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I have an extremely simple comparator circuit:
The voltage at pin 7 goes into positive saturation when a positive input voltage is presented at pin 5.
I'm simulating this using SPICE in KiCad. Here's the output:
As expected when +5V is presented at pin 5, pin 7 goes high. But why, oh why, is the current through R3 (shown with the red I(R3) line) listed as negative, and similarly positive when the input is -5V?
I feel like I might have misunderstood some fundamental concept of opamps.
Pin 1 and pin 2 are unshown on resistors in most simulators but, nevertheless the simulator will use one pin or the other as the reference for current direction. Which one does your simulator use as the positive pin? Try rotating the resistor so that pins are reversed and note the new current polarity.
R1.nA and R1.nB, defining current as positive flowing into the specified terminal. But it isn't possible to tell which terminal is which by inspecting the schematic graphically.
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May 17, 2020 at 23:33