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Floating base pin of optocoupler output transistor ?

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:

optocoup.jpg


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

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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

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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

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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

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