Few questions about 4n25

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
Hi,
about the Optoisolator Transistor 4n25:
1. Does it keep the ON status after remove the trig high? does it need a 0 to reset?
2. what's the Voltage drop between pin5 to pin4?
3. how to use pin6?
Thanks
Adam
 

PeteHL

Joined Dec 17, 2014
363
Hi,
about the Optoisolator Transistor 4n25:
1. Does it keep the ON status after remove the trig high? does it need a 0 to reset?
2. what's the Voltage drop between pin5 to pin4?
3. how to use pin6?
Thanks
Adam
1. The 4N25 is made up of an infrared LED illuminating a silicon NPN phototransistor. When sufficient current flows through the LED, then the transistor is turned on. If you remove current to the LED, then the transistor turns off.

2. Resistance of the collector(connected to pin 5) to emitter (connected to pin 4) junction of the transistor is very high (1 Megohm?) when the transistor is off, and has negligible resistance when the transistor is on. So what voltage you find between pins 5 and 6 depends on the circuit that you have connected to those pins and the state of the transistor.

3. Pin 6 is connected to the base of the transistor. Possibly Pin 6 would allow you to control the threshold of turning the transistor on/ off, but I don't really know. No doubt other members could show how to use Pin 6.

-Pete
 

Delta Prime

Joined Nov 15, 2019
1,191
Pin six can be used to adjust sensitivity of optic coupler incorporate potentiometer in between Base pin 6 to ground to capture high frequency pulses. 4N25 is also a widely used optocoupler / optoisolator / photocoupler, It is a general purpose optocoupler.
Pin 3"NC” means it is not connected with the internal circuitry of the device.
4N25 replacement and equivalent optocouplers are 4N26, 4N28, PC817, PC816, 4N27, 4N35, 4N36 & H11Ax series.
used as voltage detector in AC and DC operated circuits, output of microcontroller to operate other devices and applications from microcontrollers, controlling relay, controlling mosfet and other high power transistors, high voltage circuits, controlling AC and DC high voltages etc.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
1. The 4N25 is made up of an infrared LED illuminating a silicon NPN phototransistor. When sufficient current flows through the LED, then the transistor is turned on. If you remove current to the LED, then the transistor turns off.

2. Resistance of the collector(connected to pin 5) to emitter (connected to pin 4) junction of the transistor is very high (1 Megohm?) when the transistor is off, and has negligible resistance when the transistor is on. So what voltage you find between pins 5 and 6 depends on the circuit that you have connected to those pins and the state of the transistor.

3. Pin 6 is connected to the base of the transistor. Possibly Pin 6 would allow you to control the threshold of turning the transistor on/ off, but I don't really know. No doubt other members could show how to use Pin 6.

-Pete
Thanks.
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
Pin six can be used to adjust sensitivity of optic coupler incorporate potentiometer in between Base pin 6 to ground to capture high frequency pulses. 4N25 is also a widely used optocoupler / optoisolator / photocoupler, It is a general purpose optocoupler.
Pin 3"NC” means it is not connected with the internal circuitry of the device.
4N25 replacement and equivalent optocouplers are 4N26, 4N28, PC817, PC816, 4N27, 4N35, 4N36 & H11Ax series.
used as voltage detector in AC and DC operated circuits, output of microcontroller to operate other devices and applications from microcontrollers, controlling relay, controlling mosfet and other high power transistors, high voltage circuits, controlling AC and DC high voltages etc.
Thanks.
 

Ian0

Joined Aug 7, 2020
3,765
With no LED current, the transistor can be used like any other transistor, it actually has rather good Hfe (about 600 if I remember correctly)
"Why would you want to do that?", I hear you ask, when you could just buy a transistor.
When component and assembly costs are at a premium - I once used a 4N25 transistor as a microphone amplifier for a sound-to-light unit and used its LED as the input from an external sound source.

The 4N25 looks like a transistor with a photodiode connected between base and collector, so a resistor between base and emitter would reduce the base voltage and make the CTR lower (It's low enough already). I think it also makes it respond (especially turn off) more quickly, as it shunts away the stored charge.
The base connection, if left unconnected, is a great place for it to pick up interference. If you're using an optoisolator it usually means that there is mains somewhere close by. That's probably why newer optos (PC817) don't have a base connection.
The best thing to do with the base connection is to connect it to emitter with a capacitor. That tends to reduce interference, but makes it even slower, but if you wanted it to be quick, you wouldn't be using a 4N25.

What puzzles me is why did the bloke that invented it think we needed the base connection?
 

Thread Starter

LAOADAM

Joined Nov 21, 2018
658
With no LED current, the transistor can be used like any other transistor, it actually has rather good Hfe (about 600 if I remember correctly)
"Why would you want to do that?", I hear you ask, when you could just buy a transistor.
When component and assembly costs are at a premium - I once used a 4N25 transistor as a microphone amplifier for a sound-to-light unit and used its LED as the input from an external sound source.

The 4N25 looks like a transistor with a photodiode connected between base and collector, so a resistor between base and emitter would reduce the base voltage and make the CTR lower (It's low enough already). I think it also makes it respond (especially turn off) more quickly, as it shunts away the stored charge.
The base connection, if left unconnected, is a great place for it to pick up interference. If you're using an optoisolator it usually means that there is mains somewhere close by. That's probably why newer optos (PC817) don't have a base connection.
The best thing to do with the base connection is to connect it to emitter with a capacitor. That tends to reduce interference, but makes it even slower, but if you wanted it to be quick, you wouldn't be using a 4N25.

What puzzles me is why did the bloke that invented it think we needed the base connection?
Thanks.
very detail, helpful.
 
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