David Weinshenker wrote: > [EMAIL PROTECTED] wrote: > >>Wouldn't this shunt the spike current straight towards the battery and >>to the other FETs? > > > That's why you want some low-impedance > bypass capacitance across the DC line, > near the switches themselves.
No, the supply current drops to zero at the same time that the FET current does, the motor current simply freewheels around the motor and its diode; there is no DC return path while the FET is off. The FET does see its input rise to one diode drop above the positive supply while the freewheel current decays, but the supply current drops to zero along with the FET current. Only if there is subtantial inductance in the line leading from the battery to the mottor/diode/fet circuit would there be any L dI/dT at the top of the motor- the bypass cap would absorb that. With reasonable line routing (keep it away from iron, use twisted pairs instead of common returns) the supply line inductance will be very low and the bypass cap unneeded. The freewheel diode for the inductor solves 99% of the problem. We use freewheel diodes on all our solenoid circuits, mostly to protect the relays driving them. For convenience, these are installed in the pigtail's connector about a foot from the coil- this adds the pigtail's inductance to that of the coil, but is trivial. I don't bother with bypass caps. (We use relays driving the high side of grounded coils, but that's just to reduce the opportunities for shorting out the power supply. One problem with low-side drive circuits is that you have low-impedance connections to system voltage running all over the place, energized even when the device is off. One slip with a tool to the grounded frame and Yikes!) Doug Jones EE in a former life... _______________________________________________ ERPS-list mailing list [EMAIL PROTECTED] http://lists.erps.org/mailman/listinfo/erps-list
