Re: Stepper power supplies, unregulated and Zener selection

Re: Zener watt rating

You can always beef up the zener with a power transistor. Connect the
zener cathode to the transistor collector and it's anode to the base,
connect a resistor from the anode/base connection to the emitter, and
connect the transistor collector/emitter across the power supply. The
resistor should be sized to allow enough current through the zener to
get it past the "knee" threshold, say 1mA? So that would be about 680
ohms.

I might be nervous enough about it to add a fuse, as power transistors
tend to fail shorted.

What is the capacitance across a zener? IE, will a 680 ohm series
resistor cause an appreciable delay? Perhaps the base-emitter
capacitance of the power transistor will compensate for the zener
capacitance? Or perhaps the zener capacitance may cause the power
transistor to turn on due to fast transients regardless of voltage? Or
perhaps I'm analyzing this too much? ;')

Or perhaps this is moot, and the instantaneous current rating will be
more of a limit than the wattage?

My $.02, adjusted for inflation.

Steve