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the problem

I have another problem with PSpice for TI2020. I want to build a PID controller using operational amplifiers. Up to the red line drawn, the circuit works as it should. However, if I add the op amp with a non-inverting basic circuit (U6), the simulation crashes and the error message is Overlow Multiply. What am I doing wrong?

What I was thinking

Yesterday I tested the individual components (P, I and D components) one after the other. Today I added the inverting adder.

The voltage source outputs a voltage from a mathematical function, which is: f(t)=0.05t²+0.55t. That is, at t = 0 the value is 0 and at t = 5 the value is 4. I do this with a quadratic function to see if the differentiator part is working.

For the individual components, I thought that the respective gain = 1, for test reasons, hence my (unrealistic-precise) capacitance values. I installed three LM321LV here.

Then I thought to myself today: The U5 must be able to add those three ones up, so I installed a THS4281 with a ±15V supply.

As mentioned, everything works fine up to here. If I add the U6 to the back, the simulation crashes.

Schematic

Daniel
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1 Answers1

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If it's a solver issue (which it looks like to me) try changing the value of R14 to something like 10k or 100k. Solvers like signals all being on the same 'scale' A large 10meg resistor will lead to really small currents which the solver is having a hard time 'resolving'. Another thing you could do is find the solver settings and increase the accuracy or change the type of solver.

Another thing you may want to consider is switching to LT spice, it's free and has a better solver

Voltage Spike
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    As you suspected, it was the resistor values. I increased R15, R16 and R11 to 3kΩ and R6 to 2300Ω. Thank you. – Daniel Feb 01 '22 at 19:41
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    This trick works with any numerical solver, some work better than others, but you want all the signals to be in the simmilar range or it can be hard to come up with a solution when quantization happens. – Voltage Spike Feb 01 '22 at 20:37