Forward / Off / Reverse Mode (4QD Series)

The 4QD series has a speed input and a forwards/reverse input. Also, being designed for vehicle use, it also has high pedal lockout which prevents the controller working is a demand speed is present when the controller is enabled. For the use described here it is normal to have a speed permanently selected and to apply two signals to the controller: one makes the controller run forwards at the set speed and the other makes the controller run in reverse at the set speed.

The HPLO effectively may seem to effectively prevent such operation. However the 4QD series also has an inhibit input which can quite simply be used to give this style of working. The scheme is shown on the diagram. The inhibit input is active low – pulling it to 0v will inhibit the controller.

The 10K between pins A and F inhibits the controller when the 3 way switch is off. However, moving the switch to forward or to reverse applies 11v via one or other diode to the inhibit input, removing the inhibit. The inhibit input over-rides the HPLO circuit, so the controller will automatically start up in forward or in reverse as selected.

There is one small modification to be made – the inhibit is configured as a footbrake inhibit so deceleration (when inhibit is applied) is fast: this is not required for this application so it is necessary to bridge out a transistor on the 4QD controller.

This can be easily done by linking out a transistor as shown on the diagram left. The transistor is very near the A and D presets. Link out as shown by soldering a piece of insulated wire between the outside two pads – though it will not matter if all three pads of the transistor are linked.

Travel Limits

With this scheme, it is very easy to fit limit switches. Simply fit normally closed switches in series with the two diodes. The limit switch which opens when reverse is operated must open the diode from the reverse switch, so that when the limit switch open, the inhibit input is no longer pulled up and the controller stops. Clearly the inhibit can still be removed by the forward diode.

Input voltages

There may be a call to operate this system from a logic voltage other than the internal 12v line (fed from pin B, via a diode).

The reverse line is happy with higher voltages but the inhibit must never have any higher voltage fed to it – the controller will malfuncion. So include a diode in the inhibit line to prevent reverse voltage feed.