Help understanding open collector comparator

Question

I am working on a simple circuit based on the LM311 comparator. It has an open collector, which is not something that I've used before, and I have a few questions. First, my understanding of the open collector model is that the internal comparator switches between open and connected to ground, requiring a source at the output to create a current. If that is the case, what is the purpose of the VCC rail voltages? Finally, I've tried modeling the circuit in multisim but I keep getting 5v at the output regardless of what the + and - inputs are. I assume that is due to my pull-up resistor, but I think I am just misunderstanding somewhat how this comparator is meant to be setup. My circuit is below, I've tried tying pin 4 (VCC-) to ground, and pin 8 (VCC+) to my 5v VCC, but I get the same result regardless.

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• richbai90: In case anyone stumbles upon this later. There are a few problems with this circuit. 1. The rail voltages need to be connected As pointed out in the answer 2. The reference divider at the inverting input is always less than the non-inverting 3. The probe is probing the power source not the output as pointed out in the answer 4. The emitter (pin 1) must be tied to ground for the comparator to function

Answer 1

The comparator still has Vcc pins because the comparator still needs power to run.

Also, you are probing the +5V power supply, not the output which is Pin 7. So of course you would always see 5V. Durr moment.