onsemi H21A1

Overview

H21A1 from Fairchild Semiconductor is a phototransistor-based optical interrupter switch comprising a GaAs infrared LED coupled to a silicon phototransistor in a 4-pin DIP plastic housing. It delivers IC(ON) = 1.0 mA at IF = 20 mA and VCE = 5 V, with 30 V VCEO, 8 µs turn-on time, and operates from –55°C to +100°C. It is used for position sensing and object detection in industrial encoders and printer paper-path monitoring.

For engineers reviewing the H21A1 datasheet, pinout, applications, or equivalent options, key selection considerations include its 1.0 mA on-state collector current, 8 µs/50 µs switching speed, 30 V output breakdown rating, opaque housing for ambient light rejection, and mechanical gap design enabling reliable optical interruption.

Technical Context

The H21A1 integrates a gallium arsenide infrared emitter and silicon NPN phototransistor in a single molded package with a fixed optical gap. Its coupling efficiency is optimized for mechanical resolution and ambient light immunity, not spectral tuning or high-speed data transmission.

It operates in DC or pulsed mode with no internal biasing or signal conditioning circuitry. The phototransistor output stage requires external load resistance (e.g., 2.5 kΩ) and supply voltage (≤5 V), and exhibits saturation voltage ≤0.40 V under specified drive conditions.

Key Specifications

Pinout & Package

Package: 4-pin dual-in-line plastic housing with 0.3" lead spacing and integral optical gap; dimensions per Fairchild DS300290 (19.2 mm × 8.0 mm × 12.0 mm).

Key Features

Applications

Equivalent & Alternatives

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

Compared with LTH-1550 and TCST1103, the H21A1 offers lower cost and standard DIP compatibility but trades off speed and shielding-making it optimal for cost-sensitive, moderate-speed industrial detection where board space and assembly simplicity are prioritized.

Availability

H21A1 is available at Aetrix Electronics and suitable for industrial encoder position sensing, printer paper path detection, automated assembly line counting, and door/gate safety interlock applications requiring stable component supply across extended production lifecycles.

Supply support for H21A1 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-reliability optocouplers and discrete power devices.

The H21A1 belongs to Fairchild's legacy optical interrupter family designed specifically for robust, low-cost mechanical position and presence detection in industrial automation and office equipment.

FAQ

What is the maximum continuous forward current for the H21A1 emitter?

The H21A1 emitter supports a maximum continuous forward current (IF) of 50 mA at TA = 25°C. Above 25°C, power dissipation must be derated linearly at 1.33 mW/°C. This rating ensures long-term LED reliability under sustained operation and aligns with typical drive circuits using series resistors referenced to 5 V supplies. Always verify thermal layout when operating near this limit in the H21A1 design.

Does the H21A1 require an external base connection for the phototransistor?

No, the H21A1 phototransistor is a two-terminal (collector–emitter) device with no base terminal exposed; it operates solely on photocurrent generated by the coupled IR LED. This simplifies circuit design by eliminating base bias networks or sensitivity adjustments. The H21A1 functions as a self-contained optically triggered switch-no external base connections are present or required in the H21A1 pinout or application.

What is the typical on-state collector current (IC(ON)) of the H21A1 under standard test conditions?

The typical on-state collector current IC(ON) of the H21A1 is 1.0 mA when driven with IF = 20 mA and measured at VCE = 5 V. This value is confirmed in the Fairchild DS300290 datasheet and reflects the device's guaranteed minimum performance under pulsed conditions. Designers should use this figure to size pull-up resistors and validate logic-level compatibility in the H21A1 interface circuit.

Can the H21A1 operate reliably at –40°C ambient temperature?

Yes, the H21A1 is fully specified from –55°C to +100°C for both operating and storage conditions. At –40°C, normalized output current remains ≥0.8× the 25°C value (per Figure 2), and leakage current stays below 100 nA. No derating or special startup sequencing is required-the H21A1 maintains functional integrity across this full range, making it suitable for outdoor or refrigerated industrial deployments.

Is the H21A1 RoHS compliant and halogen-free?

The original Fairchild H21A1 was manufactured prior to full RoHS implementation and is not certified RoHS-compliant or halogen-free. It contains lead in solderable terminations and may include brominated flame retardants in the housing. For new designs requiring compliance, consult ON Semiconductor's updated equivalents (e.g., FODM8071) or confirm material declarations via official traceable lot documentation for the H21A1.