Re: [CAD_CAM_EDM_DRO] Re: Value of Zener diodes

grantfair2001 wrote:

>Jon - thanks again.
>
>The drive I am using is not a chopper type unipolar one. The power
>supply is is current limited to the rated current limit of the stepper
>being used (the circuit is from the National Semiconductor LM317
>datasheet, though it omits a few important details), and the voltage
>is set at 24 to 30 volts.
>

Is there a separate current source for each phase of the stepper?

> Although a high voltage will be evident at
>the start of a coil charging, by the time full current is supplied for
>a step, the voltage will be the much lower (as a rule) rated coil
>voltage. I haven't actually looked at the voltage with a scope, but
>that is the theory.If all steppers I would use were low voltage
>perhaps the zener would be unnecessary? Would a zener take care of
>spikes which might be relatively high (I guess you refer to these)?
>Some steppers I have are specified at 12 or 24 volts, so a zener would
>definitely make sense with them, I guess.
>
>

When the phase switches, the center tap would be expected to rise toward
the DC supply voltage as current falls. So, the other side would still
go to ~ Vsupply*2.

> Again the current limited supply implies an average voltage
>somewhat above the rated coil voltage, but when the driver switches
>off current to a coil, I think the voltage at the tap is at the
>recommended coil voltage, so in many cases the power rating of the R's
>would be lower than your calculation suggests.
>
>

Well, that would be true BEFORE the transistors switch, but the snubbing
circuitry is activated AFTER the transistors switch. But, since this is
NOT a chopper-type circuit, I'm not sure I see the need for the Zeners,
etc. at all.

The real question, of course, is why you are making such an out-of-date
design, and spending so much effort to get a stepper driver that is going to
produce vast amounts of heat in lots of expensive semiconductors, when
the bipolar drive is so much better. Second, the advantages of
microstepping
are so great, it is just not worthwhile to spend much effort making any
drive
without it.

Jon