0
\$\begingroup\$

I have a LM311P which I want to use as a comparator in a different project. To test the working of the comparator I created a simple circuit as shown in the diagram below:

enter image description here

As you can see in the diagram, I have used 5V on the non inverting terminal as a reference voltage which is fixed. The 9V source is connected through a potentiometer which allows me to control the voltage between about 1V and 9V. The VCC of the LM311 is connected to a 12V supply (I also used a 5V supply for this but that one didn't work either). Pin 1 and 4 are connected to the ground and the pin 7 (output) is connected to an indicator LED.

The problem I am having is there is no output. I varied the potentiometer all the way from both extremes but still the LED does not glow. I checked the LED to see if it is broken and it works fine when directly connected to a power source. I even changed the LM311 chip itself on suspicion of physical damage, but still the same result.

Why is this happening and how can I fix it?

\$\endgroup\$

1 Answer 1

4
\$\begingroup\$

Pin 7 is an open-collector output. It can sink current, but it cannot source current.

Reverse the LED and exchange the inputs.

\$\endgroup\$
1
  • 1
    \$\begingroup\$ And power the LED from +12, rather than return it to GND \$\endgroup\$ Aug 8, 2023 at 12:22

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.