4N35 optocoupler issue

[kaning]

I want to use optocoupler 4N35 as switch to turn on a 5v fan.
im using generic design of led input 5v in series with 1k resistor.
i turn on led with 5v and i get reading of 5v on the other but when i connect the fan to the 5v on the other side i get only 2,3v... why such a big drop? internal resistance?

i would apreciate some help, thnks in advance.

 

[Audioguru]

The datasheet for the 4N35 shows that its LED forward voltage is typically 1.18V at 10mA. You have a 1k series current-limiting resistor that provides a current of only (5V - 1.18V)/1k= 3.8mA.
The datasheet spec's the minimum Current Transfer ratio as 100% so the transistor output current could be as low as 3.8mA. The datasheet does not show typical curvfes with an output current higher than 10mA.
How much current does the fan need? 50mA? 100mA? Then you must have this opto-coupler drive an external transistor that can turn on your fan.

 

[kaning]

it needs 80mA, can i use regular npn transistor so 4n35 feeds base 5v on collector and fan on emiter to ground
?

many thansk

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this combination worked, thanks a lot for the tip. just to be sure the same i did this switch for fan i want to use a 4n35 to feed a microcontroller with exactly 5v. i do exactly the same way right?

 

[Audioguru]

If you use the external transistor an emitter-follower then its emitter output is about 0.7V less than its base voltage.