# Basic Simscape Electrical Circuit Not behaving as expected in Operational Amplifier Adder Circuit

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Captain Karnage on 12 Oct 2022
Answered: Captain Karnage on 28 Oct 2022
I'm having trouble getting a complex Simscape Electrical circuit working. It's not behaving as I'm expecting and I suspect it's just that I haven't used Simscape Electrical enough to understand the nuances. I decided I would go back to basics and do some simple circuits and build up from there. I was able to successfully simulate a basic voltage & current divider and a full-wave bridge rectifier. Both used the default settings and gave the expected results.
When I start getting to any more complex circuits, I'm running into issues. I even attempted to make a simple op-amp adder circuit. However, it just keeps going to the negative rail voltage. Please see the attached model. This one uses the "ideal" op-amp (with supposedly no voltage "rail") and it's outputting -90-V. I did try the Finite-Gain Op-Amp limited to +/- 15V and got a -15V result (slightly > than -15V actually).. My inputs are 3V and 6V, so I expect a result of 9V. It's a pretty basic circuit. The circuit (minus the batteries and sensors / scopes) came straight out of a textbook. What is wrong in my setup?

Captain Karnage on 28 Oct 2022
For anyone else looking at this, here's the answer:
1. The attached circuit had positive feedback - I forgot to flip the op-amp to put the - terminal on the top to match the circuit I was duplicating. The feedback needs to go to the (-) terminal, not the (+).
2. I forgot to adjust the feedback resistor resistance. In order to have a direct adder, you have to have R1 = R2 = RF. Since RF is 10k and R1 and R2 were 1K, I was getting -10 x (6 + 9) = -90V, which was exactly what I should have expected.
3. Op-amp adders will always invert, that's why it's negative. To flip it positive, would need another stage.
This was less re-learning simulink & simscape and more double-checking my values to make sure they were correct..

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