[SOLVED] Why so much power thru opamp?

[Zak28]

Simple circuit yet huge power thru opamp. Whats wrong here?

 

[FvM]

We can't check if the expression posted in the screenshot is actually representing U1 power, nor if the LM675 model is connected correctly.

For a useful answer, you can zip the simulation files (schematic, OP symbol and model) and post it. Or check the simulation details yourself by looking at individual current and voltage waveforms.

 

[Zak28]

...you can zip the simulation files...

Sure, heres the file:
View attachment circuits.zip

The pic detailed the correct op amp model, I replaced LM675T with the one in the pic just left the spice directive.

Greatly appreciated.

 

[Easy peasy]

the high current is through D1, D2. When U2 is hi U1 is lo, 29V supply, check the true zener voltage of these .... (D1, D2)

 

[FvM]

Apparently an inappropriate LM675 model. It generates internal voltage higher than power supply, dumping into substrate diodes. Must not use the model with saturated output.

Where did you get the model?

the high current is through D1, D2.

No.

 

[Zak28]

It does NOT matter which oamp is used, look in the first pic, the ad8512 was implace and huge power flowed.

 

[FvM]

It does NOT matter which oamp is used, look in the first pic, the ad8512 was implace and huge power flowed.

Sorry, that's BS. I can insert a different OP and the circuit is working fine. You have imported LM675 model in the original picture, there's apparently no AD8512 model, the symbol name doesn't matter.

If you have an AD8512 model showing the same effect, please post it. It's not provided with Ltspice.

 

[Zak28]

Measure oamp power in this circuit, ltspice provided oamp.
View attachment oamp.zip

 

[FvM]

O.K., I didn't notice that AD8512 is now included with Ltspice (due to the Linear/ADI merge). Problem is however similar, inappropriate OP model. Current is driven by G1 into D1 and D2 (supply rail clamp). It's a functional model not covering 2nd stage saturation.

*
* SECOND STAGE & POLE AT 50 Hz
*
R5   9  98  8.16E4
C3   9  98    39E-9
G1   98  9    5  6  7.8E-1
V2   99  8    1.8
V3   10 50   1.8
D1   9   8    DX
D2   10  9    DX
*

You may want to complain at the ADI forum.

- - - Updated - - -

As a simple test, open the AD8512 test fixture, change R1 to 2K (results in clipping output voltage). Watch the supply currents.

 

[Zak28]

...Problem is however similar, inappropriate OP model...

I was right about this, I have yet to see a normally behaving opamp in ltspice.

 

[FvM]

I received a comment from Helmut Sennewald, Yahoo Ltspice forum, he mentioned that older models (like the AD8512 one) often did not model supply currents, regardless which manufacturer. Following this interpretation, you should avoid any assumption about OP supply currents.

I didn't yet observe unrealistic high supply currents with any originally supplied Linear OP model, but I didn't yet check for useful quiescent current.