Motor Circuit acting weird

Thread Starter

haunsdoger

Joined May 1, 2015
5
Hello,
I've attached my circuit that shows a simple schematic of a motor control circuit.
I am using a PIC processor to supply Pin-3 either ground or 5 volts (Logic 0 or 1)
to control the motor via a 2N2222A NPN transistor. The 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 - this square wave is not coming from the Processor (base of transistor).

Am I missing something here - It seems like a very simple circuit? Do I have the wrong transistor?
Thanks for your time - any suggestions would be helpful.
George
 

Attachments

Alec_t

Joined Sep 17, 2013
12,007
What you are seeing is probably back-emf voltage spikes from the motor's inductance. D1 is doing nothing useful; it should connect between the collector and the +5V rail, to suppress the spikes.
 

MaxHeadRoom

Joined Jul 18, 2013
22,866
Personally i would have used a fet, 2n7000 etc.
With the circuit impedance and a 3v motor, I don't think you need to worry about dropping the voltage, unless you want to cut the max rpm.
I see no immediate reason why the transistor would fail.
Is the motor free wheeling or controlling a load?
What is the nature of the mechanics, if any?
Max.
 

Thread Starter

haunsdoger

Joined May 1, 2015
5
What you are seeing is probably back-emf voltage spikes from the motor's inductance. D1 is doing nothing useful; it should connect between the collector and the +5V rail, to suppress the spikes.
Thank you Alec_t for the reply - I will modify the circuit and see if that works - I bet it just might.
 

Thread Starter

haunsdoger

Joined May 1, 2015
5
Personally i would have used a fet, 2n7000 etc.
With the circuit impedance and a 3v motor, I don't think you need to worry about dropping the voltage, unless you want to cut the max rpm.
I see no immediate reason why the transistor would fail.
Is the motor free wheeling or controlling a load?
What is the nature of the mechanics, if any?
Max.
Thank you for your reply
I must use on-hand parts so that is what I have. Fet sounds nice though - wish I could use one but ...
This is a freewheeling load at this time. Basically the motor turns a cam that will select an amount of liquid to pass by
lining up two holes. Very little torque is required as it is very small and easy to turn.
The transistor never failed due to limited current - but it did get hot so I think it was not operating in the
saturation zone somehow even though the base current was high. I think I will also remove the
Diode you suggested and see what goes - I'm just worried about the warning they gave with the motor
"Do not apply overvoltage". Thanks for your help! George
 

Thread Starter

haunsdoger

Joined May 1, 2015
5
If not sure, briefly measure the actual voltage across the motor.
Max.
It looks ok so far - I've added the cap across the motor to reduce the spikes on the supply (which caused the PIC to do weird stuff).
I've removed the diode in series. The snubber was moved to the supply instead of ground and that seemed to help. It has been
running (on 2 seconds off 2 seconds) for over two hours. I'll let it run for a day. This circuit will only be active once in a great while
so I'm going to call it fixed! In the future, I'll get a FET to drive the circuit. The voltage across the motor is 4.95vdc.
Thanks again for your interest and help - George
 
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