finally got caught up w/ some other stuff and had a question in regards to this:

the datasheet - http://www.fairchildsemi.com/ds/4N/4N33M.pdf

claims forward current up to 80ma and forward voltage of 1.2-1.5V. i am considering that my main input will be 13V max, so the R2 resistor for 1.3V/5ma to the incoming side will be of 2.7KΩ, why does the schematic say 10KΩ is necessary? is my math incorrect or am i missing something? are the leds in these to be used like a normal led? even if 15V was coming in, i am still well under anything near max if i am reading the datasheet correctly

edit: also, i don't have to have a common ground between the sides according to the datasheet....again if i am reading it correctly...just wondering why that schematic was created that way? what should i expect for a V drop across the diode? 1V? if i am using only +DC, is there any possibly way for it to go under 0V since the original power is coming from a wallwart, but i just didn't think they could go less than 0V.

thanks,
bob

 

Where's that schematic from?

 

it was on an arduino page

 

Which Arduino page? Post a link.

 

let me go through my history and find it...just out of curiosity, why do you need the link? i am not making up where i found it at...the original had 2 schematics on it, i took off the other circuit..

 

here it is - http://www.arduino.cc/playground/uploads/Learning/Level_shifting_4_arduino.pdf

 

I'm curious to why it was designed that way, the tied ground defeats using an optocoupler and the 10K is very high. What voltage range do you want for the input?

Edit, thanks for the link.

13V is this for an automotive application? Some designs would use a pair of transistors as a constant current source for the LED.

 

i kind of thought the same thing after i read the data sheet, and i just want 12-14Vmax, that is why i am a bit confused. the arduino can handle 5Vinput on its side so i was too wondering why such a high resistance on that side too.

 

Take a look at the optos used on this PLC trainer. (see the users manual PDF)
http://www.mikroe.com/eng/products/view/419/picplc16-v6-plc-system/

 

will take a look, thanks

 

Don't connect the grounds, and for 14V a 1K resistor should be ample.

 

is there an issue w/ the grounds connected as i did follow that arduino schematic at the moment...i just thought i read the data sheet wrong . i can change it on the next board i make and will. thanks for clearing it up.

now is there any way for a complete +dc power to get -? is the diode necessary? i know they are cheap and not a problem, but for education is the diode necessary?

 

I'm curious to why it was designed that way, the tied ground defeats using an optocoupler and the 10K is very high. What voltage range do you want for the input?

Edit, thanks for the link.

13V is this for an automotive application? Some designs would use a pair of transistors as a constant current source for the LED.

not an automotive application, just happens that the V is 12V.

 

The diode is only needed if you have an AC input or reverse the +/- inputs.

 

thanks, may as well leave the diode in case +/- gets reversed appreciate the info

 

this schematic would be more accurate:

 

Again get rid of the connection between 2 & 4 of the 4N33

 

Hello,

Here is the corrected circuit:

I also corrected the current for the input led.
In the datasheet is given that it should be between 10 and 50 mA.
Now it will be about 8 mA at 5 Volts and 23 mA at 12 Volts.

Bertus

 

excellent. just to verify, and i couldn't find it on the datasheet (maybe i did see it but didn't know??), but how long can the led stay on? are they designed to be on for say days at a time like a regular led? of course, staying under the 50ma as suggested and now put into place

 

excellent. just to verify, and i couldn't find it on the datasheet (maybe i did see it but didn't know??), but how long can the led stay on? are they designed to be on for say days at a time like a regular led? of course, staying under the 50ma as suggested and now put into place

Bertus' design should be more than capable of withstanding 12V continuously on its input for many years without failure of the OPTO.

hgmjr