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Why is this happening. My wiring diagram looks similar to this picture I created in LTspice. Can anyone help me come up with ideas to try and get this working? Thanks for reading.
The LM has an open collector output so it can not actively drive a positive voltage. You need to add a pull-up resistor of about 10k between the output pin and your positive supply rail. You might want to try making your drawings easier to read, since that makes it much easier for us to follow the circuit's ins and outs. In general, inputs come from the left and the top of the drawing, and outputs go out the bottom and the right, kinda like this, after adding the pullup to the comparator's output:.
Just for grins - since you want to learn about comparators - it might be fun to take a look at the two circuits below and figure out why VOUT2 switches and VOUT1 doesn't, even though VIN is the same input to both circuits.
Left click on the graphic to make it go full-screen and, by the way, here's the LTspice circuit list just in case you want to play with the circuit s :. Sign up to join this community. The best answers are voted up and rise to the top. Home Questions Tags Users Unanswered. Comparator help LmN Ask Question. Asked 4 years, 2 months ago. Active 4 years, 2 months ago.
Viewed times. Matthew Tubbs Matthew Tubbs 5 1 1 bronze badge. You need a pullup resistor. Reading the datasheet would have quickly cleared this up, there's even a nice "typical application" schematic. Thank you for pointing this out and I will go back over the datasheet to get a better understanding of how to read it.
Active Oldest Votes. Steve G Steve G 4, 1 1 gold badge 10 10 silver badges 23 23 bronze badges. I mistakenly thought the inverted its output. In general, inputs come from the left and the top of the drawing, and outputs go out the bottom and the right, kinda like this, after adding the pullup to the comparator's output: Just for grins - since you want to learn about comparators - it might be fun to take a look at the two circuits below and figure out why VOUT2 switches and VOUT1 doesn't, even though VIN is the same input to both circuits.
They helped a lot. I will look into your question. After testing the pull up resistor, I am left wondering when I invert the inputs I do not get 0v like I was expecting. Why is this? Can you describe what you mean by "invert the inputs" by editing your question, please? You'll never get 0V out with GND equal to zero volts, no matter what, because of the saturation resistance of the comparator's output transistor. Unfortunately, that ohm pullup I showed is more than likely causing the problem, and it should be more like 10k for an LM Sorry 'bout that For a better handle on what's going on with an LM, take a a look at the data sheet , particularly at Figure 3.
One more question thought please? After I get my desired voltage out of my Comparator, how do I stabilize it so that I can use it on a load? Say I want to use the 12v Vout to make my horn go off on my car. The horn is about 15 ohms. If you have a new question relating to a car horn, you should ask it separately.
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Datasheet LM2903N - National Semiconductor IC, COMPARATOR DUAL, 2903, DIP8
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