# How to calculate node voltage in this circuit

I came across this circuit snippet as part of a larger schematic. It's intended to adjust the trip point of the comparator as a means of testing the overall circuit. My question is: how does one calculate the value of R3 to achieve a specific voltage at node A when switch S1 is closed? It looks like a combination of parallel resistance and resistor divider analysis, but I can't solve it. Thanks!

• Step 1: replace the 14 V source, R1, and R2 with a thevenin equivalent circuit. Commented Sep 25, 2023 at 0:09
• Please post a link or citation to the original document.. Folks here are more willing to answer then, Commented Sep 25, 2023 at 0:27
• Eric, discussing that circuit as a comparator (it's not, really, as there's no hysteresis present) when all you are really asking about is how to compute the Thevenin equivalent in two cases: switch closed and switch open. What are you really on about here? Commented Sep 25, 2023 at 0:49
• Need a resistor at the output ... Commented Sep 25, 2023 at 5:59
• @ThePhoton, thank you very much for your comment. It was exactly what I needed. You gave me just enough information to know what to search for. That search led me to learn something new, and that knowledge answered my question. Much appreciated! Commented Sep 25, 2023 at 16:33

## 2 Answers

From the datasheet ... Outputs are "Open Collector".

So, if you wire a resistor at output, it will respond ...
If my R2 = 22 kOhm (max), it will be ok.

• I probably shouldn't have included the comparator in my schematic at all, as it's not really the focus of my question. In the larger schematic, the comparator's output has no connection to its inputs (no feedback); it simply switches the next portion of the circuit. I'm sorry if including it caused confusion about what I was asking. All I really wanted to know was how to calculate R3 to achieve a desired voltage at node A. The comparator isn't involved. Commented Sep 25, 2023 at 16:33

Thanks to a comment posted by @ThePhoton, I found a tutorial at All About Circuits on how to use Thevenin's Theorem to perform a DC network analysis. To do that, I re-drew my circuit (below) to eliminate the comparator (it was an unnecessary distraction from my question in the first place), then went through the steps in the tutorial. Luckily, my re-drawn circuit was a very close match for the circuit used in the tutorial.