1. Arduino Application: Speed control of small DC Motors
ME 120
Mechanical and Materials Engineering
Portland State University

2. Learning Objectives
Be able to describe the use of a transistor as a high speed switch
Be able to build a breadboard circuit that uses a transistor as a high speed switch
Be able to explain the role of a snubber diode
Be able to implement PWM speed control of a DC motor
Additional references:
Circuit 12 (p. 64) in the SIK manual
ME 120 course notes on PWM
motors/overview

3. Using a transistor as a high speed switch

4. Transistor as a switching device
Each Arduino output channel has a 40 mA limit
Only current to power a very small DC motor
Arduino is not designed as a power supply
Maximum current draw for an Arduino is 200 mA
Use the Arduino as the brain
Let another switching element be the brawn

5. Use an NPN Transistor as a switch
This device is designed for use as a medium power amplifier and switch requiring collector currents up to 500 mA

6. Use Digital I/O pin to switch LED on/off
Digital I/O pin → LED

7. Code to control brightness of an LED
int LED_pin = 11; // PWM pin LED or motor control
void setup() {
pinMode( LED_pin, OUTPUT ); }
void loop() {
int duty, pot_pin=0, reading;
reading = analogRead(pot_pin); // read potentiometer
duty = map(reading,0,1023,0,255); // rescale to 8-bit
duty = constrain(duty,0,255); // be safe
analogWrite(LED_pin,duty); // set duty cycle
In the following examples, the Arduino code does not need to change when the electrical circuit is changed. The Arduino code only needs to use a single digital output pin, which in this code is LED_pin.

8. Use a Transistor to switch LED on/off
Digital I/O pin → Transistor → LED

9. NPN Transistors as Switches
Transistors can be used as switches: By applying relatively small voltage to the Base, electrical current will flow from the collector to the base.
C is the collector.
B is the base
E is the emitter

10. NPN Transistors as Switches
When used as a switch, ICE, the current from the collector to the emitter is large compare to IBE, the current from the base to the emitter.
C is the collector.
B is the base
E is the emitter

11. What is a snubber diode, and why should I care?

12. Simplest DC Motor Circuit
Connect the motor to a DC power supply

13. Current continues after the switch is opened
Opening the switch does not immediately stop current from flowing in the motor windings

14. Reverse current
Charge build-up can cause damage

15. Motor Model Simple model of a DC motor:
Windings have inductance and resistance
Electrical energy is stored in the windings – the inductance effect
We need a way to safely dissipate electrical energy when the switch is opened after the motor has been running

16. Flyback diode or snubber diode
Adding a diode in parallel with the motor provides a path for the dissipation of stored energy when the switch is opened

17. DC motor speed control circuit
The circuit for DC motor speed control uses the idea from the LED brightness control circuit. Replace the LED circuit with the DC motor and snubber diode
(Digital pin ~11)
flyback diode
DC motor
Motor Pin
330Ω
+5V
transistor
(Analog pin A0)
Pot. Pin
Potentiometer