onsemi H21A4

Overview

H21A4 from Fairchild Semiconductor is a gallium arsenide infrared LED coupled with a silicon photodarlington in a 4-pin DIP plastic interrupter housing, delivering 0.15× normalized output current at IF = 5 mA / VCE = 5 V, 1.0 mA on-state collector current at IF = 20 mA, and 0.40 V saturation voltage - used for position sensing and object detection in industrial encoders and printer paper-path monitors.

For engineers reviewing the H21A4 datasheet, pinout, applications, or equivalent options, key selection criteria include its 55 V BVCEO rating, 8 µs turn-on time, 50 µs turn-off time, and mechanical gap-based optical interruption capability in ambient-light-rejecting opaque housings.

Technical Context

The H21A4 integrates an IR emitter and photodarlington detector in a single molded package with a fixed 0.103" (2.60 mm) nominal gap. Its photodarlington output provides high current gain but slower switching than phototransistors - confirmed by ton = 8 µs and toff = 50 µs at RL = 2.5 kΩ.

It operates across –55°C to +100°C, supports 50 mA continuous emitter current and 20 mA collector current, and achieves ambient light rejection via opaque housing and 0.035" apertures - validated in Figures 1–6 of DS300291.

Key Specifications

Pinout & Package

Package: 4-pin dual-in-line (DIP) plastic housing with integral optical gap; dimensions per DS300291 - body width 0.422" (10.7 mm), length 0.957" (24.3 mm), gap height 0.103" (2.60 mm) nominal.

Key Features

Applications

Equivalent & Alternatives

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

Compared with H21A5 and PC817XNIP0X, the H21A4 offers the lowest-cost, drop-in-ready solution for standard 0.103" gap applications requiring 1 mA output and industrial temperature range - avoiding both over-specification and mechanical rework.

Availability

H21A4 is available at Aetrix Electronics and suitable for industrial encoders, printer paper-path monitors, and automotive throttle position sensors requiring stable component supply, long-lifecycle support, and RoHS-compliant sourcing.

Supply support for H21A4 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; known for high-reliability optoelectronics and power management solutions.

The H21A4 belongs to the H21A series of optical interrupter switches designed specifically for mechanical position and presence detection in cost-sensitive industrial and office equipment.

FAQ

What is the maximum continuous forward current for the H21A4 emitter?

The H21A4 emitter supports a maximum continuous forward current of 50 mA, as specified in the Absolute Maximum Ratings table. This rating applies at TA = 25°C with linear derating of 1.67 mW/°C above that temperature. Exceeding 50 mA risks irreversible degradation of the GaAs LED. Always use a series current-limiting resistor calculated for the target drive current - for example, 20 mA at 5 V supply requires ~200 Ω assuming VF ≈ 1.0 V.

Does the H21A4 require external biasing for the photodarlington output?

No, the H21A4 photodarlington output operates in common-emitter configuration with PIN 4 (emitter) grounded. A pull-up resistor on PIN 3 (collector) is required to generate a logic-high when the beam is blocked. The device delivers 1.0 mA IC(ON) at IF = 20 mA, so a 4.7 kΩ pull-up to 5 V yields ~0.4 V saturation - compatible with TTL and most 5 V microcontrollers. No base bias network is needed.

What is the mechanical gap dimension of the H21A4 package?

The H21A4 features a nominal mechanical gap height of 0.103" (2.60 mm), as documented in the PACKAGE DIMENSIONS drawing on page 1 of DS300291. This fixed gap defines the minimum detectable object thickness and must be aligned with the moving shutter or encoder wheel during PCB placement. Tolerance is ±0.010" unless otherwise specified, and the gap is formed by molded plastic walls enclosing the optical path between emitter and detector.

Can the H21A4 operate reliably at 100°C ambient temperature?

Yes, the H21A4 is rated for continuous operation from –55°C to +100°C ambient temperature (TOPR), per the Absolute Maximum Ratings table. At 100°C, its normalized IC(ON) drops to ~0.6× the 25°C value (per Figure 2), and VCE(SAT) increases slightly (per Figure 3), but remains within specification. Ensure power dissipation stays within derated limits: PD ≤ 150 mW − 1.67 mW/°C × (TA − 25°C).

How does the H21A4 differ from the H21A6 in terms of output current?

The H21A4 delivers 1.0 mA IC(ON) at IF = 20 mA, while the H21A6 delivers 4.0 mA under identical conditions - a 4× higher output current. This difference arises from internal photodarlington gain tuning, not package or pinout changes. Both share identical pinout, gap dimension, BVCEO (55 V), and timing specs (ton = 8 µs, toff = 50 µs). Choose H21A4 where 1 mA suffices to minimize power and cost; select H21A6 only when driving heavier loads or compensating for aging or contamination.