PC817 optocoupler: pinout, CTR ranks, ratings and a worked interface

The top carries PC817, a rank letter, a factory mark and a three-digit date code, with a dot over pin 1. The rank letter is the part you need: A, B, C or D, or nothing for an unsorted part. Sharp sells the through-hole DIP-4 and an SMT gullwing version with the same pins.

Sharp's current sheet covers the PC817X series and rates it 80 V collector to emitter. Older PC817 documents and the many 817-numbered parts from other makers do not all carry the same ratings, and we could not read their sheets on the makers' own sites for this page. If your part is unbranded or old stock, do not assume 80 V; find the sheet for the exact maker, or keep the output side well under the lowest rating you can confirm.

On the LED side: 50 mA forward current and only 6 V in reverse. On the transistor side: 80 V and 50 mA, with 150 mW of dissipation and 200 mW for the whole package. Sharp derates the LED current above 55 °C and the collector power above 25 °C (its Fig. 1 and Fig. 3).

The 5 kV is an isolation test: 5 kV rms between the two sides for one minute, in 40 to 60 % humidity. It is not a working voltage, and the sheet gives no continuous one. What sets the safe working voltage on a real board is the gap and creepage between the traces under the part, and mains work belongs in a design that a qualified person has checked.

CTR, the current transfer ratio, is collector current divided by LED current. Sharp grades every part at 5 mA of LED current and 5 V across the transistor. An unsorted part gives anywhere from 2.5 to 30 mA there, a CTR of 50 to 600 %. The four ranks narrow it, as Figure 1 shows. The first page of the sheet sums the range up as 50 to 400 %, but its own rank table runs to 30 mA, or 600 %; design to the table.

Two things move CTR after you buy the part. It falls as the package warms: Sharp's Fig. 9 shows about 82 % of the room-temperature value at 60 °C, at 5 mA. And the LED ages: Sharp's design notes tell you to allow for 50 % degradation over five years. The sheet also warns that CTR varies more below 1 mA of LED current, so keep the LED at a few milliamps.

• Passing a logic signal between circuits with separate grounds: a 24 V PLC input into a 5 V microcontroller, or a microcontroller pin into a separately powered driver board.
• Breaking a ground loop between two boards, or keeping a noisy motor supply's ground away from a sensor's.
• Slow signals: switches, alarms, enable lines and serial links up to a few thousand bits per second. It is too slow for fast serial or PWM above a few kHz.

For switching mains loads, a triac-output optocoupler or a solid-state relay is the part; see the solid-state relay.

A microcontroller pin lights the LED, and the transistor pulls an input on another board low. The output side has its own 5 V supply and its own ground. We design for the worst part we could be sent: unranked, five years old, in a warm box.

• Buy a ranked part (PC817C or PC817B) when you can. Their guaranteed minimum is 2.6 or 4 times the unranked one, which allows a smaller, faster pull-up.
• Keep the two grounds apart. A single wire between them, often through a USB cable or a shared supply, undoes the isolation without anyone noticing.
• The LED takes only 6 V in reverse. If the input can swing negative, put a small diode across it, cathode to pin 1.
• If an output is slow or never quite reaches 0 V, measure the LED current first, then halve the pull-up only if the rank allows it.
• Leave room on the board between the input and output traces. The isolation depends on that gap as well as on the package.

Within Sharp's range, any ranked part replaces an unranked one, and the SMT version replaces the DIP. We could not read another maker's 817 sheet on its own site for this page, so other numbers are not listed; compare their pinout, VCEO and CTR table against the Sharp figures before you swap.

1. Meter on diode test, red on pin 1, black on pin 2: the LED reads about 1 V. Reversed, open.
2. Pins 3 and 4 should read open both ways with the LED dark.
3. Wire up the example: 680 Ω from 5 V into pin 1, pin 2 to ground, and 10 kΩ from a second 5 V supply to pin 4 with pin 3 on that supply's ground. Pin 4 should sit near 5 V, and drop under 0.2 V when the LED is lit.
4. Check the isolation last: with nothing connected, the meter on its highest resistance range should read open between any input pin and any output pin.

The phototransistor and optocoupler lesson covers what happens inside, and continuity and diode test explains the meter.

Drive the LED at about 5 mA (680 Ω from 5 V, 330 Ω from 3.3 V), buy rank B or C, and size the pull-up from the minimum CTR halved for ageing. For an unranked part on a 5 V output, that means 10 kΩ and signals slower than a few kHz. If you need more speed, use a higher rank and a smaller pull-up, or a logic-output optocoupler.

PC817s are sold loose and on small breakout boards by most Indian electronics shops. Ask for the rank letter and the maker; a bag of unmarked or unranked 817s needs the unranked design above. Our guide to buying electronic components in India compares ten online sellers and covers how to spot fakes.