Problem contolling motor with PC parallel port.

[JohnJohn20]

Hi. I thought it would be straight forward to use a darlington pair transistor (TIP120 in my case) to amplify the current from a parallel port DATA pin to turn a 2N3055 transistor to turn a motor on.

When HIGH, the DATA pin puts out a voltage of ~3.6 Volts which turns on the transistor and it works well. But when the DATA pin is low it still has an output voltage of ~0.08V rather than zero volts as I was expecting. Due to the high gain of the transistor, this is enough to keep the motor turning (albeit at a reduced speed).

How can I stop that LOW state voltage turning on the motor?

I tried putting a few rectifyer diodes in between the DATA pin and the base of the TIP 120 to drop the voltage so the LOW state .08V would have no effect but that made no difference. Perhaps a 2V zener would be better?????

have no way of posting the circuit but basically I have a wire from the data pin going through a 4.7K resistor into the base of the TIP 120 with the emitter going into the base of the 2N3055. The emitter of the 2N3055 goes to GND/0V and the collectors of both transistors go to the motor, of which the other end is connected to 12V. My diagram below doesn't display properly but if you click the "Reply with Quote" link you can see it.

| +12V
__|__
| |
| M |
|_____|
|
+------------------+-----+
| C | |
| / | |
LPT1 B |/ | |
D0 ---/\/\/\/\/\----| TIP120 | |
4K7R |\ | C |
| \ | / |
| E B |/ |
+---------------| _|__
|\ /\
2N3055 | \ |
| E |
| |
GND ---------------------------------+-----+---0V

Thanks. Chris.

 

[Genovator]

I guess you should try using a pull down resistor of around 4.7k in the gate of TIP120. It will help to drain the excess 0.08v to ground.