onsemi H21A6

Overview

H21A6 from Fairchild Semiconductor is a gallium arsenide infrared LED coupled with a silicon photodarlington in a plastic optical interrupter package, functioning as a slot-type photo-interrupter switch. It delivers 4.0 mA on-state collector current at IF = 20 mA and VCE = 5 V, features 55 V BV_CEO, 0.40 V V_CE(SAT), and operates across –55°C to +100°C for position sensing in industrial encoders.

For engineers reviewing the H21A6 datasheet, pinout, applications, or equivalent options, key selection criteria include its 0.60 normalized current transfer ratio (IC/IF), 8 µs turn-on time, 50 µs turn-off time, and mechanical gap design enabling reliable opaque-flag detection in motion control systems.

Technical Context

The H21A6 integrates an IR emitter and photodarlington detector in a single molded housing with a fixed optical gap, optimized for ambient light rejection and mechanical resolution. Its photodarlington output provides high current gain, enabling direct interface with logic-level inputs without external amplification.

It operates under pulsed input conditions (PW = 100 µs, PRR = 100 pps) and supports load resistances up to 10 kΩ while maintaining stable switching performance. The device is rated for 50 mA emitter forward current and 20 mA collector current, with thermal derating applied above 25°C.

Key Specifications

Pinout & Package

Package: 4-pin DIP-style plastic optical interrupter with 0.315" (8.0 mm) gap width, opaque housing, and 0.035" apertures. Dimensions conform to Fairchild DS300291 mechanical drawing.

Key Features

Applications

Equivalent & Alternatives

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

Compared with H21A5 and TCST2103, the H21A6 offers the highest CTR and fastest switching among these three interrupters, making it optimal for new designs requiring minimal LED drive, fast response, and DIP-compatible layout - especially where ambient light rejection and wide temperature operation are critical.

Availability

H21A6 is available at Aetrix Electronics and suitable for industrial encoder position sensing, printer paper jam detection, automotive throttle position feedback, and factory automation limit switch applications requiring stable component supply and long-term manufacturability.

Supply support for H21A6 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 semiconductor manufacturer, acquired by ON Semiconductor in 2016; its legacy optocoupler and optointerrupter products remain widely deployed and supported.

The H21A6 belongs to the H21A series of optical interrupter switches designed specifically for non-contact position and presence detection in cost-sensitive industrial and consumer motion-control systems.

FAQ

What is the maximum continuous forward current for the H21A6 emitter?

The H21A6 emitter supports a maximum continuous forward current (I_F) of 50 mA at T_A = 25°C. Derating applies above 25°C at 0.83 mA/°C. This rating ensures reliable LED lifetime when used with appropriate current-limiting resistors in applications such as motor encoders and paper path sensors. Always verify I_F against actual operating conditions in the H21A6 datasheet.

Does the H21A6 require external biasing for the photodarlington output?

No, the H21A6 photodarlington output operates in common-emitter configuration and requires only a collector pull-up resistor (typically 2.5–10 kΩ) to generate a logic-compatible output. PIN 4 (emitter) is grounded, and PIN 3 (collector) sources the switched signal. No base biasing or additional transistors are needed - the H21A6 is designed for direct interface with TTL or CMOS inputs.

What is the mechanical gap dimension of the H21A6 package?

According to Fairchild DS300291, the H21A6 has a nominal mechanical gap width of 0.315" (8.0 mm), with tolerance ±0.010". This fixed gap accommodates standard flag thicknesses used in rotary encoders and linear position sensors. The gap geometry is integral to the H21A6's function as an optical interrupter switch and must be preserved in mechanical design.

Can the H21A6 operate reliably at 100°C ambient temperature?

Yes, the H21A6 is rated for continuous operation from –55°C to +100°C (T_OPR). At 100°C, its normalized output current remains ≥0.6× room-temperature value per Figure 2, and leakage stays within spec. For sustained 100°C use, ensure adequate PCB copper area for thermal dissipation and verify V_CE and I_F derating per the H21A6 absolute maximum ratings table.

Is the H21A6 RoHS compliant and lead-free?

The original H21A6 was manufactured prior to full RoHS implementation and contains lead in the solderable terminations and internal die attach. While not RoHS-compliant by modern standards, it remains available for legacy repair and industrial maintenance. For new designs requiring RoHS compliance, consult ON Semiconductor's updated optointerrupter portfolio - the H21A6 itself does not meet current RoHS Directive 2011/65/EU requirements.