Porter 40: Use with PWM Input

The Porter 40 controller sense the current through the pot input to switch the controller on and to give pot fault protection. If the voltage across the pot is greater than about 7v, the pot fault protection will operate. This works properly with a standard pot input but, if a pwm input signal is used, there are problems as the ignition and pot fault will also be switched by the pwm.

It is therefore often necessary to pre-filter the pwm fed to the Porter 40, so is sees a d.c. voltage and not a pwm waveform.

To further illustrate what happens, the input circuit is as below.

Q2 is a current source supplying 640µA to the pot (320µA if the 10K link is broken). As soon as the pot is advanced from zero to increase the voltage on Q13’s base, Q13 conducts and turns on the circuits ignition.

Q4 is off and will not be operated until Q2’s emitter rises to about 7v – indicating th pot or wiring is faulty. This causes the circuit to ramp down to zero speed and switch off.

If plain unfiltered PWM is used to feed the controller, both Q2 and Q4 will be switched on/off at the {PWM input frequency and the controller will not operate. It is therefore necessary to filter the input PWQM so it is essentially d.c. – though a significant level of ripple will cause little problem.

Be aware that the input must be from a low impedance source, low enough to easily sink the current supplied by Q2.