TIP31 (TIP120) as DC Motor driver compare to MOSFET

Question

I'm trying to drive DC motor with PWM using PIC P16F1824. Some books suggest using TIP31/TIP120 as the driver. So I have PWM output pin connected Base pin TIP31, the Emitter pin connected to Ground. I have a LED, the positive pin connected to positive pin from the battery, and the negative pin from the LED connected to Collector pin TIP31.

For LED, it works fine. Using PWM, I could see different strengths on the LED.

BUT, when I connected the DC motor, I could only hear noise sound. The Motor normally run with 200 mA. If I connect the battery directly to the motor, it works fine.

My battery current is 900mA (2 x AAA, 3V). I measured the PWM output from the IC is 5mA, and the current from the Collector pin is 50mA. One of the Motor pin connected to the battery positive pin(900 ma), and the other pin connected to the Collector pin.

My questions:

1. Why the Motor doesn't run, although it appears it has enough current?

2. What are the difference between NPN Transistor (TIP31) with MOSFET and Motor Driver?

Answer 1

The drawing you just added shows that the bypass cap accross the PIC is missing, there are no caps on the crystal, and there is nothing to limit the current out of the PIC into the transistor. Each one of these can cause flaky operation.

EDIT: I just noticed the motor seems to be connected to ground, not power.

EDIT2: And the emitter is connected to power instead of ground.

Answer 2

First, get the motor just to be on/off controllable by the PIC. Once that is working you can try inbetween values using PWM.

From your description, it sounds like you're missing the resistor that should be between the PIC output and the NPN base. Without the resistor, the PIC output pin current is likely exceeded, which can cause it to behave unpredictably. The TIP31 has a minimum gain of 25 at 1A. To get 200mA collector drive, it therefore needs at least 8mA base current, preferably rather more. At only 3V Vdd and figuring 700mV for the B-D drop, that leave 2.3V accross the resistor. 2.3V / 10mA = 230 Ohms.

I think what's going on is that the circuit is too weak to driver the motor properly. The motor may take 200mA when running, but will take a lot more when starting and when there is a load on it. In short, a simple TIP31 driven directly from the PIC isn't good enough for this low Vdd voltage.

You could use another transistor to provide more base drive. In this case, a low voltage FET like IRLML2502 sounds like a better idea. Don't forget to put a reverse diode accross the motor else the transistor and possibly the PIC will get fried.