Optocoupler Selection Guide: Types, Parameters, and Alternatives

What is an Optocoupler?

An optocoupler (or optoisolator) transfers electrical signals between two isolated circuits using light. They're essential for safety isolation, ground loop elimination, and high-voltage switching.

Popular Optocoupler Comparison

Part Number	Channels	Isolation	CTR	Output	Speed	Best For
PC817	1	5000Vrms	50-600%	Transistor	2µs	Power supply feedback, general isolation
TLP185	1	3750Vrms	100-300%	Transistor	3µs	Compact SOP-4, space-saving
TLP291	1-4	2500Vrms	100-600%	Transistor	3µs	Multi-channel isolation
4N35	1	2500Vrms	100%	Transistor	10µs	Classic, hobby projects
MOC3021	1	7500Vpk	—	Triac	—	AC load / SSR trigger
MOC3063	1	7500Vpk	—	Triac ZC	—	Zero-cross AC switching
6N137	1	2500Vrms	—	Logic	75ns	High-speed digital isolation
EL357N	1	3750Vrms	200-400%	Transistor	4µs	PC817 SMD alternative

PC817 is the most popular optocoupler in the world — used in nearly every switching power supply.

Current Transfer Ratio (CTR) — Why It Matters

CTR = (output current) / (input current) × 100%

CTR Grade	Range	Best For
Low (50-100%)	A/B grade	High LED drive available
Medium (100-200%)	C/D grade	Balanced, most common
High (200-600%)	GR/Y grade	Low LED drive, high load

Important: CTR degrades over time and with temperature. Design with 20-30% margin. We've seen CTR drift cause problems when boards heat up — check the curves first.

Chinese Optocoupler Alternatives

Western Part	Chinese Alternative	CTR Match	Savings
PC817	EL357N (Everlight)	Same grades	20-30%
PC817	OC817 (Orient)	Same grades	30-40%
TLP185	EL185 (Everlight)	Similar	30%
4N35	EL4N35 (Everlight)	Same	40%
MOC3063	EL3063 (Everlight)	Same	30%

Everlight (亿光) from Taiwan is the largest optocoupler manufacturer globally by volume. Their EL series is often cheaper than Japanese brands (Toshiba, Sharp) with comparable quality. When PC817 lead times stretch, EL357N is our usual drop-in — same grades.

Selection by Application

Application	Recommended	Why
Power supply feedback	PC817 / EL357N	Industry standard, widely available
Signal isolation (slow)	4N35	Enough speed for GPIO / sensor signals
Multi-channel isolation	TLP291-4	4 channels in one SOP-16
AC switching	MOC3063	Zero-cross detection built-in
Digital bus isolation	6N137	10Mbaud, enough for SPI/UART
Industrial I/O modules	EL357N	SMD, cheap, compact

Where to Buy

PartsCube Global stocks PC817, EL357N, TLP185, 4N35, MOC3063, and Chinese alternatives.