Floating base pin of optocoupler output transistor ?

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hm_fa_da

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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.
 

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
 
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.
 
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.
 
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