connecting a small motor to arduino using a transistor

Question

I want to connect a small motor (water pump) to an arduino, and I am not sure about the circuit.

The pump is small, works with 3.3V, but consumes 0.1A so I decided to use a transistor. The one I have is BF547, an NPN type small transistor. It supports currents up to 100 mA.

I measured the hfe (beta) of the transistor, and it is 370. The Vcc is 3.3V, and the current through the motor is 0.1 A, therefore:

A. the Ib equals to Ic/hfe = 0.1A/370 giving Ib = 0,00027A

B. with the Vb at 3.3V(pin voltage) and Vbe at 0.7 (datasheet) the Rb = (Vb-Vbe)/Ib = 9558 Ohm (10kOhm)

My questions are:

1. Did I calculate the base resistance correctly?
2. Do I need any additional resistor for the collector, next to the pump? Is it possible that more current flows through the collector-emitter?
3. According to the datasheet of BF457 100mA is the max Ic it can take. Should I limit the pump by adding a small (10 Ohm) resistor to it (33 Ohm[motors resitance] + 10 Ohm for 3.3 V gives a current of 76 mA)

I am attaching the schematics (linear voltage regulator is there only to stabilise the Vcc provided by an old dc power source I found)

Thanks a lot for any suggestions!

Answer 1

I like your thought process (even though it might not be completely correct), I've seen how little thought were put in for such simple circuit design in the Arduino community, including from some Youtuber celebrity who teaching Arduino and claims to have an EE degree.

BF547 is an RF amplifier transistor, not for switching application, it would be better to use an NPN transistor such as 2N3904 which is designed for switching application and has a Ice of 200mA.

Your resistor value is too high. If your motor is designed to operate at 100mA 3.3v condition, you would want your driver to provide AT LEAST that current. Assuming your are driving the transistor with 5V out from an Arduino, a 1k bias resistor would limited the Ibe to (5 - 0.7Vbe)/1000=4.3mA, take a very conservative DC current gain of 30 (it typically in the range of 30 - 80 for switching transistor, and hfe could be in the range of 100 - 300 when frequency go up, but this will not be the case for Arduino even you drive the motor using PWM), which will capable to deliver Ice of 4.3 x 30 = 129mA. This should meet your Motor requirements.

You would need a protection diode connected between 3.3V and collector with its cathode connect to 3.3V and its anode connect to the collector of the transistor. An 1N4148 (max If=200mA) would be okay for this specific motor, or even better, use any Schottky diode with current and voltage limit higher than your application).

Answer 2

4.6V - 1.2V (2 diode drops of 0.7V) = 3.4V, close enough for the motor if you are using a MOSFET as a driver. You can use a UIS rated MOSFET with a Vgs of less than 4 and it will work perfectly. the transistor will give you an additional voltage drop of about 0.7 Volts and needs a base resistor. If you want to guarantee the motor will not start during reset and setup() add a 10K resistor from the port pin to ground.