Hey there, I'm trying to test the TL081 opamp model I downloaded for LTspice IV, so I made a simple summer circuit as shown on the pic.
The problem is also shown in the pic, Vin/off seems to be very large and therefore the circuit just doesn't work (I expect Vs to be 3+2=5 V but I'm getting -51 V). Any suggestions? Thanks!

 

Welcome to the forum!
You've got the inputs to the inverting and non-inverting inputs swapped over.

 

Well, thanks for the answer... but it's still happening the same thing, as you may see. Maybe I'm still having something wrong there?

 

Well, thanks for the answer... but it's still happening the same thing, as you may see. Maybe I'm still having something wrong there?

Hi,

Strictly speaking that's not really a summing circuit.

To make it a summing circuit I think you need R1=2k, and another 1k from the non inverting terminal to ground.

 

hi javi,
It works for me, is your model OK.?
E

 

Am I being nit picky or is V- setup wrong?

 

hi javi,
It works for me, is your model OK.?
E

Ok, I am looking at Eric's V- setup and OP definitely has V- setup wrong.

 

but it's still happening the same thing

Strange. It works ok in my LTS sim for various opamps (didn't trythe '081).

 

Strange. It works ok in my LTS sim for various opamps (didn't trythe '081).

hi alec,
I tried other OPA types, also no problems

E
TL801 model attached

 

Thank you very much, maybe there was something wrong with my model... I tried the same with a TL082 and worked!

 

Thank you very much, maybe there was something wrong with my model... I tried the same with a TL082 and worked!

Check the pinout assignment in the model file. It may be different than what LTspice uses for the op amp symbol. Look at a different LTspice op amp model for comparison.

 

Hi,

In the original diagram the inverting and non inverting terminals are swapped. To get it to work at all you need to swap those as found in Eric's drawing. I had to look really close to see this as the (+) and (-) inputs show up very very tiny on my screen.