Arduino Transistor Switching: High-Power Device Control Tutorial

This section covers practical implementation of PWM motor speed control. PWM (Pulse Width Modulation) controls motor speed by varying the average voltage through duty cycle adjustment. The oscilloscope displays the PWM signal at the transistor base, while motor speed responds proportionally to duty cycle changes. The flyback diode protects against voltage spikes from the motor's inductive load. The TIP120 pinout is: Base (control input), Collector (connected to motor and diode anode), Emitter (ground). Practical verification shows the motor accelerating from low to high speed as duty cycle increases, demonstrating successful PWM control implementation.

Transistors in common-emitter configuration serve as switches for controlling high-power loads from microcontrollers. When saturated, collector-emitter voltage drops to ~0V, allowing loads like motors (1A) or high-power LEDs to operate safely. PWM (Pulse Width Modulation) controls LED brightness and motor speed by varying the duty cycle, changing the average voltage delivered proportionally from 0V to the source voltage. This enables proportional control rather than simple on/off switching.

A DC motor is controlled using PWM through a BC337 NPN transistor driver. The motor's emitter connects to ground, collector to the motor, and a flyback diode provides protection. By adjusting the PWM duty cycle, the average voltage applied to the motor changes, directly controlling its speed. Lower duty cycles reduce speed while maintaining torque, and higher duty cycles increase speed up to maximum. The demonstration shows clear speed adjustment from minimum to maximum by varying the duty cycle. This simple transistor-based PWM circuit provides an effective solution for motor speed control and LED brightness adjustment in practical applications.

Pulse Width Modulation (PWM) allows precise control of motor speed by varying the duty cycle of the control signal. The Arduino's analogWrite() function generates PWM signals (0-255 range) that are sent to the transistor base. Higher duty cycles result in more average current flowing through the motor, increasing speed. The potentiometer provides an analog input (0-1023 range) that is mapped to the 0-255 PWM range using the map() function. This technique enables smooth, variable-speed motor control without complex circuitry.

H-Bridge motor control operates through transistor saturation and cutoff states. When Q2 saturates and Q1 cuts off, current flows from Q3 through the motor in one direction. Reversing the input logic reverses current flow. PWM (Pulse Width Modulation) controls motor speed by applying variable voltage through PWM signals on one input while maintaining the other high. This creates variable average voltage across the motor. The circuit can control motors up to 1A with standard TIP122 transistors, or up to 3A by replacing freewheeling diodes with higher-current diodes like 1N5402.

Optocouplers (PC817) provide electrical isolation between high-voltage primary and low-voltage secondary sides. The optocoupler receives feedback from the TL431 and transmits it optically to the controller's feedback pin. A 1kΩ resistor limits current through the optocoupler LED. The complete circuit integrates all stages: input protection, rectification, filtering, switching, and regulation. Once assembled, the circuit demonstrates proper operation with regulated outputs and stable switching action.

A MOSFET driver circuit using an optocoupler (PC817) provides galvanic isolation between the control circuit (microcontroller or digital logic) and the power stage, preventing ground loop interference and protecting the controller when driving inductive loads like DC motors, relays, or solenoids. The circuit uses a PC817 optocoupler with a 1kΩ resistor for the LED input, and a complementary push-pull configuration with BC337 (NPN) and BC327 (PNP) transistors to drive an N-channel MOSFET (such as IRF640) from a 12V power supply. This topology allows the controller to operate at 5V or 3.3V while safely switching higher voltage loads.

An optocoupler (also called optoisolator or photocoupler) is an electronic component that optically couples signals between two circuits while providing complete electrical isolation, making it ideal for applications requiring galvanic isolation such as connecting low-voltage circuits to high-voltage circuits, protecting sensitive electronics from voltage spikes and noise, and preventing ground loops; it consists of an infrared LED on one side and a photosensitive device (such as a phototransistor, photodarlington, photo-SCR, or photo-triac) on the other side, where the LED converts electrical signals to light and the photosensitive device converts the light back to electrical signals, with key specifications including forward current/voltage, current transfer ratio (CTR), switching characteristics, isolation voltage, and common-mode noise rejection.

Optocouplers provide galvanic isolation with isolation voltages up to 7500V, protecting control circuits from high-voltage loads. In advanced applications, optocouplers can be used with relays to create double galvanic isolation, where the optocoupler controls a relay, providing multiple layers of protection for sensitive microcontroller signals.

An optocoupler (optopair) consists of a light-emitting diode (LED) and a phototransistor in a single package, providing galvanic isolation between circuits. The LED emits infrared light when current flows through it, activating the phototransistor. The LED operates at low voltages (1-3V), while the phototransistor handles high voltages (tens to hundreds of volts). Four-pin optocouplers have two pins for the LED and two for the phototransistor. Current-limiting resistors are essential for both sides to prevent damage. The LED resistor calculation uses R_LED = (Vsupply_LED - VF_LED) / IF_LED, while the phototransistor resistor calculation simplifies to R_phototransistor ≈ Vsupply_phototransistor / I_C. Reading datasheets requires identifying pinout configurations and locating key parameters including maximum forward current (IF), maximum collector-emitter voltage (VCEO), forward voltage drop (VF), isolation voltage rating, and timing characteristics. Optocouplers enable safe control of high-voltage loads (up to hundreds of volts) by driving power transistors (P-channel or N-channel MOSFETs) while protecting the controlling circuit from voltage spikes and transients.

