# Basics: How to interpret forward current?

Discussion in 'Homework Help' started by Nitrogen, Jul 31, 2013.

1. ### Nitrogen Thread Starter New Member

Jul 28, 2013
2
0
I'm an electronics novice, doing it as a hobby.

I've got a project where I'm trying to connect an optocoupler to a 12v logic signal line.

The optocoupler (Vishay 4N25) says it has a maximum forward current of 60mA.

I want to know if the optocoupler will draw too much current from the logic line and disrupt the other devices also listening in on the data signal.

Now because of my flakey electronics knowlege I'm not too sure of the following:

The optocoupler does not dictate the current passing through it right?

The current passing through the optocoupler is the line voltage (12v) divided by resistance (? ohms) - from what I can remember from school. So where do I find out the resistance of the optocoupler, or do they mean resistance of the whole circuit?

2. ### cornishlad Active Member

Jul 31, 2013
212
26
The input led in your opto coupler has no resistance as such, but a forward turn on voltage of what looks like 1.25v. The current through it must be set by an external resistor. 12v - 1.25v means 10.75v to drop. It may work ok in your application with 20Ma or less. So by ohms law 560 ohms might be a start.
If your line can't support that current you'l need a driver ic or a transistor to drive it.

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