Transistor circuit to toggle motor on/off not working (Arduino)

Question

I've been trying to create a simple transistor circuit with a 2N3904 that will toggle a geared DC motor on and off without drawing power from the arduino - I've followed the schematic seen in the 2nd image.

(Breadboard view)

(Schematic)

The circuit isn't behaving as I expect it to. For testing purposes, I've been feeding the 5V from the arduino into the base pin (before the resistor) to activate the 'switch' of the BJT. This only powers the motor when I have a low valued resistor at the base (~10ohms). When I use 1k resistor, the motor does not turn on. Also, I have used a digital pin set at high to attempt to power the base pin and toggle the motor on/off, but this does nothing, regardless of the value of the resistor.

Perhaps my circuit is incorrect? All I want the circuit to do is turn the motor on and off when I feed it a high signal from the arduino.

Notes: The red wire coming into the node at the top is from a 9V battery. The ground bus on the breadboard has both the arduino ground and battery ground connected to it.

Answer 1

Your motor that you linked to is a 4.5V 190~250 mA (No Load) motor. At 9v, the current probably increases. You are overdriving it by 200%. And any load/weight will cause it to increase in current requirements as well. Stall current is probably 10x that at least.

You are missing the protection diode across the motor, that can easily kill the transistor.

The Transistor you are using is a 100mA standard, 200mA Absolute Maximum. One of those motors by itself without any load, can easily kill that transistor.

The base resistor is calculated as (Base Voltage - Base-Emmiter Voltage) / Current required. Base Voltage is the Arduino pin, so 5v, Vbe depends on the collector current which is 200 mA here, so typically 1V. Current required is calculated as Collector Current (200mA) / Hfe (From datasheet, 10~30). On the safe side, lets go with 10, so 200 / 10 = 20mA needed at the base.

(5V - 1V) / 20mA or 4V / 0.02A = 200Ω resistor. A 1kΩ resistor would only allow 4mA at the base, which times the Hfe of 10, would only allow 40mA at the collector, probably no where enough to tun on the motor.

TLDR: You need the protection diode, your 9v power source is too high, and your transistor is too weak for the motor you are using. And you need a bigger resistor at the base because the motor requires more current then you are figuring. A common 2n2222 transistor with a 470Ω resistor would do much better.

Edit: Not making the pin an output also puts a damper on things. Answer, Arduino pins default to input.

Answer 2

The most likely explanation is that the motor needs more current than the transistor wants to deliver. Have you measured the motor current?

For that matter, it may be that the motor requires more current than your 9V battery can supply. Have you run the motor directly off the 9V battery?

The second most likely is that back EMF from the motor has popped the transistor. Substitute a small incandescent bulb for the motor, and see if you can shed some light on the subject.

Put a reverse-biased diode across the motor. I'd probably grab a 1N4001 or something similar.

Answer 3

For what it's worth, I think that N-channel MOSFETs with logic level gates are much easier to use than NPN transistors for these kinds of driving. A one kiloohm resistor would work fine for the base, as there's only a small amount of charge needed to make the MOSFET conduct, and then it will draw basically nothing.

The BS170 (similar to 2N7000) comes in a 500 mA version that's great for general purpose experimentation, and you can buy 25 for a few bucks. The IRLB8721 N-channel MOSFET is great for driving bigger things (easily over 10 Amperes without heat sinking) and is about a buck each. For good measure, you probably want a 220 Ohm resistor into the gate of that, to turn it on nice and quick.

Answer 4

You may have invested too much faith into the transistor. When I look at the datasheet for the 2N3904, it has a max collector current of 200mA, which is what your DC motor is getting. DC motors are high current devices, try to find out the current needed to turn this motor.

The collector current of the transistor will be a function of the current to the Base of the transistor. Remember that B*I(b) = I(c).

When you use the 5V pin from the Arduino, you are connecting directly to the output of the on-board LDO. This will be much better suited to drive your DC motor, which is why this pin will work (Higher base current equals higher collector current).

However the Arduino can only output a max of 40mA per I/O pin, I haven't checked the max current of the LDO but I'm sure it's much higher.

Here's the schematic for reference: Arduino Schematic

Anyway, the most pragmatic advice I can give is that your transistor should only be used to drive little toy motors. If you have a DC motor of value, it is necessary to be using a relay or a more robust transistor.