To determine if a heatsink is needed for a transistor, calculate the junction temperature using the thermal circuit model where power dissipation (P) acts as current, temperature as voltage, and thermal resistance (Rth) as resistance; if the calculated junction temperature exceeds the maximum rating from the datasheet, a heatsink is required. The heatsink selection process involves calculating the total thermal resistance (Rth(j-a) = Rth(j-c) + Rth(c-h) + Rth(h-a)), where Rth(j-c) is from the transistor datasheet, Rth(c-h) depends on mounting quality with thermal paste, and Rth(h-a) is specified in heatsink catalogs. For a desired junction temperature, solve for the required Rth(h-a) using Tj = Ta + P × Rth_total, then select a heatsink with equal or lower thermal resistance.

The transistor dissipates power equal to the product of collector-emitter voltage and collector current (P = Vce × Ic). As the potentiometer cursor is lowered, the voltage across the transistor increases while current decreases, but power dissipation can become significant. For example, with 0.3A current and 71V voltage, the transistor dissipates approximately 21W, requiring a heatsink. The power dissipation depends on load resistance—lower resistance allows more current, increasing dissipation. A 500Ω load is generally acceptable, but a 100Ω load would cause considerable power dissipation. Proper load calculation is essential to prevent transistor overheating.

MOSFETs (Metal-Oxide Semiconductor Field Effect Transistors) are voltage-controlled switching devices that function as variable resistors, where the resistance between drain and source (RDS-on) decreases rapidly once the gate-source voltage exceeds a threshold voltage, making them ideal for high-current applications compared to BJTs; when using MOSFETs, engineers must check the threshold voltage (VGS) for logic-level compatibility, calculate power dissipation using P = I² × RDS-on, and assess thermal requirements using the formula (Tjunction_max - Tambient) / θja to determine if a heat sink is necessary.

Power dissipation in a transistor is calculated as: P = IC × VCE. This power is dissipated as heat in the transistor. The maximum power dissipation occurs at the Q-point. Understanding power dissipation is crucial for thermal management and selecting appropriate transistors for specific applications.

Thermal resistance (Rth) measures opposition to heat flow, measured in Kelvin per Watt (K/W). For TO-220 packages without heatsinks, RthJC is approximately 90 K/W, meaning 1W causes 90°C temperature rise. The total thermal path includes: junction-to-case (RthJC), case-to-heatsink (RthCS), and heatsink-to-ambient (RthSA). To select a heatsink, calculate required RthSA using: RthSA = (Tj_max - Tamb) / P - RthJC - RthCS. For example, with Tj_max=150°C, Tamb=30°C, P=25W, RthJC=2.5 K/W, and RthCS=1 K/W, the required RthSA is 1.3 K/W.

An H-bridge circuit uses four transistors (two PNP and two NPN) arranged in a bridge configuration to control the direction of current flow through a DC motor, allowing it to rotate in both directions; the circuit includes diodes in antiparallel with each transistor to protect against back EMF generated by the motor when it is switched off, and resistors at the bases to limit current and protect the transistors.

An H-bridge is an electronic circuit topology consisting of four switches arranged in an H-shape that enables bidirectional control of DC motor rotation by switching the polarity of voltage applied to the motor terminals; it can operate in four main states—forward rotation (switches 1 and 4 closed), reverse rotation (switches 2 and 3 closed), coasting (all switches open), and braking (both ground or both VCC switches closed)—with speed control achieved through PWM signals when using MOSFETs instead of mechanical switches, though designers must avoid shoot-through conditions where opposing switches conduct simultaneously.

DC motors are widely used in electric bicycles, scooters, and small vehicles. The H-Bridge circuit, consisting of four transistors, enables bidirectional motor control by allowing users to select which transistors activate to determine rotation direction. Motor direction is controlled by reversing voltage polarity: positive to one terminal and negative to the other causes clockwise rotation, while reversing polarity causes counterclockwise rotation. The H-Bridge can also control speed through Pulse Width Modulation (PWM), where varying the duty cycle changes the average voltage delivered to the motor.

A MOSFET H-bridge circuit uses four MOSFETs arranged in an H-configuration to control the direction of a DC motor by switching the polarity of the voltage applied to the motor terminals; when one pair of MOSFETs is activated, the motor rotates in one direction, and when the opposite pair is activated, the motor rotates in the reverse direction.

An H-bridge circuit uses four NPN transistors (such as 2N4401) to enable bidirectional control of a DC motor from an Arduino, where applying digital high to one transistor pair and low to the other causes the motor to spin in one direction, while reversing these signals reverses the rotation, and PWM on the control lines adjusts motor speed.