IL4108 Optoisolator Triac Output Equivalent & Substitute Parts

Part Overview

The IL4108 is an optoisolator with triac output manufactured by Vishay Semiconductor Opto Division. This component provides galvanic isolation up to 5300Vrms with a single triac output channel in a 6-DIP package. The device is actively produced and widely used in AC switching applications requiring electrical isolation. Substitute parts may be required due to inventory constraints, regional availability, or design flexibility in applications where the specified electrical parameters remain within acceptable operating ranges.

Substiute Parts

IL4108

Vishay Semiconductor Opto DivisionIn Stock: 1690IL4108 Datasheet

Current PartRFQ

FOD4108

onsemiIn Stock: 2092FOD4108 Datasheet

MFR RecommendedRFQ

FOD4118

onsemiIn Stock: 1606FOD4118 Datasheet

MFR RecommendedRFQ

Key Parameters

Substitute Part Grouping Explanation

Substitute parts for the IL4108 are qualified based on the following critical parameters that determine functional equivalence:

• Output Type: Triac (required for AC switching applications)
• Zero Crossing Circuit: Yes (required for synchronized switching)
• Number of Channels: 1 (single channel isolation)
• Package / Case: 6-DIP (0.300", 7.62mm) (mechanical and pin compatibility)
• Mounting Type: Through Hole (PCB assembly compatibility)
• Voltage - Isolation: Minimum 5000Vrms (meets or exceeds isolation requirement)
• Voltage - Off State: 800 V (triac blocking voltage)
• Static dV/dt (Min): 10kV/µs (noise immunity specification)
• Operating Temperature: -55°C ~ 100°C (thermal operating range)
• Current - Hold (Ih): 500µA (triac holding current specification)

The substitute parts FOD4108 and FOD4118 (both onsemi) meet these critical parameters. Minor variations in Current - LED Trigger (Ift), Turn On Time, and Current - DC Forward (If) are within acceptable ranges for triac optoisolator applications and do not prevent functional substitution.

Parameter Comparison

Engineering Selection Recommendations

All three parts—IL4108, FOD4108, and FOD4118—are actively produced and ROHS3 compliant. Both substitute parts meet the critical isolation voltage, package, and functional requirements for triac optoisolator applications.

FOD4108 and FOD4118 differ primarily in LED trigger current specification (2mA vs. 1.3mA maximum). Both maintain the required 500µA holding current and 10kV/µs static dV/dt rating. The isolation voltage of 5000Vrms on the onsemi parts is 300Vrms lower than the IL4108 specification but remains within standard industrial isolation requirements for most AC switching applications.

Turn-on time increases from 35µs to 60µs on substitute parts. This difference is typical among triac optoisolator variants and does not affect zero-crossing detection functionality. LED forward current specification is reduced on substitute parts (30mA maximum vs. 60mA), which may require circuit design consideration for LED drive current limiting.

Both onsemi parts carry CSA and UL approvals, matching the regulatory compliance profile of the IL4108.

Frequently Asked Questions (FAQ)

Q: Can FOD4108 or FOD4118 be used as direct pin-for-pin replacements for IL4108?

A: Yes. All three parts share identical 6-DIP package geometry (0.300", 7.62mm) and through-hole mounting. Pin configuration is compatible for triac optoisolator applications with single-channel output and zero-crossing detection.

Q: What is the significance of the 300Vrms isolation voltage difference between IL4108 and the onsemi substitutes?

A: The IL4108 provides 5300Vrms isolation while FOD4108 and FOD4118 provide 5000Vrms. Both exceed standard industrial isolation requirements (typically 3000Vrms minimum for mains-rated applications). The difference does not affect functional compatibility in applications designed for 5000Vrms or lower isolation specifications.

Q: How does the 25µs difference in turn-on time affect circuit performance?

A: IL4108 turn-on time is 35µs while FOD4108 and FOD4118 are rated at 60µs. Both values are within acceptable ranges for zero-crossing triac switching. The longer turn-on time on substitute parts does not degrade zero-crossing detection performance and is typical among triac optoisolator product variants.

Q: Why is the LED forward current specification lower on onsemi parts?

A: FOD4108 and FOD4118 specify 30mA maximum DC forward current compared to 60mA on IL4108. This reflects different LED design specifications. Circuit design must ensure LED drive current does not exceed 30mA on substitute parts. Current-limiting resistor values may require adjustment based on the specific drive circuit.

Q: Are there differences in LED trigger current between FOD4108 and FOD4118?

A: FOD4108 specifies 2mA maximum LED trigger current (matching IL4108), while FOD4118 specifies 1.3mA maximum. FOD4118 requires lower trigger current for triac activation. Selection depends on available drive current capability in the application circuit.

Q: Do all three parts support the same operating temperature range?

A: Yes. IL4108, FOD4108, and FOD4118 all operate across -55°C to 100°C, ensuring thermal compatibility across industrial and commercial temperature ranges.