Re: Value of Zener diodes

Thanks for your reply. You are right, I am thinking of unipolar motors.

I am using TIP125s in one controller, and they are good for 100 volts.
In another, I am using mosfets good to 60 volts.

When you say "so a 40 V Zener diode could be used. but, this is
pretty dangerous", what is the danger?

Also, when you say the resister/diode combination require the R to be
based on the current involved, how is this calculated? Is this just to
ensure the R's ability to dissipate the power, or is there more?

Grant

--- In CAD_CAM_EDM_DRO@yahoogroups.com, Jon Elson <elson@p...> wrote:

> grantfair2001 wrote:
>
> >Jon-
> >
> >How is the value of the zener established? I am using a current
> >limiting power supply which operates at up to about 30 volts.
> >
> >
> 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