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.
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.
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. Search our catalog or submit your BOM.
Need help sourcing these components?
PartsCube Global stocks all alternatives mentioned in this guide. Search our catalog or submit your BOM for a quote.
Chat on WhatsApp