What value power resistor do I need?

Gollywog

Could someone please tell me what numbers to use in selecting a value for a main power resistor in a section of my circuit? The resistor must disapate the energy generated by a permanent magnet motor when the PWM drive is stopped. The (Curtis 1505) PWM boards I'm using cause the voltage at the supply terminals to increase above the voltage supplied by the DC-DC converter that powers them when ever the PWM stream to the motor is stopped. I'm trying to use a 1n3324 (50W 30V) zener in series with the resistor to hold the voltage on the main supply line to a max of 34V so I don't cook the AD5290 digital pot chips that need to be on the same supply. I just blew $35 for two 1n3324s so I _really_ don't want to exceed 50W. One the other hand, a 36V spike on the supply line is going to toast my AD5290 pot chips. The maximum possible voltage that "could" appear on the supply line (if the zener/resistor path were NOT there) depends on many things but mainly on the very impedance I am trying to caculate.

Any help greatly appreciated!

kfurem

I would figure what you want by the max zener current rating, which would determine your resistance value, then your power would be I*I*R. However, you can probably go a little lower than that on the power rating since you don't draw the power continuously or least least not have to put in a safety margin. The manufacturer's data sheet should let you know how much you can overload the resistor--but it cold get pretty hot.