Opto Isolator Circuit

[labview1958]

I have completly removed the +5V supply for the input of the opto-isolator. I have digital signals coming into the opto-isolator from the DAQ card. The "old" +5V line , I have connected to the ground input of the DAQ card. It is working fine. I really do not need the external +5V supply!

[labview1958]

How come my opto iosoltor can work without the +5V input from a power supply?

[philba]

Ask yourself "what is a signal?". What does a high output on the pin represent electrically? Is there any power there? <p>Well, the answer is yes. of course. When you output a 1 (or high signal) on a pin on your output port, the voltage on that pin will be around 5V. If you connect that pin with ground (0V), current will flow. <p>How much current is dictated by one thing - the amount of resistance of the connection to ground. However, if the resistance to ground is very low and ohms laws tells you that the current would have been above the current rating of the output port, you may note an odd smell. Quickly followed (or perhaps preceded) by the output voltage dropping to 0.<p>This is why the led in the opto can be light up when there is a 1 on your output port but needs a resistor to drop the voltage and restrict the current.<p>Hopefully this is an aha! moment for you...