# opto isolator

Discussion in 'General Electronics Chat' started by jonisonvespa, Jan 23, 2013.

Mar 25, 2012
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2. ### SPQR Member

Nov 4, 2011
379
49
Hi,

Why would you want to put that much voltage across an LED?
The max voltage is 3V.
The typical current from the datasheet is 50 mA, and the maximum 1A.
You'll need to use a resistor to drop the current anyway, why not just a voltage divider?

3. ### crutschow Expert

Mar 14, 2008
23,131
6,860
The input is an LED (diode) so you need a series resistor at the input to limit the current. For a nominal input current of 20mA you would need a resistor value of (24-1.3) / 20mA ≈ 1.3kΩ

4. ### SPQR Member

Nov 4, 2011
379
49

On the datasheet it gives a maximum of 3V as input to the LED, can you drive the LED without a divider to drop the voltage from 24V?

5. ### Brownout Well-Known Member

Jan 10, 2012
2,364
1,000
That's not a specification for driving the input. Look at it closly: that spec shows the forward voltage for a specified current, and it's given as a characteristic of the input device. What you are worried about is the "Absolute Maximum Ratings" chart. Forward current should be nominally 50mA, and Peak Forward Current is 1A. But don't design for peak forward current. Use the nominal "Forward Current" spec.

6. ### SPQR Member

Nov 4, 2011
379
49
Oh....Ok...thanks.

So then the input is like any LED, I should be mostly concerned with the current.

Thanks again!

7. ### mcgyvr AAC Fanatic!

Oct 15, 2009
5,394
1,194
make sure its rated 1W or more.

8. ### Brownout Well-Known Member

Jan 10, 2012
2,364
1,000
Yup. Use the equation given by Crutschow above and you'll be fine.