onsemi H21B6

Overview

H21B6 from Fairchild Semiconductor is a gallium arsenide infrared emitting diode coupled with a silicon photodarlington in a plastic optical interrupter housing, designed for mechanical position sensing and object detection. It delivers 2.0 mA on-state collector current at 2 mA input, 10 mA at 5 mA input, and 25 mA at 10 mA input, with 55 V V_CEO, 1.0 V V_CE(SAT), and operates from −55°C to +100°C. It is used in industrial encoder systems requiring reliable edge-triggered signal interruption.

For engineers reviewing the H21B6 datasheet, pinout, applications, or equivalent options, key selection criteria include its 2.0–25 mA I_C(ON) range across input currents, 45 µs turn-on time at 750 Ω load, 250 µs turn-off time, opaque housing for ambient light rejection, and 0.35" aperture width for precise mechanical alignment.

Technical Context

The H21B6 functions as a self-contained opto-isolated switch: infrared LED emission triggers a photodarlington output stage, providing high DC current gain (I_C/I_F up to ~2500× at I_F = 10 mA) without external biasing. Its open-collector output requires an external pull-up resistor and supports direct interfacing with TTL/CMOS logic inputs.

It uses a fixed-gap mechanical interrupter architecture with a 0.35" aperture, enabling deterministic switching via physical obstruction. The device achieves ambient light immunity through opaque housing and spectral filtering inherent to GaAs LED / Si photodarlington pairing, eliminating need for external shielding in most industrial enclosures.

Key Specifications

Pinout & Package

Package: 4-pin DIP (dual in-line package) with 0.3" row spacing, plastic opaque housing, 0.35" optical gap, and lead finish compatible with RMA flux and methanol cleaning.

Key Features

Applications

Equivalent & Alternatives

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

Compared with H21B5 and PC817XNIP0X, the H21B6 provides highest output current drive and widest temperature range, making it optimal for demanding industrial motion control where robustness and margin are critical.

Availability

H21B6 is available at Aetrix Electronics and suitable for industrial rotary encoders, conveyor belt object detection, printer paper jam sensors, and automotive throttle position feedback requiring stable component supply and long-term lifecycle support.

Supply support for H21B6 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 optocoupler and optointerrupter portfolio remains widely deployed in industrial systems.

The H21B6 belongs to Fairchild's H21Bx photodarlington optical interrupter family, engineered specifically for high-gain, ambient-light-immune mechanical position sensing in cost-sensitive industrial automation equipment.

FAQ

What is the maximum continuous forward current rating for the H21B6 emitter diode?

The H21B6 emitter diode has a maximum continuous forward current (I_F) rating of 50 mA at 25°C ambient temperature. Derating is required above 25°C at 1.33 mW/°C. This rating ensures long-term reliability of the GaAs infrared LED while supporting pulsed operation up to 60 mA for transient detection events. Always verify thermal design when operating near this limit in enclosed environments.

Does the H21B6 require an external base connection for the photodarlington output stage?

No, the H21B6 photodarlington output stage is internally connected and requires no external base terminal. It operates as a two-terminal (collector/emitter) open-collector device. Pin 3 is the collector and Pin 4 is the emitter; no third connection is needed. This simplifies PCB layout and eliminates base biasing complexity compared to discrete phototransistor solutions.

Can the H21B6 be used in a 24 V DC control circuit?

Yes, the H21B6 can be used in 24 V DC control circuits because its V_CEO rating is 55 V, providing adequate margin against transients and ripple. When used with a properly sized pull-up resistor (e.g., 2.2 kΩ), the H21B6 delivers a clean logic-low output (<1.0 V) and logic-high level near 24 V. Confirm that the load does not exceed 25 mA maximum I_C(ON) at the chosen LED drive current.

What is the aperture width of the H21B6 and why does it matter mechanically?

The H21B6 has a 0.35" (8.9 mm) aperture width, defined by the physical gap between emitter and detector within the opaque plastic housing. This dimension sets the maximum thickness of an interrupting flag or slotted disk that can reliably pass through the gap while maintaining optical coupling. It also determines mechanical alignment tolerance during assembly-tighter apertures improve resolution but increase sensitivity to misalignment.

How does ambient temperature affect the H21B6's on-state collector current?

Ambient temperature affects the H21B6's I_C(ON) nonlinearly: output current decreases as temperature rises above 25°C, with normalized I_C(ON) dropping to ~70% at 100°C (per Figure 2). This reduction is due to photodarlington gain degradation and LED efficiency loss. For high-temperature applications, design with worst-case I_C(ON) at max T_A-e.g., use 10 mA I_F to ensure ≥17.5 mA output at 100°C.