Datasheet for the 4N25 phototransistor optocoupler, a classic six-pin DIP isolator used in power supplies, interfaces and control circuits. The 156 KB PDF provides the pinout, LED forward current and voltage, current transfer ratio, isolation voltage and switching times. When repairing, it lets you test the LED side with a diode meter, confirm the output transistor responds to drive current, and pick a compatible substitute such as another 4N-series part, checking CTR and isolation ratings match the original application.
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