4N26FM Optoisolator - Equivalent & Substitute Parts

Part Overview

The 4N26FM is an optoisolator transistor with base output manufactured by onsemi, designed for high-voltage isolation applications. This component features a single channel with 7500Vpk isolation rating and transistor output configuration in a 6-SMD surface mount package. The part is currently listed as obsolete, making identification of equivalent and substitute components essential for ongoing design support and production continuity.

Key Parameters

Parameter	Value
Manufacturer Part Number	4N26FM
Manufacturer	onsemi
Category	Optoisolators
Number of Channels	1
Voltage - Isolation	7500Vpk
Current Transfer Ratio (Min)	20% @ 10mA
Output Type	Transistor with Base
Voltage - Output (Max)	30V
Vce Saturation (Max)	500mV
Operating Temperature	-55°C ~ 100°C
Mounting Type	Surface Mount
Package / Case	6-SMD, Gull Wing
Product Status	Obsolete

Substitute Part Grouping Explanation

Substitution eligibility for the 4N26FM is determined by the following critical parameters:

Number of Channels: Must be 1 channel
Output Type: Must be Transistor with Base
Voltage - Isolation: Minimum 7500Vpk (or equivalent Vrms rating)
Current Transfer Ratio (Min): Minimum 20% @ 10mA
Vce Saturation (Max): Maximum 500mV
Input Type: DC
Voltage - Forward (Vf) (Typ): 1.18V nominal
Current - DC Forward (If) (Max): 60mA maximum
Package Configuration: 6-SMD or compatible DIP alternatives

The 4N26FM has two identified substitutes: the 4N26SM (Fairchild Semiconductor) and the 4N26 (Texas Instruments). Both maintain the core functional parameters required for optoisolation applications, though with variations in isolation voltage rating, package type, and operating temperature range.

Parameter Comparison

Parameter	4N26FM (onsemi)	4N26SM (Fairchild)	4N26 (Texas Instruments)
Voltage - Isolation	7500Vpk	4170Vrms	5000Vrms
Current Transfer Ratio (Min)	20% @ 10mA	20% @ 10mA	20% @ 10mA
Turn On / Turn Off Time (Typ)	2µs, 2µs	2µs, 2µs	Not specified
Output Type	Transistor with Base	Transistor with Base	Transistor with Base
Voltage - Output (Max)	30V	30V	70V
Vce Saturation (Max)	500mV	500mV	500mV
Voltage - Forward (Vf) (Typ)	1.18V	1.18V	1.3V
Current - DC Forward (If) (Max)	60mA	60mA	60mA
Operating Temperature	-55°C ~ 100°C	-40°C ~ 100°C	-55°C ~ 100°C
Mounting Type	Surface Mount	Surface Mount	Through Hole
Package / Case	6-SMD, Gull Wing	6-SMD, Gull Wing	6-DIP (0.300", 7.62mm)
Product Status	Obsolete	Active	Active

Engineering Selection Recommendations

4N26SM (Fairchild Semiconductor): This substitute maintains identical package configuration (6-SMD, Gull Wing) and switching characteristics (2µs turn-on/off times). The isolation voltage rating of 4170Vrms is lower than the 4N26FM's 7500Vpk specification. This substitute is suitable for applications where the lower isolation voltage meets system requirements. The part is currently active in production with bulk packaging availability.

4N26 (Texas Instruments): This substitute provides the highest isolation voltage rating (5000Vrms) among available options and supports higher maximum output voltage (70V vs. 30V). However, it uses a through-hole 6-DIP package rather than surface mount configuration, requiring PCB layout modifications. The operating temperature range matches the original (-55°C ~ 100°C). This part is active in production with extensive inventory availability.

Selection between substitutes depends on isolation voltage requirements, PCB assembly capability (surface mount vs. through-hole), and output voltage specifications for the target application.

Frequently Asked Questions (FAQ)

Q: Can the 4N26SM directly replace the 4N26FM without PCB modifications?

A: Yes. Both parts use identical 6-SMD, Gull Wing surface mount packaging and footprint. No PCB layout changes are required for mechanical compatibility.

Q: Is the 4N26 (Texas Instruments) a direct replacement for the 4N26FM?

A: No. The 4N26 uses a 6-DIP through-hole package, which requires different PCB footprint and assembly process. PCB redesign is necessary for this substitution.

Q: What is the isolation voltage difference between the 4N26FM and 4N26SM?

A: The 4N26FM provides 7500Vpk isolation, while the 4N26SM provides 4170Vrms isolation. The 4N26SM has lower isolation voltage capability. Verify that your application's isolation requirements are met by the lower rating before substitution.

Q: Are all three parts compatible with the same circuit design?

A: The 4N26FM and 4N26SM are compatible without circuit modifications due to matching electrical parameters and package type. The 4N26 requires circuit board redesign due to its through-hole package but offers higher output voltage capability (70V vs. 30V).

Q: What is the minimum operating temperature difference between these parts?

A: The 4N26FM and 4N26 both operate from -55°C, while the 4N26SM operates from -40°C. For applications requiring -55°C operation, the 4N26SM is not suitable.

Q: Do all three parts have the same current transfer ratio specification?

A: Yes. All three parts specify a minimum current transfer ratio of 20% @ 10mA, ensuring consistent optical coupling performance across substitutes.

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