Re: [CAD_CAM_EDM_DRO] Value of Zener diodes

grantfair2001 wrote:

>Jon-
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>How is the value of the zener established? I am using a current
>limiting power supply which operates at up to about 30 volts.
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I'm not sure what you are asking, here. If you are asking what voltage
of Zener diode to use in a unipolar stepper drive, you need to determine
what voltage the power transistors can handle. If you have a 30 V
power supply, and 100 V transistors, you don't have much margin.
When the other transistor turns on, the off side will rise to 60 V
just due to the transformer action of the center-tapped winding.
The collapse of the magnetic field of the switched-off winding
(mostly due to leakage inductance) could theoretically be allowed
to cause the off end to rise to 100 V, so a 40 V Zener diode could
be used. but, this is pretty dangerous. Now, if the driver has
200 V transistors, you could make some use of that extra margin,
and use Zeners up to 100 V or so. You'd also need to put a
regular diode to prevent the Zener from becoming forward biased
when the transistor is on.

But, I'm not real sure this will improve the drive's performance
dramatically. The worst thing is just a diode from the transistor/
winding end to the power supply, without a resistor. this recirculates
the current until the motor resistance allows it to die. Instead of
a Zener diode, a series resistor and diode does about the same,
but the resistor needs to be chosen based on the motor's current.
The Zener does it right independent of motor current.

Jon