Infineon BC817K-40: Key Features and Application Circuits
The Infineon BC817K-40 is a widely used NPN bipolar junction transistor (BJT) housed in a compact SOT-23 package. As a general-purpose amplifier and switch, it is a fundamental component in modern electronic design, prized for its reliability and performance in a broad range of low-power applications.
Key Features
The BC817K-40 is characterized by several critical parameters that make it a versatile choice for designers:
High Current Gain: The `-40` suffix denotes a DC current gain (hFE) classification between 250 and 600 at IC=2mA, VCE=5V. This high gain allows for significant output current control with a very small input base current.
Low Saturation Voltage: With a collector-emitter saturation voltage (VCE(sat)) typically around 0.5V at IC=500mA, the transistor operates efficiently as a switch, minimizing power loss and heat generation in the on-state.
High Current Capability: It can handle a continuous collector current (IC) of up to 500 mA, making it suitable for driving relays, LEDs, motors, and other peripherals.
Small Form Factor: The SOT-23 surface-mount package is ideal for space-constrained PCB designs, enabling high-density board layouts.
Application Circuits
Two of the most common circuit configurations for the BC817K-40 are the low-side switch and the digital logic level shifter.
1. Low-Side Switch for Peripheral Driving
This is the most frequent use case for the BC817K-40. Here, the transistor acts as a switch to control a load (e.g., an LED, relay, or small DC motor) by connecting it between the collector and the positive supply rail (Vcc). The emitter is connected to ground.
Operation: When a small control signal (typically 3.3V or 5V from a microcontroller GPIO pin) is applied to the base resistor (Rb), it allows a base current to flow, saturating the transistor. This creates a low-resistance path from the collector to the emitter, effectively connecting the load to ground and turning it on. The base resistor is crucial for limiting the base current to a safe value. A flyback diode is essential when driving inductive loads like relays or motors to protect the transistor from voltage spikes generated when the current is suddenly switched off.
2. Digital Logic Level Shifting
Microcontrollers often operate at different voltage levels (e.g., 3.3V and 5V). The BC817K-40 can be used to interface a 3.3V logic signal to control a 5V circuit.
Operation: The 3.3V signal from a microcontroller is applied to the base through a resistor. When the input is HIGH, the transistor saturates, pulling the output line (connected to the collector via a pull-up resistor to 5V) down to near ground (LOW). When the base input is LOW, the transistor is off, and the pull-up resistor pulls the output up to 5V (HIGH). This inverts the logic signal but provides a simple and effective way to shift voltage levels from 3.3V to 5V. An additional inverter can be used if the logic polarity must be preserved.
The Infineon BC817K-40 stands as a testament to the enduring utility of the BJT. Its optimized blend of high current gain, robust current handling, and low saturation voltage in a miniature package ensures its continued relevance. For designers seeking a reliable, cost-effective solution for switching and amplification in consumer electronics, industrial controls, and IoT devices, the BC817K-40 remains an excellent and fundamental choice.
Keywords: NPN Transistor, SOT-23, Low-Side Switch, Saturation Voltage, Current Gain
