4N26 — General-Purpose 6-Pin Phototransistor Optocoupler (DIP-6)

Overview

The 4N26 (along with 4N25, 4N27, and 4N28) is an industry-standard general-purpose 6-pin DIP optocoupler (optoisolator) manufactured by Vishay, onsemi, Lite-On, and Everlight. It consists of a gallium arsenide (GaAs) infrared light-emitting diode optically coupled to a silicon NPN phototransistor in a 6-pin through-hole DIP-6 and surface-mount SMD-6 package.

Providing up to $5000\text{ V}_{RMS}$ of galvanic isolation between the input LED and output transistor, the key distinguishing feature of the 4N26 compared to 4-pin optocouplers (like the PC817) is that its phototransistor base terminal is brought out to Pin 6. This allows designers to connect a base-emitter bleed resistor to dramatically speed up turn-off switching times or bias the base for linear AC zero-crossing detection. The 4N26 is widely used in MIDI instrument input isolation, AC mains zero-cross detectors, SMPS feedback networks, industrial PLC 24V inputs, and microprocessor ground-loop breaking.

Quick reference


Device Type	GaAs IR LED to Silicon NPN Phototransistor Optocoupler
Package	6-pin DIP (DIP-6 / PDIP-6) / SMD-6
Galvanic Isolation Voltage	$5000\text{ V}_{RMS}$ (1 minute test)
Collector-Emitter Breakdown ( $V_{CEO}$ )	$30\text{ V}$ min
Input LED Forward Current ( $I_F$ )	$60\text{ mA}$ max ( $10\text{mA} \dots 20\text{mA}$ typical)
Current Transfer Ratio (CTR)	$\ge 20\%$ min ( $50\% \dots 100\%$ typ at $I_F = 10\text{mA}$ )
Phototransistor Base Terminal	Accessible on Pin 6 (Allows base-drain resistor speedup)
Switching Speed ( $t_{on} / t_{off}$ )	$2.0\ \mu\text{s} / 2.0\ \mu\text{s}$ typ
4-Pin Alternative	PC817 (4-pin optocoupler without base access)

Pinout (DIP-6 Package)

text

                            ┌───┴───┐
             (LED Anode)  A 1│ 1   6│ B (Phototransistor Base)
           (LED Cathode)  K 2│ 4N26│5│ C (Phototransistor Collector)
        (No Connection)  NC 3│ DIP-6│4│ E (Phototransistor Emitter)
                            └───────┘

Pin	Name	Type	Description
1	`ANODE (A)`	LED Input (+)	Infrared LED anode (Driven with current via series resistor)
2	`CATHODE (K)`	LED Input (-)	Infrared LED cathode (Connect to ground or open-collector driver)
3	`NC`	No Connect	Unconnected internal lead
4	`EMITTER (E)`	Transistor Out	Phototransistor emitter (Connect to ground or pulldown resistor)
5	`COLLECTOR (C)`	Transistor Out	Phototransistor collector (Connect to pull-up resistor or load)
6	`BASE (B)`	Transistor Base	Phototransistor base (Leave floating, or tie resistor to Pin 4 to speed up switching)

Typical Application Circuit: Standard Microcontroller Galvanic Logic Isolator

text

  +5V Transmitting Logic                       +3.3V / +5V Isolated Logic
           │                                                │
    [ 220Ω - 330Ω ]                                 [ 4.7kΩ - 10kΩ Pull-Up ]
           │                                                │
           ├───► [Pin 1: ANODE]                             ├───► Isolated Logic Out (to MCU GPIO)
           │        4N26                                    │
  MCU GPIO ┴───► [Pin 2: CATHODE]                  [Pin 5: COLLECTOR]
                                                        4N26
                                                   [Pin 4: EMITTER] ──► Isolated GND
                                                   [Pin 6: BASE]
                                                        │
                                          [ Optional 100kΩ - 470kΩ ] ──► Isolated GND (Speeds up turn-off)

Comparison: 4N26 vs 4N25 vs 4N35 vs PC817

Parameter	4N26	4N25	4N35	PC817
Package	6-Pin DIP	6-Pin DIP	6-Pin DIP	4-Pin DIP (Compact)
Base Pin Out	Yes (Pin 6)	Yes (Pin 6)	Yes (Pin 6)	No (Floating Base)
CTR Minimum	$20\%$	$20\%$	$100\%$ (High CTR)	$50\%$
Isolation Voltage	$5000\text{ V}_{RMS}$	$5000\text{ V}_{RMS}$	$5000\text{ V}_{RMS}$	$5000\text{ V}_{RMS}$

Common mistakes

Leaving Pin 6 (Base) floating in high-noise environments without awareness: When Pin 6 is left open, the high base impedance makes the phototransistor sensitive to external electrostatic or magnetic noise pickup. In noisy industrial environments, placing a $100\text{k}\Omega \dots 1\text{M}\Omega$ resistor between Pin 6 (Base) and Pin 4 (Emitter) bleeds off stray charge and substantially increases switching speed.
Exceeding LED reverse voltage ( $V_R = 6.0\text{V}$ ): In AC mains detection circuits, always place a standard 1N4148 diode in anti-parallel across Pins 1 and 2 to protect the GaAs LED from reverse voltage breakdown on the negative AC half-cycle.

Notes

MIDI Standard: The classic MIDI specification specifically prescribes 6-pin/8-pin optocouplers (such as 4N26, 4N28, or 6N138) on all MIDI IN DIN-5 jacks to ensure absolute ground isolation between musical instruments.

Interface	Infrared LED (Anode, Cathode), Phototransistor (Emitter, Collector, Base)
Package	6-pin DIP (DIP-6 / PDIP-6) / 6-pin SMD