in terms of being able to take abuse, but electrolytics have a finite life.
In all the (old) stuff ive repaired most has been failed electrolytics.
Colin =^.^=
dude, if the 2k2 discharges the cap, then thats where the energy gets dissipated. No its not in the cap, but its still gone, and you had to add the resistor to dissipate it in :)
Cheers Terry
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