Power conversionBench electronicsTest and identify
Optocoupler
Also called optoisolator, PC817, 4N35.
An LED and a phototransistor in one package, used so a supply can regulate across the isolation barrier. A PC817-class 4-pin opto is one of the few ICs worth desoldering, because the pinout is standard and the part is robust.
If you are new to this category
These parts change wall power into the lower voltage a device uses. A sealed supply with a label is the safer kind. An open board from a lamp can still be connected to the wall on the side that looks like low voltage. Isolated means the output is not electrically connected to the wall. If you are not sure, treat the output as live.
Read the label out loud: volts, amps, and the polarity symbol. Then measure. A number you remember from a similar brick is not a measurement.
Left: a brick. The label is your spec, and the plastic is the safety barrier. Right: a board taken out of a lamp or a charger. Unless you have checked that the output is isolated from the prongs, do not let it touch a project you will handle while it is plugged in.
Mains voltage. Line voltage, and the DC bus just after the rectifier, both kill. A disconnected cord is not a discharged supply.
| What to look for | Usual figure |
|---|---|
| PC817 | Pin 1 anode, 2 cathode, 3 emitter, 4 collector. CTR varies by grade. |
| Use | Feedback across isolation, sensing a mains-side event without sharing a ground |
Volts are electrical pressure, amps are how much current is flowing, and ohms are how hard the part resists that flow. A higher voltage than the part was built for is how salvage parts die on the bench. If a cell in the table says “read the stamp”, that stamp is the whole specification.
What people actually use it for
- Isolated sensing
- Replacing a dead opto of the same type in a supply
Leave this one when
It came off a supply that died in a surge and you have no way to know the LED is tired. CTR fades.
Buying a new one is the better idea when
A new PC817. Keep the salvage one as a known-good spare only after it passes the test.
How to tell if this one still works
Diode-test the LED side. Then a series resistor and a pull-up on the transistor, and watch it switch.
Tools you will want: Meter, 5 V.
Words on this page
Short meanings, so the rest of the entry is readable if you are not already doing this every weekend.
- mains
- Ordinary household wall power. It can kill. Unplug before you open anything that used it, and do not experiment on live wiring.
- isolation
- The barrier between wall power and the low voltage you can touch. If that barrier is missing, the 'low' side can still shock you.
- series
- Connected in a chain. Cells in series add their voltages. LEDs in series all go dark if one of them opens.
Where people actually find it
0 notes, most useful first. “Leave it” means the part is in that machine and you should not take it: it is dangerous, worn out, or only useful if you already know that exact repair.
| From | Call | How many | How often | Where it sits |
|---|