[theshadow27]

Al - Thanks for your detailed answer. The problem with using the 884 is not that it is under powered, but rather that it is over powered. I was trying to use a Victor as a common example of an H-Bridge, however I am actually trying to use a larger motor on a cheap RC car that has been adapted to computer remote control. It has no external motor drivers for me to tap into - everything is potted in one epoxy smear.

I have attached what I have so far. The outputs are around six volts, the positive side is constant and the negative side is switched between float and ground. There is a brake function that I would like too keep. The circuit needs to perform 3 functions – decode direction, extrapolate PWM width, and properly drive the new h-bridge.

To get direction, there are two diodes (one per side). If there is a positive voltage (presumably the 6 volts constant from the controller) present at either of them, it will override a pull-down resistor, and bring the line attached to ¼ AND gate (acting as a buffer here) which goes to logic H. If there is anything less then .7 volts before the diode, the buffer input will be logic L. The respective top/bottom output indicates which direction to turn (logic H = + output on motor)

To get pulse width, a diode with a pull-up resistor is found on each side. As the PWM is pulsed between GND and float, the base of the PNP transistor alternates between ~5.3 volts and GND. This transistor is set up as an inverter, so that when there is a ground signal at the input, the output from the transistor is logic H, and when the input floats, the transistor outputs logic L. The output of each transistor is a positive voltage representation of the pulse width modulation of the opposite of each respective side (i.e. channel 1’s pwm will be reflected in channel 2’s output, and channel 2’s pwm will be reflected in channel 1’s output)

The last step is to take this data and drive the h-bridge. The first ½ of the 74AC11008 is configured so that it is impossible to short circuit the bridge (i.e. turn on Q1 and Q3 or Q2 and Q4), while also integrating the PWM signal. This works because the P channel FETs are active when their base is negative, while the N channels are active when the base is positive.

As for the brake function – when both inputs go high, low, or float, the motor is shorted through itself. At very low PWM rates (down to %0/%100) as long as there is still positive on one the one side, the motor will coast, but as soon as both inputs go to the same state it brakes.

I appreciate you all taking the time to look over this. JD