This can be suitable for most LED & motor control applications. It can handle up to 8A current spikes for short periods of time. Look for the maximum collector-emitter voltage ( V CE) and collector current ( I C) ratings specified in the transistor’s datasheet.įor example, the TIP120 NPN Darlington transistor has an I C maximum rating of 5A according to its datasheet. Ensure that the transistor you select can handle the voltage levels and current demands of your load. You need to consider the voltage and current requirements of your project. Selecting a Transistor For Your Arduino Project Now, the load R L (can be a motor, LED, or whatever) will have no voltage difference across its terminals, and no current will flow through it because the circuit is opened. We drive the Vin voltage to the transistor’s base pin to LOW (0v) using any IO pin of our microcontroller (Arduino), this will prevent any current from flowing through the base of the transistor causing it to go into the cut-off region (the transistor will not conduct and becomes like an open switch).Ĭonsequently, the Vout will be equal to the V CC which is the supply voltage of your circuit. The transistor is fully turned OFF, and the base-emitter & base-collector junctions are reverse-biased. In this region, the BJT acts as an open switch, preventing any significant current flow from the collector to the emitter. Transistor Switching OFF (Cut-OFF Region) Now, the load R L (can be a motor, LED, or whatever) will have a voltage difference across its terminals, and current (I C) will flow through it because the circuit is closed. We apply the Vin voltage to the transistor’s base pin using any IO pin of our microcontroller (Arduino), this base voltage will push a small current (I B) through the base of the transistor causing it to go into saturation region (the transistor becomes like a closed switch).Ĭonsequently, the Vout will be equal to the V CEsat of the transistor (around 0.2v) which is nearly zero. The transistor is fully turned ON, and the base-emitter & base-collector junctions are forward-biased. In this region, the BJT operates as a closed switch, allowing a large current to flow from the collector to the emitter ( I C). Transistor Switching ON (Saturation Region) Instead, we’ll be using the transistor as a switch, which means we’ll bias the BJT to operation in either Saturation (ON) or Cut-off (OFF). We’re not interested in the linear region (active mode) of the transistor as we’re not going to use it as an amplifier or any other signal generation or processing application. The IV characteristics curve of a BJT transistor is shown below and it does indicate the 3 operating regions (modes) of a transistor. How Do Transistors Work?Ī BJT ( Bi-Polar Junction Transistor) can be operated in 3 different regions (modes) by controlling the base voltage (biasing voltage) which we apply to the base pin of the transistors. We’ll discuss how to properly do it hereafter in this tutorial. We just need to set (bias) the transistor properly to work as a switch (ON/OFF). But also the output voltage is only 5v which is very low to drive some output devices like 12v relays, DC motors, or 12v LEDs.įor both reasons, high output current delivery capability & high output voltage, we typically use transistors with Arduino to achieve such requirements with ease. Moreover, the digital output signal that you can get out of Arduino IO pins is (0v to 5v) which is not only insufficient in terms of current delivery capability. Instead, we use the digital output signal from the Arduino IO pins to trigger a transistor (that works as an electronic switch) to turn ON or OFF the high-current load device (e.g. Why Use Transistors With Arduino?īecause the IO pins of the Arduino are not designed to deliver high-current to drive loads like motors or power LEDs. Therefore, we typically use transistors with Arduino as output drivers for high-current load devices (such as Power LEDs, DC Motors, Stepper Motors, etc). All Arduino IO pins can source or sink up to 200mA combined at any given time.Īs a rule of thumb, the Arduino IO pins are not capable of driving any sort of output load devices other than small LEDs, a buzzer, or any tiny device that require a few mA of current to operate. This is the maximum absolute current that can be sourced from or sunk to any Arduino’s IO pin at any given time. The Arduino’s IO pins are able to source or sink up to 40mA of current per IO pin.
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