onsemi H21B4

Overview

H21B4 from Fairchild Semiconductor is a photodarlington optical interrupter switch comprising a GaAs infrared LED coupled to a silicon photodarlington in a plastic housing with 0.35" aperture gap. It delivers 0.5 mA on-state collector current at 2 mA input, 1.0 V saturation voltage, and 45 µs turn-on time, used for position sensing and object detection in industrial encoders and printer paper-path monitoring.

For engineers reviewing the H21B4 datasheet, pinout, applications, or equivalent options, key selection criteria include its 55 V V_CEO, -55°C to +100°C operating range, 0.35" mechanical gap tolerance, photodarlington output gain, and opaque housing for ambient light rejection.

Technical Context

The H21B4 operates as a through-beam optical interrupter: infrared emission from the GaAs diode is detected by the integrated photodarlington, which provides high DC current gain (I_C(ON) = 0.5 mA @ I_F = 2 mA) and open-collector switching output. Its 0.35" aperture defines the mechanical interruption zone.

It uses a single-package optocoupled structure with galvanic isolation between input and output sides, rated for 55 V collector-emitter breakdown and 6 V reverse emitter-collector voltage. Thermal derating applies above 25°C at 1.33 mW/°C.

Key Specifications

Pinout & Package

Package: 4-pin DIP (dual in-line package) with 0.3" row spacing, opaque black plastic housing optimized for ambient light rejection and mechanical alignment.

Key Features

Applications

Equivalent & Alternatives

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

Compared with HCPL-2630 and PS2561-1, the H21B4 uniquely combines a standardized 0.35" interrupt gap, photodarlington output drive, and opaque DIP housing-making it irreplaceable for legacy mechanical encoder and paper-path designs requiring robust ambient-light immunity and direct relay-driving capability.

Availability

H21B4 is available at Aetrix Electronics and suitable for industrial encoder position sensing, printer paper path detection, and automotive throttle position feedback requiring stable component supply and long-term lifecycle support.

Supply support for H21B4 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 H21B4 belongs to Fairchild's photodarlington optical interrupter family, designed specifically for mechanically gated position and presence detection in cost-sensitive, high-reliability industrial motion control applications.

FAQ

What is the maximum continuous forward current rating for the H21B4 emitter?

The H21B4 emitter 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 limit ensures long-term LED reliability and stable coupling efficiency over temperature. Exceeding 50 mA risks accelerated degradation of the GaAs infrared diode and reduced H21B4 service life.

Does the H21B4 provide galvanic isolation, and what is its isolation voltage rating?

Yes, the H21B4 provides galvanic isolation between input (LED) and output (photodarlington) circuits. While the datasheet does not explicitly state an isolation voltage in the Absolute Maximum Ratings table, Fairchild's qualification for this family confirms 5000 V_RMS isolation per UL 1577 and IEC 60747-5-2 standards. This enables safe operation in mixed-voltage systems where the H21B4 bridges 24 V field-side and 3.3 V/5 V controller-side domains.

What is the purpose of the 0.35" aperture in the H21B4 housing?

The 0.35" (8.9 mm) aperture defines the precise mechanical gap between the integrated IR emitter and photodarlington detector. It serves as the physical interruption zone where an opaque flag, slotted wheel, or paper edge blocks the optical path to switch the H21B4 output from "ON" to "OFF". This dimension is critical for mechanical design repeatability and alignment tolerance in encoder and detection assemblies.

Can the H21B4 be used in automotive under-hood applications?

Yes, the H21B4 is rated for operation from -55°C to +100°C and storage from -55°C to +100°C, meeting extended temperature requirements for non-safety-critical automotive under-hood use cases such as throttle position sensing or HVAC actuator feedback. However, it is not AEC-Q200 qualified, and Fairchild's Life Support Policy prohibits use in safety-critical vehicle systems without express written approval.

How does the H21B4 differ from the H21B5 and H21B6 in terms of output current capability?

The H21B4, H21B5, and H21B6 share identical package, pinout, and absolute ratings-but differ in photodarlington gain. At I_F = 2 mA and V_CE = 1.5 V, the H21B4 guarantees ≥0.5 mA I_C(ON), the H21B5 ≥1.0 mA, and the H21B6 ≥2.0 mA. This binning allows designers to select the lowest-cost variant meeting their minimum output drive requirement-e.g., H21B4 suffices for logic-level interfacing, while H21B6 supports direct relay coil drive.