onsemi H21A3

Overview

H21A3 from Fairchild Semiconductor is a phototransistor-based optical interrupter switch comprising a GaAs infrared LED coupled to a silicon phototransistor in a molded plastic housing with 3.2 mm gap. It delivers IC(ON) = 4.0 mA at IF = 20 mA, VCE = 5 V; VCEO = 30 V; and operates from −55°C to +100°C. It is used in position sensing, object detection, and encoder applications where mechanical interruption of an optical path triggers state change.

For engineers reviewing the H21A3 datasheet, pinout, applications, or equivalent options, key selection criteria include guaranteed minimum current transfer ratio (CTR ≥ 0.60), saturation voltage ≤ 0.40 V, turn-off time ≤ 50 µs, and compatibility with 5 V logic-level interfacing in industrial motion control systems.

Technical Context

The H21A3 implements a monolithic optocoupled structure with emitter and sensor die aligned across a fixed 3.2 mm air gap. Its phototransistor output stage operates in common-emitter configuration with base left unconnected, enabling direct digital switching without external biasing.

It uses pulsed drive conditions (PW = 100 µs, PRR = 100 pps) for electrical/optical characterization, and its CTR is specified under IF = 20 mA, VCE = 5 V - a condition reflecting typical microcontroller GPIO-driven operation with series current limiting.

Key Specifications

Pinout & Package

Package: 4-pin DIP-4 with 3.2 mm optical gap, opaque black housing, 0.100" (2.54 mm) lead pitch. Dimensions conform to JEDEC MS-012AC.

Key Features

Applications

Equivalent & Alternatives

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

Compared with TCST1103 and EE-SX1041, the H21A3 provides the highest guaranteed CTR (≥ 0.60), widest optical gap (3.2 mm), and through-hole DIP-4 package optimized for manual prototyping and rugged industrial serviceability - making it preferred for high-reliability mechanical sensing where signal margin and field maintainability are critical.

Availability

H21A3 is available at Aetrix Electronics and suitable for position sensing, rotary encoding, printer paper detection, and conveyor belt object counting requiring stable component supply across extended temperature ranges and long product lifecycles.

Supply support for H21A3 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 optoelectronics portfolio remains widely deployed in industrial and automotive systems.

The H21A3 belongs to Fairchild's H21A-series optical interrupters, designed specifically for robust, low-cost, ambient-light-resistant mechanical position and motion detection in harsh environments.

FAQ

What is the minimum guaranteed current transfer ratio (CTR) for H21A3?

The H21A3 has a minimum guaranteed current transfer ratio (CTR) of 0.60, measured at IF = 20 mA and VCE = 5 V. This value ensures that the phototransistor delivers at least 4.0 mA of collector current under those conditions. The CTR specification is critical for designing reliable digital interfaces without amplification. H21A3 maintains this CTR across its full operating temperature range of −55°C to +100°C, making it suitable for demanding industrial applications where thermal stability matters.

Can H21A3 be driven directly from a 3.3 V microcontroller GPIO?

Yes, H21A3 can be driven directly from a 3.3 V microcontroller GPIO using an appropriate current-limiting resistor. With a forward voltage (VF) ≤ 1.7 V at 60 mA, a 3.3 V source can deliver ~20 mA with a 75 Ω series resistor. This current satisfies the IF = 20 mA test condition used to guarantee the 4.0 mA IC(ON) output. The H21A3 datasheet confirms operation under pulsed conditions matching typical MCU output capability, and its low VF reduces power dissipation in the driving circuit.

What is the maximum allowable continuous forward current for the H21A3 LED?

The maximum continuous forward current (IF) for the H21A3 LED is 50 mA at TA = 25°C. Above 25°C, power dissipation must be derated linearly at 1.33 mW/°C. This limit ensures long-term reliability of the GaAs infrared emitter. Exceeding 50 mA risks accelerated degradation of optical output and reduced CTR over time. For typical operation delivering 4.0 mA output, 20 mA drive current is recommended - well within the 50 mA absolute maximum and aligned with the device's characterized performance points.

Does H21A3 require external base connection for the phototransistor?

No, the H21A3 phototransistor is configured with an unconnected base terminal - it operates as a two-terminal photosensor (collector and emitter only). This simplifies circuit design by eliminating base bias components and reducing sensitivity to stray capacitance. The device relies solely on incident infrared light from the integrated GaAs LED to generate collector current. All published electrical characteristics, including IC(ON), VCE(SAT), and switching times, are measured with the base left open, confirming its intended baseless operation.

What is the optical gap dimension of H21A3 and why does it matter?

The H21A3 features a fixed 3.2 mm optical gap between emitter and detector, defined by the molded plastic housing. This dimension determines the maximum thickness of opaque objects that can fully interrupt the optical path - such as 3 mm metal flags or plastic encoder discs. A wider gap increases mechanical tolerance and simplifies mechanical integration but slightly reduces coupling efficiency. The 3.2 mm gap balances robustness and performance, supporting reliable detection of standard industrial components while maintaining sufficient CTR margin for long-term operation.