onsemi BSR50

Overview

BSR50 from Fairchild Semiconductor is an NPN Darlington transistor optimized for high-current switching applications up to 0.5 A with DC current gain ≥1000 at 150 mA, V_CEO = 45 V, and V_CE(sat) = 1.3 V at I_C = 500 mA / I_B = 500 µA. It is commonly used in relay drivers, lamp drivers, and low-frequency power switches in industrial control modules.

For engineers reviewing the BSR50 datasheet, pinout, applications, or equivalent options, key selection considerations include its Darlington configuration for high h_FE, TO-92 package thermal limitations (R_θJA = 200 °C/W), saturation voltage behavior across load current, and safe operating area defined by P_D = 625 mW at T_A = 25°C.

Technical Context

The BSR50 integrates two cascaded NPN transistors in a monolithic Darlington pair, delivering minimum h_FE of 1000 at I_C = 150 mA and 2000 at I_C = 0.5 A. Its breakdown ratings-V_CEO = 45 V, V_CBO = 60 V, and V_EBO = 5 V-are specified under standard test conditions with I_C or I_E fixed at microampere levels.

Thermal performance is constrained by its TO-92 plastic package: R_θJC = 83.3 °C/W and R_θJA = 200 °C/W, requiring derating above 25°C at 5.0 mW/°C. Saturation voltages are characterized at two operating points: V_CE(sat) = 1.3 V (I_C = 500 mA, I_B = 500 µA) and 1.6 V (I_C = 1.0 A, I_B = 4.0 mA).

Key Specifications

Pinout & Package

BSR50 is housed in a standard TO-92 plastic package with 3 leads. Pin 1 is Emitter, Pin 2 is Collector, Pin 3 is Base - confirmed per Fairchild Rev. A datasheet mechanical drawing and terminal identification.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar NPN Darlington transistor applications.

Compared with MPSA13 and ULN2003A, the BSR50 offers superior V_CEO margin (45 V) and predictable h_FE range (1000–2000) in a minimal footprint, making it preferable for single-load industrial switching where voltage robustness and thermal simplicity outweigh multi-channel or integrated-diode benefits.

Availability

BSR50 is available at Aetrix Electronics and suitable for relay drivers, lamp dimming interfaces, DC motor on/off control, and industrial sensor output stages requiring stable component supply and long-term industrial-grade availability.

Supply support for BSR50 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Fairchild Semiconductor was a U.S.-based semiconductor company specializing in power management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016.

The BSR50 belongs to Fairchild's general-purpose bipolar transistor product line, designed specifically for cost-sensitive, medium-current linear and switching applications in industrial controls and appliance electronics.

FAQ

What is the maximum continuous collector current for BSR50 in typical PCB mounting?

The BSR50 has an absolute maximum I_C rating of 1.5 A, but continuous operation is limited by thermal dissipation. With R_θJA = 200 °C/W and P_D = 625 mW at 25°C, sustained current must remain below ~200 mA on standard FR-4 without copper pour. Derating reduces usable I_C further at elevated ambient temperatures. The BSR50 is best applied in pulsed or intermittent duty cycles above 300 mA.

Does BSR50 have integrated base resistors or protection diodes?

No, the BSR50 is a bare Darlington transistor without internal base resistors or clamp diodes. External components-including a series base resistor for current limiting and a flyback diode across inductive loads-are required for reliable operation. This distinguishes it from integrated driver arrays like ULN2003A. The BSR50 provides design flexibility but places full responsibility for drive and protection on the circuit designer.

Can BSR50 be used as a linear amplifier, or is it intended only for switching?

The BSR50 can operate in active (linear) mode, but its high h_FE and thermal limitations make it suboptimal for precision amplification. It is explicitly characterized and qualified for switching applications-especially relay and lamp driving-where saturation and cutoff states dominate. For linear use, V_CE(sat) and thermal runaway risk require careful biasing; the BSR50 datasheet does not specify small-signal parameters like f_T or noise figure, confirming its primary role as a power switch.

What is the recommended base resistor value when driving BSR50 from a 5 V microcontroller GPIO?

To ensure saturation at I_C = 500 mA, the BSR50 requires I_B ≥ 500 µA (per h_FE ≥1000). With V_BE(sat) ≈ 0.9 V, a 5 V GPIO yields ~4.1 V across the base resistor. A 6.8 kΩ resistor delivers ~600 µA, providing margin. Lower values (e.g., 4.7 kΩ) may be used for faster turn-on but increase MCU drive burden. The BSR50's base resistor must be selected based on actual load current-not just logic voltage-to guarantee deep saturation and avoid thermal overstress.

Is BSR50 RoHS compliant and lead-free?

The original Fairchild BSR50 (Rev. A, May 2002) predates mandatory RoHS implementation and was manufactured with leaded solderability. Current production equivalents sourced through authorized channels may be RoHS-compliant and lead-free, but the legacy BSR50 part number itself is not inherently RoHS-certified. Engineers specifying BSR50 for new designs should verify compliance status with Aetrix Electronics' latest lot documentation or request Pb-free alternatives such as the ON Semiconductor NSS12500UFR2G (if functionally aligned).