onsemi H21B5

Overview

H21B5 from Fairchild Semiconductor is a photodarlington optical interrupter switch comprising a GaAs infrared LED coupled to a silicon photodarlington in a plastic DIP-4 package with 0.35" aperture gap. It delivers 1.0 mA on-state collector current at 2 mA input, 5.0 mA at 5 mA input, and 14 mA at 10 mA input under V_CE = 1.5 V, enabling reliable position sensing in mechanical encoders and limit switches.

For engineers reviewing the H21B5 datasheet, pinout, applications, or equivalent options, key selection criteria include its 55 V V_CEO, 1.0 V V_CE(SAT) at low drive, 45 µs turn-on time with 750 Ω load, ambient temperature range of −55°C to +100°C, and opaque housing for ambient light rejection in industrial motion control systems.

Technical Context

The H21B5 operates as a current-transfer device where infrared emission from the GaAs LED optically triggers the silicon photodarlington output stage, providing high DC current gain without external biasing. Its photodarlington configuration yields higher sensitivity than phototransistor-based interrupters but with slower switching (t_on/t_off up to 45/250 µs).

Designed for mechanical interruption via opaque flag insertion into the 0.35" gap, it functions as a digital on/off sensor with no electrical isolation required between input and output circuits. The DIP-4 package integrates emitter anode/cathode and detector collector/emitter terminals with standardized lead spacing for through-hole PCB mounting.

Key Specifications

Pinout & Package

Package: 4-pin dual-in-line plastic (DIP-4) with gull-wing leads, 0.3" row spacing, 0.1" lead pitch, and integrated optical gap.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar optical interrupter applications.

Compared with H21A5 and PC817XNIP0X, the H21B5 uniquely combines integrated mechanical gap, photodarlington sensitivity, and industrial temperature rating-making it optimal for ruggedized position detection where reliability trumps raw speed or isolation.

Availability

H21B5 is available at Aetrix Electronics and suitable for rotary encoders, printer jam detection, industrial limit switches, and conveyor object counting requiring stable component supply and long-term lifecycle support.

Supply support for H21B5 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 analog and power IC manufacturer acquired by ON Semiconductor in 2016; its legacy optoelectronics portfolio remains widely deployed in industrial systems.

The H21B5 belongs to Fairchild's photodarlington optical interrupter family, engineered specifically for cost-sensitive, high-reliability mechanical position sensing in harsh environments where ambient light immunity and wide temperature operation are essential.

FAQ

What is the maximum continuous forward current rating for the H21B5 emitter diode?

The H21B5 emitter diode has a maximum continuous forward current (I_F) rating of 50 mA at T_A = 25°C. Derating is required above 25°C at 1.33 mW/°C. This rating ensures long-term LED reliability while supporting typical drive currents of 2–10 mA used in most interrupter applications. Exceeding 50 mA risks accelerated degradation of the GaAs infrared emitter in the H21B5.

Does the H21B5 require an external base connection for the photodarlington output stage?

No, the H21B5 photodarlington output stage is internally connected and requires no external base terminal or biasing. Pins 3 (collector) and 4 (emitter) form a two-terminal output device that behaves like a high-gain, open-collector switch. This simplifies circuit design compared to discrete phototransistor solutions and eliminates base leakage path concerns in the H21B5.

What is the saturation voltage specification for the H21B5 under typical operating conditions?

The H21B5 specifies V_CE(SAT) ≤ 1.0 V when I_F = 10 mA and I_C = 1.8 mA, and ≤ 1.5 V when I_F = 60 mA and I_C = 50 mA. This low saturation voltage ensures efficient switching into logic-level loads and minimizes power loss in the output stage of the H21B5, especially important in battery-powered or thermally constrained designs.

Can the H21B5 be used in environments with strong ambient light exposure?

Yes-the H21B5 features an opaque plastic housing that blocks visible and near-infrared ambient light, and its GaAs LED emits at ~940 nm, matched to the peak spectral response of the silicon photodarlington. This combination provides high signal-to-noise ratio in daylight or fluorescent-lit factories, making the H21B5 suitable for unshielded installations where ambient light rejection is critical.

What is the recommended soldering profile for the H21B5 during PCB assembly?

The H21B5 supports wave soldering at 260°C for 10 seconds (flow) and hand soldering at 240°C for 5 seconds (iron), with soldering iron tip kept ≥1.6 mm from the housing. RMA flux and methanol/isopropyl alcohol cleaning are recommended. These parameters prevent thermal damage to the internal die bonds and epoxy encapsulation of the H21B5 during reflow or manual assembly.