Re: [CAD_CAM_EDM_DRO] Re: Value of Zener diodes

grantfair2001 wrote:

>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?
>
>

Hmmm, maybe I was miscalculating!
The center tap of a chopper-type unipolar drive is at the DC supply
voltage. When one transistor turns on, grounding that end, the other
end goes up to 2 X the DC supply voltage by simple transformer
action, plus a little more, due to the collapsing of the leakage flux.
If you limited this peak voltage with a 40 V Zener going back to the
DC supply, that would cause the DC supply + 40 V to be felt by the
transistor. If your DC supply was 30 V (as you mentioned), + the 40 V
Zener, that would clip the voltage to 70 V, which sounds reasonable.

(My error was I was adding 40 V to the 60 V transformer-action peak
at the off side, and that was EQUAL to the transistor's rating. But, that
is not how the circuit works. It is the Zener plus the supply voltage.)

>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?
>
>

No. The value of the resistor is determined by R=E/I. So, you figure how
much voltage you can tolerate above the DC supply (again, 40 V is a good
number) and your motor current (was it 3 A?) and R=40/3 = 13.3 Ohms.
So, you'd use a 12 Ohm resistor. The power rating depends on the step
rate, and the fact that the resistor could never see a duty cycle greater
than 50 % anyway (unless you combined currents from both ends with
the standard diodes that are necessary anyway). Assuming you used
4 resistors, and the steps were coming fast and furious, so the current
hardly has time to die away, P=I^2*R (/2 for the duty cycle) so you
get 3^2*12/2 = 54 Watts. Actually, a 25 Watt resistor will probably
handle it ok, due to the exponential decay of the current. Hmm, 4
25 Watt resistors! Note, also, that the Zener diodes would have
to dissipate a similar amount of heat! Are you starting to see why
nobody makes serious motor drives unipolar? You start out thinking
you will save money with only HALF the transistors, but then all these
uncontrolled inductive pulses start REALLY complicating the drive
when you try to tame the windings.

Jon