You need one of these "Constant Current" controllers per motor. You have to limit current based on the current to each motor. It's a brute force way to do what modern chooper drives do anyway.
You can't simply replace a bipolar design with a FET. Bipolars turn on with current and on the PNP shown the base only need have enough current flow from the base to turn on the device. The FET uses voltage and will put the FET in the linear range (high power dissipation) unless full turn-on voltage is acheived. Drive signal for on/off need to have crisp rise and fall times.
Takes a lot of electronics to fix a bad design |