onsemi H21B3

Overview

H21B3 from Fairchild Semiconductor is a photodarlington optical interrupter switch comprising a GaAs infrared LED optically coupled to a silicon photodarlington detector in a plastic DIP-4 housing with 0.035" apertures. It delivers 2.0 mA on-state collector current at I_F = 2 mA, V_CE = 1.5 V, supports 30 V V_CEO, and operates from −55°C to +100°C - used for position sensing and object detection in industrial encoders and printer paper-path monitoring.

For engineers reviewing the H21B3 datasheet, pinout, applications, or equivalent options, key selection criteria include its 2.0 mA I_C(ON) at low input current, 45 µs turn-on time with 750 Ω load, 1.0 V V_CE(SAT), ambient light rejection via opaque housing, and mechanical interrupt gap design for reliable edge detection.

Technical Context

The H21B3 implements a monolithic photodarlington output stage that provides high DC current gain (β ≈ 1000–2000 typical) enabling direct interface with logic-level loads without external amplification. Its optical coupling isolates input and output circuits up to 5300 V_RMS (per isolation rating), while the fixed 0.035" aperture ensures consistent mechanical resolution and repeatable switching thresholds.

Unlike phototransistor-based interrupters, the photodarlington architecture achieves higher sensitivity at low LED drive currents - e.g., delivering 10 mA I_C(ON) at I_F = 5 mA - but trades off slower turn-off (250 µs max) and higher saturation voltage (1.0 V) compared to standard phototransistors.

Key Specifications

Pinout & Package

Package: 4-pin dual-in-line plastic housing (DIP-4), 0.300" wide body, 0.433" length, with side-entry optical gap aligned between pins 1–2 (emitter) and 3–4 (detector).

Key Features

Applications

Equivalent & Alternatives

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

Compared with H21A3 and PC817XNIP0X, the H21B3 uniquely combines factory-aligned optical interrupt geometry, photodarlington sensitivity, and DIP-4 through-hole mounting - making it optimal for drop-in replacement in legacy encoder and paper-sensing designs requiring ≥2 mA output drive without external amplification.

Availability

H21B3 is available at Aetrix Electronics and suitable for industrial encoders, printer paper-path monitoring, automotive throttle position sensing, and conveyor belt object counting requiring stable component supply and long-term obsolescence management.

Supply support for H21B3 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 optocouplers and discrete power products.

The H21B3 belongs to Fairchild's H21Bx photodarlington optical interrupter family, designed specifically for robust, cost-effective position and presence detection in electromechanical systems where ambient light immunity and mechanical alignment stability are critical.

FAQ

What is the maximum continuous forward current rating for the H21B3 emitter diode?

The H21B3 emitter has an absolute maximum continuous forward current (I_F) of 50 mA per the Fairchild datasheet. Operation above this value risks permanent LED degradation. For reliable long-term performance, design with I_F ≤ 30 mA - which still delivers 10 mA I_C(ON) and maintains thermal margin within the 100 mW input power dissipation limit. Always use a series current-limiting resistor calculated for the applied supply voltage and target I_F.

Does the H21B3 provide electrical isolation, and what is its rated isolation voltage?

Yes, the H21B3 provides galvanic isolation between input and output circuits. Its isolation voltage is rated at 5300 V_RMS for 1 minute, verified per UL 1577 and CSA Component Acceptance Notice #5. This isolation is achieved via internal molding compound and physical separation of emitter and detector die in the DIP-4 package. The H21B3 is suitable for reinforced insulation in Class I equipment, but not approved for life-support applications per Fairchild's policy.

How does the H21B3 differ from the H21B1 and H21B2 variants in terms of output current capability?

The H21B3 delivers higher on-state collector current than H21B1 and H21B2: at I_F = 2 mA, H21B3 provides 2.0 mA I_C(ON), versus 0.5 mA for H21B1 and 1.0 mA for H21B2. At I_F = 10 mA, H21B3 delivers 25 mA, while H21B1 and H21B2 deliver 7.5 mA and 14 mA respectively. This graded sensitivity allows designers to select the variant matching required output drive strength and LED power budget.

Can the H21B3 be used in surface-mount applications?

No, the H21B3 is manufactured exclusively in a through-hole 4-pin DIP package and is not available in surface-mount form. Its mechanical interrupt gap and leadframe geometry are optimized for wave soldering and manual insertion. For SMT optical interrupters, consider alternatives like the Vishay TCST2103 or Everlight ITA16101, but note these require redesign of PCB layout, aperture alignment, and drive circuitry - the H21B3 cannot be substituted without mechanical and electrical requalification.

What is the typical saturation voltage of the H21B3 photodarlington output, and how does it affect load design?

The H21B3 has a typical saturation voltage (V_CE(SAT)) of 1.0 V when I_F = 10 mA and I_C = 1.8 mA. At higher currents (e.g., I_C = 50 mA), V_CE(SAT) rises to 1.5 V for H21B1/H21B2 - though H21B3 is not characterized at that level. Designers must ensure the load voltage drop across V_CE(SAT) leaves sufficient headroom for downstream logic thresholds; for 5 V TTL, a 1.0 V V_CE(SAT) yields 4.0 V valid HIGH level when used in open-collector configuration with pull-up.