Part of the Day · PC817 — DIP-4 photocoupler (transistor output, 5kV isolation)

Part of the Day · 2026-11-08

A photocoupler: when the infrared LED inside lights up, the phototransistor facing it turns on. Input and output are not electrically connected (5kV isolation), so it is used to separate a microcontroller from AC-mains or motor circuits. It is the most common 4-pin part for switching-power-supply feedback and PLC inputs.

3D view

3D drawing based on the Through-Hole drawing dimensions in the Sharp datasheet (OP18002EN). Two leads on each side; the ● dot side is pin 1 (anode). The height where the leads leave the body and their splay angle are approximate. Not to scale.

Actual size · top view

↓ Actual size, left edge aligned to 0 on the ruler

One ruler tick = 1mm. On PC this is actual size on a 24-inch FHD monitor at 100% zoom; on iPhone it adjusts automatically. For other screens, use “Calibrate ruler” below.

Enlarged (DIP-4 · top view · approx. 9×)

Pins 1 Anode (A) · 2 Cathode (K) — input LED / 3 Emitter (E) · 4 Collector (C) — output transistor (the ● dot marks pin 1)

Package DIP-4

Actual size · Right side view

Body thickness 3.5mm · body top to lead shoulder 4.15mm · lead 3.2mm · lead width 0.5mm (Sharp OP18002EN drawing)

Enlarged · Right side view · approx. 14×

Seen from the right (pins 3 and 4) — below the wide shoulders, the thin leads go into the PCB holes. The body sits slightly above the shoulders.

Key specifications

Values are from the Sharp datasheet.

Marking and pins

Typical uses

• Switching power supply (SMPS) output-voltage feedback — paired with a TL431 to pass the secondary-side voltage to the primary-side IC
• PLC and industrial inputs — isolating 24V sensor and switch signals from the microcontroller side
• Signal isolation when a microcontroller drives relay, triac or motor circuits
• Passing digital signals between two circuits with different grounds

Companion & similar parts

• PC817X1NSZ1B — CTR rank A of the same part (IC 4.0~8.0mA) — the digit after X is the rank
• PC817X3NSZ1B — CTR rank C (IC 10~20mA)
• PC817XNNIP1B — SMT (gull-wing) version of the same part, 2,000 per reel
• PC851XNNSZ1H — Listed in the same DIP-4 general-purpose row of Sharp's product lineup

The character after X in the part number is the CTR rank (N = no rank selection 2.5~30mA, 1~4 = A~D, 5~0 = combined ranks). Through-hole parts come in sleeves of 100, the SMT version on reels of 2,000. Below IF 1mA the CTR variation grows, and the LED output drops with long use (datasheet: 50% over 5 years), so design with margin. Besides PC817, the chip is also marked with the rank letter, factory mark and date code.