onsemi H21A2

Overview

H21A2 from Fairchild Semiconductor is a phototransistor-based optical interrupter switch comprising a GaAs infrared LED coupled to a silicon NPN phototransistor in a 4-pin DIP plastic housing with 0.125" (3.2 mm) pin spacing and 0.119" (3.0 mm) body width. It delivers 2.0 mA on-state collector current at IF = 20 mA, VCE = 5 V, with 8 µs turn-on and 50 µs turn-off times, and operates across –55°C to +100°C for position sensing in motor encoders.

For engineers reviewing the H21A2 datasheet, pinout, applications, or equivalent options, key selection criteria include guaranteed IC(ON) ≥2.0 mA at 20 mA input, VCEO = 30 V, saturation voltage ≤0.40 V, thermal derating of 1.33 mW/°C above 25°C, and mechanical gap dimensions optimized for opaque flag interruption.

Technical Context

The H21A2 implements a monolithic optocoupled interrupter architecture: an infrared emitter (IF max = 50 mA, VF typ = 1.7 V) optically aligned to a phototransistor detector (VCEO = 30 V, ICEO ≤100 nA) across a fixed air gap. Its switching behavior is governed by photon flux density and base-emitter junction sensitivity, not external biasing.

Electrical isolation is inherent-no galvanic connection exists between input and output sides. Timing performance (ton = 8 µs, toff = 50 µs) is specified under pulsed conditions (PW = 100 µs, PRR = 100 pps) with RL = 2.5 kΩ, and exhibits predictable temperature dependence per Figures 2–4 in DS300290.

Key Specifications

Pinout & Package

Package: 4-pin dual-in-line plastic housing (DIP), 0.125" (3.2 mm) pin pitch, 0.119" (3.0 mm) body width, 0.422" (10.7 mm) length, with molded optical gap. Pin 1 marked by dot or chamfered corner.

Key Features

Applications

Equivalent & Alternatives

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

Compared with HCPL-2630 and EE-SX1071, the H21A2 offers the best balance of cost, DIP mountability, and guaranteed 2.0 mA output for direct TTL interfacing-making it optimal for high-volume motor encoder and paper-path detection where moderate speed suffices.

Availability

H21A2 is available at Aetrix Electronics and suitable for motor encoder position sensing, printer paper jam detection, industrial limit switch replacement, and conveyor belt object counting requiring stable component supply and consistent optical interrupter performance across temperature extremes.

Supply support for H21A2 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; known for high-volume optoelectronics and power management ICs.

The H21A2 belongs to Fairchild's legacy optical interrupter family designed specifically for cost-sensitive, mechanically robust position and presence sensing in industrial automation and office equipment.

FAQ

What is the guaranteed minimum on-state collector current for H21A2?

The H21A2 guarantees IC(ON) ≥2.0 mA when driven with IF = 20 mA and biased at VCE = 5 V, as specified in the Electrical/Optical Characteristics table of DS300290. This value is measured under pulsed conditions (PW = 100 µs, PRR = 100 pps) and ensures reliable switching into standard logic inputs. The H21A2 datasheet confirms this parameter applies exclusively to the H21A2 variant-not H21A1 or H21A3.

Can H21A2 be used in place of H21A1 or H21A3?

No-H21A2 is not interchangeable with H21A1 or H21A3 due to differing optical coupling gains: H21A1 specifies IC(ON) ≥0.15 mA, H21A2 ≥2.0 mA, and H21A3 ≥4.0 mA at identical test conditions. Substituting H21A2 for H21A1 may overload downstream circuitry; using H21A1 instead of H21A2 risks insufficient output current. Each H21A2 variant must be selected based on required output drive strength.

What is the maximum continuous forward current rating for the H21A2 emitter?

The H21A2 emitter has a maximum continuous forward current (IF) rating of 50 mA, as stated in the Absolute Maximum Ratings table. Exceeding this value risks LED degradation or catastrophic failure. For reliable 20 mA operation-the condition under which IC(ON) is specified-a series resistor of ~165 Ω is recommended with a 5 V supply, accounting for VF ≈ 1.7 V. Derating applies above 25°C ambient.

Does H21A2 provide electrical isolation between input and output?

Yes, the H21A2 provides functional electrical isolation via optical coupling: the infrared LED (pins 1–2) and phototransistor (pins 3–4) share no conductive path. However, DS300290 does not specify isolation voltage rating (e.g., 5 kV), creepage, or clearance-so H21A2 is unsuitable for safety-critical isolation. It is intended for noise rejection and level-shifting, not regulatory-compliant galvanic separation.

What are the recommended soldering conditions for H21A2?

H21A2 is rated for hand soldering at 240°C for 5 seconds and flow soldering at 260°C for 10 seconds, per Note 2 in DS300290. RMA flux and methanol/isopropyl alcohol cleaning are recommended. Soldering iron tip must stay ≥1/16" (1.6 mm) from the housing to avoid lens distortion or internal bond damage. These limits ensure integrity of the epoxy lens and die attach without compromising optical alignment.