What Is the PC817? Optocoupler: Pinout, Isolation and Uses
The PC817 is one of the most common optocouplers (opto-isolators), with an LED and a phototransistor inside. It isolates two circuits electrically (~5000 V) while transferring the signal with light. Pinout DIP-4: 1 Anode, 2 Cathode, 3 Emitter, 4 Collector. It is used in SMPS feedback (with the TL431), relay/triac triggering and breaking ground loops; a current-limiting resistor on the input LED is a must.
The PC817 is one of the most common optocouplers (opto-isolators). It contains an LED and a phototransistor; it isolates two circuits electrically while transferring the signal with light. For the concept, see what is an optocoupler.
What is the PC817 used for?
Isolating a microcontroller from a high-voltage/noisy side: SMPS feedback (with the TL431), relay/triac triggering, level shifting and breaking ground loops. The input LED is driven by current; the output transistor passes the isolated signal.
PC817 pinout (DIP-4)
| Pin | Function |
|---|---|
| 1 | Anode (LED +) |
| 2 | Cathode (LED −) |
| 3 | Emitter (output transistor) |
| 4 | Collector (output transistor) |
The input (1–2) and output (3–4) sides are electrically isolated from each other (~5000 V).
Highlights
| Parameter | Value |
|---|---|
| Type | Phototransistor-output optocoupler |
| Isolation | ~5000 Vrms |
| CTR (current transfer ratio) | ~50% – 600% (by grade) |
| Channels | 1 (single) |
Watch out
A current-limiting resistor must be placed on the input LED (otherwise the LED burns out). CTR varies with grade and age; leave margin in the design. The phototransistor is not ideal for fast switching; if you need high speed use a fast optocoupler (6N137).
Equivalents and alternatives
- EL817 / LTV817: drop-in equivalents.
- PC827: the dual-channel version.
- 6N137: a high-speed optocoupler.
- 4N35: a classic alternative.
Find it in our catalog
Use the PC817 search.