onsemi H21A5

Overview

H21A5 from Fairchild Semiconductor is a gallium arsenide infrared LED coupled with a silicon photodarlington in a plastic optical interrupter package, functioning as a reflective/through-beam proximity or position sensor. It delivers 2.0 mA on-state collector current at IF = 20 mA and VCE = 5 V, features 55 V BVCEO, 0.40 V VCE(SAT), and operates across –55°C to +100°C for industrial motion detection.

For engineers reviewing the H21A5 datasheet, pinout, applications, or equivalent options, key selection criteria include its 2.0 mA IC(ON) output drive capability, 8 µs turn-on time, 50 µs turn-off time, and compatibility with 5 V logic-level interfacing in encoder and limit-switch circuits.

Technical Context

The H21A5 integrates an IR emitter and photodarlington detector in a single molded housing with a mechanical gap optimized for ambient light rejection and mechanical resolution. Its photodarlington output provides high current gain, enabling direct interface with TTL/CMOS loads without external amplification.

It operates under pulsed conditions (PW = 100 µs, PRR = 100 pps) for thermal management and achieves stable performance across –55°C to +100°C ambient range, with normalized output current varying less than ±20% over temperature at IF = 20 mA.

Key Specifications

Pinout & Package

Package: 4-pin DIP-style plastic optical interrupter housing with 0.103" (2.60 mm) nominal gap width, opaque black housing, and 0.035" apertures for precise beam alignment.

Key Features

Applications

Equivalent & Alternatives

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

Compared with H21A6 and PC817XNIP0X, the H21A5 strikes a balance between output drive strength and thermal efficiency in self-contained interrupter designs, offering optimal cost-performance for mid-speed industrial position sensing without isolation or ultra-high-current requirements.

Availability

H21A5 is available at Aetrix Electronics and suitable for industrial automation, printer subsystems, and motor control applications requiring stable component supply, long-term lifecycle support, and consistent optical performance across temperature extremes.

Supply support for H21A5 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; legacy products maintain full datasheet compliance and industrial qualification.

The H21A5 belongs to the H21A series of optically coupled interrupters designed specifically for non-contact position and motion sensing in harsh industrial environments where reliability and ambient light immunity are critical.

FAQ

What is the maximum continuous forward current rating for the H21A5?

The H21A5 has a maximum continuous forward current (IF) rating of 50 mA per its Absolute Maximum Ratings table. This value is specified at TA = 25°C with linear derating of 1.67 mW/°C above that temperature. Exceeding 50 mA risks LED degradation and reduced coupling efficiency over time. Always use a series current-limiting resistor calculated for the supply voltage and the H21A5's typical VF of 1.5 V at 20 mA.

Does the H21A5 require an external base connection for the photodarlington?

No, the H21A5 does not provide or require an external base connection. Its photodarlington output is two-terminal (collector and emitter only), with internal base driven solely by incident infrared light from the integrated GaAs LED. This simplifies PCB layout and eliminates sensitivity to stray capacitance or noise on a base node - a key advantage over discrete phototransistor solutions.

Can the H21A5 operate reliably at 100°C ambient temperature?

Yes, the H21A5 is fully rated for operation from –55°C to +100°C ambient temperature (TOPR). Its electrical parameters - including IC(ON), VCE(SAT), and leakage - are characterized across this full range, and Figure 2 confirms normalized output current remains within ±20% of room-temperature value at +100°C when driven at IF = 20 mA. Thermal design must ensure case temperature stays within limits under sustained load.

What is the typical forward voltage of the H21A5's infrared LED?

The typical forward voltage (VF) of the H21A5's GaAs infrared LED is 1.5 V at IF = 20 mA, with a maximum of 1.7 V under the same condition. This value is confirmed in the Electrical/Optical Characteristics table. Designers should use 1.5 V for nominal calculations and 1.7 V for worst-case current-limiting resistor sizing to ensure minimum drive current is maintained across manufacturing and temperature variation.

Is the H21A5 pin-compatible with other devices in the H21A series?

Yes, the H21A5 shares identical pinout, package dimensions, and mechanical footprint with H21A4 and H21A6 - all are 4-pin DIP-style optical interrupters with PIN 1 = Anode, PIN 2 = Cathode, PIN 3 = Collector, PIN 4 = Emitter. This allows direct substitution within the same socket or footprint when adjusting for different IC(ON) levels (0.15 mA, 2.0 mA, or 4.0 mA respectively) without PCB revision.