onsemi BC517

Overview

BC517 from Fairchild Semiconductor is an NPN Darlington transistor optimized for high-current-switching applications up to 1.2 A, featuring a minimum DC current gain (h_FE) of 30,000 at IC = 20 mA and V_CE = 2.0 V, V_CEO = 30 V, and TO-92 package. It serves as a high-gain interface between low-power control logic and moderate-power loads such as relays, solenoids, and LED arrays.

For engineers reviewing the BC517 datasheet, pinout, applications, or equivalent options, key selection considerations include its Darlington architecture enabling microampere-level base drive, saturation voltage of 1.0 V at IC = 100 mA/IB = 0.1 mA, thermal resistance R_θJA = 200 °C/W, and suitability for industrial control and discrete power switching where gain and simplicity outweigh speed requirements.

Technical Context

The BC517 implements a monolithic NPN Darlington pair with integrated base-emitter resistor network absent - requiring external base current limiting. Its high h_FE enables direct drive from CMOS/TTL outputs without additional amplification stages, while V_CEO = 30 V and IC_max = 1.2 A define its safe operating area for DC and low-duty-cycle pulsed loads.

Thermal design relies on R_θJA = 200 °C/W and R_θJC = 83.3 °C/W, with maximum junction temperature limited to 150 °C. The device is fabricated in Fairchild's Process 05 and rated for operation from –55 °C to +150 °C, supporting industrial ambient environments.

Key Specifications

Pinout & Package

Package: TO-92 - Standard through-hole plastic package with 3.60 mm ±0.20 mm body width, 4.58 mm ±0.20 mm height, and 1.27 mm lead pitch; compatible with manual soldering and wave-solder processes.

Key Features

Applications

Equivalent & Alternatives

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

Compared with MPSA14 and BDX53C, the BC517 delivers superior current gain and lower saturation voltage in the TO-92 form factor, making it optimal for space-constrained, low-drive, medium-power switching - whereas MPSA14 trades gain for broader availability, and BDX53C shifts to higher-power domains with different thermal and layout constraints.

Availability

BC517 is available at Aetrix Electronics and suitable for industrial control panels, relay interface modules, and LED driver circuits requiring stable component supply and long-term obsolescence management.

Supply support for BC517 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 manufacturer specializing in power management, analog, and discrete devices before its acquisition by ON Semiconductor in 2016.

The BC517 belongs to Fairchild's general-purpose bipolar transistor family designed specifically for high-gain, medium-current switching in industrial and consumer equipment where simplicity, reliability, and cost efficiency are prioritized.

FAQ

What is the maximum continuous collector current rating for the BC517?

The BC517 has a maximum continuous collector current (I_C) rating of 1.2 A at T_A = 25 °C. This value assumes adequate PCB copper area for heat dissipation and must be derated per the 5.0 mW/°C thermal coefficient above ambient temperature. Actual usable current depends on V_CE, duty cycle, and board-level thermal design - the BC517 datasheet specifies 1.2 A as the absolute steady-state limit under defined test conditions.

Does the BC517 include an internal base-emitter resistor?

No, the BC517 does not include an internal base-emitter resistor. It is a standard two-junction Darlington transistor requiring an external series base resistor to set operating point and prevent thermal runaway. Unlike digital transistors (e.g., MMBT2222A), the BC517 relies on discrete biasing - this gives designers full control over gain, speed, and saturation characteristics but mandates careful resistor selection based on target I_C and drive voltage.

What is the typical V_BE(on) of the BC517 and how does it affect drive requirements?

The BC517 exhibits a typical base-emitter on voltage (V_BE(on)) of 1.4 V at I_C = 10 mA and V_CE = 5.0 V. Because it is a Darlington pair, this represents the sum of two V_BE drops (~0.7 V each), resulting in higher forward voltage than a single transistor. Designers must account for this when sizing base resistors - for example, driving from a 3.3 V MCU GPIO requires at least 1.9 V headroom across the resistor, directly impacting minimum achievable base current and thus switching speed.

Can the BC517 be used in PWM applications?

Yes, the BC517 can be used in low-frequency PWM applications (typically ≤ 10 kHz), provided that switching losses and thermal accumulation are managed. Its relatively slow turn-off time (due to stored charge in the Darlington structure) limits high-speed use, and V_CE(sat) = 1.0 V at rated current means conduction losses remain low during on-time. For PWM, ensure gate/base drive is sufficiently strong to minimize transition time, and verify junction temperature stays within –55 °C to +150 °C under worst-case duty cycle and ambient conditions.

Is the BC517 RoHS compliant and what is its packaging format?

The BC517 Rev. A was manufactured prior to full RoHS enforcement and is not RoHS compliant - it contains lead in the die attach and lead frame plating. It is supplied in bulk TO-92 tape-and-reel or ammo-pack formats, with lead finish specified as SnPb (tin-lead). For new designs requiring RoHS compliance, consider modern alternatives like the ON Semiconductor NSS12201LT1G (SOT-23 Darlington), though package and gain characteristics differ significantly from the BC517.