4N37FM Optoisolator Equivalent & Substitute Parts

Part Overview

The 4N37FM is an optoisolator transistor with base output manufactured by onsemi, designed for high-voltage isolation applications. This component features a 7500Vpk isolation rating and operates as a single-channel optoisolator in a 6-SMD surface mount package. The 4N37FM is currently classified as obsolete, making identification of functionally equivalent substitute parts essential for ongoing design support and production continuity. Substitute parts must maintain compatibility across isolation voltage, output characteristics, and mounting requirements while accounting for differences in product status and operating specifications.

Substiute Parts

4N37FM

onsemiIn Stock: 8574N37FM Datasheet

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4N37SM

onsemiIn Stock: 127594N37SM Datasheet

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4N37

Vishay Semiconductor Opto DivisionIn Stock: 190924N37 Datasheet

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Key Parameters

Substitute Part Grouping Explanation

Substitution of the 4N37FM is determined by the following critical parameters:

• Isolation Voltage: The 4N37FM provides 7500Vpk isolation. Substitute parts must meet or exceed this isolation rating to maintain electrical safety and performance in high-voltage isolation applications.
• Output Configuration: The transistor with base output is essential for applications requiring base drive control. Substitute parts must retain this output type.
• Current Transfer Ratio: Minimum 100% @ 10mA ensures consistent optical-to-electrical signal transfer.
• Mounting Type and Package: The 6-SMD surface mount package with gull wing leads defines the physical footprint and assembly compatibility.
• Operating Temperature Range: The -55°C ~ 100°C range must be maintained or exceeded for thermal compatibility.

The 4N37FM shares a common base product number (4N37) with available substitutes. However, substitutes may differ in isolation voltage specification format (Vpk vs. Vrms), product status (active vs. obsolete), mounting technology (surface mount vs. through hole), and operating temperature minimums. These differences must be evaluated against specific application requirements.

Parameter Comparison

Engineering Selection Recommendations

4N37SM (onsemi) - Surface Mount Substitute

The 4N37SM is an active product in the same 6-SMD surface mount package as the 4N37FM. It maintains identical current transfer ratio, output voltage, forward voltage, and switching speed specifications. The 4N37SM is ROHS3 compliant, providing regulatory advantage over the obsolete 4N37FM. The primary trade-off is isolation voltage: the 4N37SM specifies 4170Vrms, which is lower than the 4N37FM's 7500Vpk rating. This substitute is suitable for applications where the lower isolation voltage meets system requirements and surface mount assembly is required. The operating temperature minimum is -40°C, which is warmer than the 4N37FM's -55°C specification.

4N37 (Vishay) - Through Hole Substitute

The Vishay 4N37 is an active product with ROHS3 compliance and the same base product number. It maintains the transistor with base output configuration and 100% minimum current transfer ratio. The Vishay part specifies 5000Vrms isolation voltage, positioned between the 4N37FM and 4N37SM. This substitute requires through-hole assembly (6-DIP package), making it incompatible with surface mount production processes. Switching speed is slower (10µs turn-on/off vs. 2µs), and maximum forward current is reduced to 50 mA. The operating temperature range matches the 4N37FM (-55°C ~ 100°C). This substitute is applicable only when through-hole assembly is acceptable and lower switching speed is tolerable.

Frequently Asked Questions (FAQ)

Q: Can the 4N37SM directly replace the 4N37FM in existing designs?

A: The 4N37SM is mechanically and electrically compatible for surface mount applications, provided the lower isolation voltage (4170Vrms vs. 7500Vpk) is acceptable for your circuit. Both parts share identical package geometry (6-SMD, Gull Wing), current transfer ratio, output voltage, and switching speed. Verify that your isolation voltage requirement does not exceed 4170Vrms before substitution.

Q: What is the difference between Vpk and Vrms isolation voltage specifications?

A: Vpk (peak voltage) and Vrms (root mean square voltage) are different measurement standards for isolation voltage. The 4N37FM specifies 7500Vpk, while the 4N37SM specifies 4170Vrms. These values cannot be directly compared without conversion factors specific to the waveform. Consult component datasheets and application requirements to determine equivalence for your specific isolation voltage needs.

Q: Why is the Vishay 4N37 slower than the onsemi alternatives?

A: The Vishay 4N37 specifies 10µs turn-on and turn-off times, compared to 2µs for both onsemi parts. This difference reflects different device design and manufacturing processes. Applications requiring fast switching response (< 10µs) should use the onsemi 4N37FM or 4N37SM rather than the Vishay 4N37.

Q: Can I use the through-hole Vishay 4N37 in a surface mount assembly?

A: No. The Vishay 4N37 uses a 6-DIP through-hole package, which is physically and mechanically incompatible with surface mount production processes. The 4N37FM and 4N37SM both use 6-SMD surface mount packages. Substitution between through-hole and surface mount packages requires PCB redesign and assembly process changes.

Q: Is the 4N37SM ROHS3 compliant while the 4N37FM is not?

A: The 4N37FM product status is listed as obsolete with no RoHS status specified. The 4N37SM is confirmed ROHS3 compliant. If RoHS3 compliance is required for your application or market, the 4N37SM is the preferred substitute among available options.

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

A: The 4N37FM and Vishay 4N37 both operate from -55°C, while the 4N37SM operates from -40°C. If your application requires operation below -40°C, the 4N37SM is not suitable. The 4N37FM or Vishay 4N37 must be used instead.