# Motor Control Circuit

Discussion in 'The Projects Forum' started by haunsdoger, May 1, 2015.

1. ### haunsdoger Thread Starter New Member

May 1, 2015
5
0

Hello,
The motor is controlled via a 2N2222A NPN transistor. The hobby motor is 3vdc and pulls about
50mA. I've placed a diode in series to drop the voltage a bit. All in all (correct me if I'm wrong),
the transistor and diode in series drops the voltage down close to the 3vdc requirement of the
motor. (5-.7-.7)= 3.6vdc. I've got a snub diode to remove any negative swings from the
motor.

This motor energizes when a positive value is placed on the base of the transistor and turns off
when the base is ground as designed. But it always fails. I have replaced all circuit components
with no results. The transistor is not shorted. The transistor can switch up to 800mA.

Here is the failure:
The voltage on the collector of the transistor goes up from 5vdc to 6-7 vdc. It does not matter if
the base of the transistor is high or low. At times there is a square wave
on the collector that slows the motor down. Funny thing is that I can ground the motor on
the collector and it will work for a while, then the transistor will not turn ON.

Am I missing something here - It seems like a very simple circuit? Do I have the wrong transistor?
George

2. ### DickCappels Moderator

Aug 21, 2008
2,779
674
Since you asked, the transistor will not necessarily drop 0.7 volts. It will be more like 10 millivolts in your circuit. That is because the collector-emitter saturation voltage is the specification that applies here. The 0.7 volt assumption sounds like base-emitter voltage, which does not directly affect the voltage across the motor because the emitter is grounded.

That means that the motor will see about 4.3 volts.

This might be related to your question about the high voltage on the collector. Would you please explain precisely what you mean by "But it always fails."? For example, does the transistor still look like two diodes with their anodes joined at the base?

By the way, possibly related, there is no protection against a positive voltage spike at the collector when the transistor is turned off. With inductors in general the spike would be positive in the circuit, not negative, though I note that there might be something related to commutation in a motor with brushes that could cause a negative spike that I don't know about.

3. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
George, what do think D1 is doing? Why is it there? Is there a better location for it?