Floating base pin of optocoupler output transistor ?

[hm_fa_da]

Hello,

I am going to use 4N26 vishay optocoupler for a project, it is a 6pin optocoupler as you see in this picture:

the output of the optocoupler - (photo transistor) has 3pins, first I used a 10K resistor between B (pin6) and E (pin4) but didn't function well, means when I turned on the LED, output doesn't switch.
I removed the resistor, then B (pin6) was floating, it started to work.
But I doubt if I use no resistor, wouldn't it make possibility of the Base pin to absorb noise ?
what's your recommendation ?! using no resistor ? using higher value resistor or ... ?
I am using this optocoupler in rs232 communication way.

 

[KlausST]

Hi,

Is there a reason
* why you choose 4N26?
* why you choose 10k?
We need to know this, to give you focussed help.

If you want us to help you should give us your requirements and expectations.
We don´t know how you tested it. No schematic, no LED current, no load voltage and current, no waveform and frequency.

RS232 communication: We don´t know whether you want to switch RS232 levels, or TTL levels, and at which baud rate.

Generally:
* if it works without a resistor... why not using it this way?
* did you read the datasheet carefully and the application note(s) provided by the manufacturer?

Klaus

 

[FvM]

Review the datasheet. Specified CTR is for floating base terminal. Base can be connected to achieve specific behaviour, but you should understand what you are doing.

 

[crutschow]

For maximum sensitivity the base is left open with no connection.
Adding a resistor to ground reduces the sensitivity, as you noted.
The bare pin is unlikely to pick up sufficient interference to affect the output.

 

[danadakk]

Floating base pin issues :

Optocoupler with phototransistor base lead

I'm thinking about using 4N25 optocoupler - it has a separate lead for base of phototransistor. How do I use it? I suppose I can't leave it floating?

electronics.stackexchange.com

Is it OK to leave the base of an optocoupler open circuit? - Page 1

Is it OK to leave the base of an optocoupler open circuit? - Page 1

www.eevblog.com

Regards, Dana.