optocoupler output not inverted

[yassin.kraouch]

i am usinf this optocoupler from fairchild https://www.fairchildsemi.com/ds/FO/FOD060L.pdf
FOD060L
when i connect the output to the collector ( Vo), with a pull down it is clear that the output is inverted ( when iput is 0 out is 1 ...), so if i need to connect the output to emitter of the output transistor, i need to use a pull down resistor, i don't know how is the value of the pull down, beacause in the datsheet they give us the value of pull up but not pull down,
any aidea ?

 

[dv_sa559]

I think you can use a transistor for revert out put pulse.
However i think there is not problem to connect pull down resistor to pin 5 instead of ground.

 

[yassin.kraouch]

Yes i know but it more simple then a transistor, juste output from the emitter not collector

 

[Raza]

simply apply the supply to collector (pin 6) and add a resistor the emitter (pin 5) you will get the result you want.
Hope this will help

 

[yassin.kraouch]

Yes i know that but the preoblem is the resistance value

 

[FvM]

In my opinion, the datasheet is very clear. The transistor has an open collector configuration with emitter tied to supply ground. I really wonder, how anyone can think about a pull-down resistor repectively connecting a resistor to the GND pin? Just read the datasheet thoroughly, and you'll hopefully understand, why the only reasonable operation mode is with a pull-up resistor, as all example circuits are suggesting.

 

[yassin.kraouch]

the problem is that i didn't find any information about that, they give the reference design when you want to output Vout from the emitter,

my problem is that i want that the output is 0 when the input is 0

 

[FvM]

they give the reference design when you want to output Vout from the emitter

You wanted to say, they didn't give a reference design, I guess. In fact, it's impossible without additional active parts.

my problem is that i want that the output is 0 when the input is 0

You'll need to add an inverter, either a NPN transistor or a logic gate. I assume, that it's not an option for you to drive the LED active low, which would of course also achieve non-inverted operation of the coupler.

 

[yassin.kraouch]

ah ok i thought that i can obtain directly an inverted signal if i connect the collector to Vcc and the emitter to the ground throught a pull down resistor, i think it is the easiest way but my problem is the resistior value, what do you think about this ?

 

[FvM]

The pin is named ground rather than Vee for a reason. As I already mentioned, it's a power supply pin. Whatever you do, you have to assure the supply voltage Vcc-Gnd to keep the specified range of 2.7 to 3.3 V. Furthermore, a quiscent current of about 7 mA will be sourced by the Gnd pin. So I can't imagine how you want to utilize the Gnd pin as a logic output.

Again, all these basic conclusions can be derived from the datasheet information.

 

[yassin.kraouch]

ok it is clear, and have you an example of an inverter, how can i use transistor ? a shematic can help

 

[FvM]

A transistor inverter would connect the base to Vo and emitter to GND, collector node is output. Pull-up resistors have to be used for Vo and the collector. But it won't hardly achieve the speed of FOD060L, which has a schottky-clamped output stage according to the schematic symbol. You can of course use a fast transistor and connect a schottky diode between B and C. But I think, a 3.3V CMOS inverter will be the better variant.

 

[dv_sa559]

Dear FVM,
As you know usually opto coupler chip only contain 2 part(opto sender & receiver) and component haven't other discrete Vcc and GND. thus we can drive both photo transistor and led by different biasing . am i true or no?
Regard

 

[FvM]

As you know usually opto coupler chip only contain 2 part(opto sender & receiver) and component haven't other discrete Vcc and GND.

usual opto couplers yes, presently discussed FOD060L no. Consult the data sheet.

 

[vimalkhanna]

Rather than do all the jugglery on the output transistor stages ,,why not think of changeover of the input LED logic??
put a 547 collector drive for the LED logic and your problem is simplified .

 

[FvM]

why not think of changeover of the input LED logic

I already mentioned the option in post #8, but was under the assumption, that it's not suitable here. It may be the case, if you want a specific output state, when the driver side loses power.

 

[3Deye]

So for this Optocoupler:
https://www.ram-e-shop.com/ds/ic/4N25.pdf

To get the output as non inverted, I can use a pull down resistor and take the output from the emmiter, correct?

 

[alexan_e]

Yes since in the mentioned optocoupler the emitter is not connected to ground internally.

Alex

 

[3Deye]

Thanks Alex.