[Alan Anderson]

If the Victor PWM input is the optically-isolated connection I think it is, limiting voltage is not as important as limiting current. Don's suggestion of a zener diode with a series resistor is fine, but it's very likely that a simple series resistor alone will be all that is necessary.

Determining the appropriate value of that resistor is a separate matter. That's where it would be nice to have a schematic, or at least a specification of what the Victor's typical PWM input current is. As a ballpark guess, for an 8 volt input I would try starting at about 1 kilohm, then reducing the value if necessary until I got consistently correct operation.