# how to calculate power in circuit

#### ericgibbs

Joined Jan 29, 2010
18,234
hi.
This is what I get, using my OPA models.
Where did you get your OPA models.??
E

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#### aryna

Joined Dec 10, 2020
19
i got it from texas instruments manufacture, form pspice lib, and i create symbol in ltpsice. Hmm, neverminded i will try figure out . maybe my PC a little bit old, idk

But can you explain why we get negative value for power and current?
and this formula for power consumption is it correct P = I(Vdd)*V(vdd)?
OR
this P = - I(Vdd)*V(vdd) ?

#### aryna

Joined Dec 10, 2020
19
oo okay

then why we get negative for power?

so for my homework i can write the value power in positive or not?

sorry for asking too many question

#### ericgibbs

Joined Jan 29, 2010
18,234
hi,
It is the current direction that LTS uses, that appears to give a negative power which it is not in this case, its positive power.
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#### aryna

Joined Dec 10, 2020
19
okay thanks a lot you really help me a lot in my assignment , may God bless you. But i will ask another question another day, maybe hehe.
Thankyou sir

#### ericgibbs

Joined Jan 29, 2010
18,234
hi,
Do well in your examinations, so one day you can help other students.
E

#### aryna

Joined Dec 10, 2020
19
hi sir , i have another question for another circuit
for V2, what exactly value that i need to put in
I need to find power consumption and vout
i attach the asc file for this circuit.
Hope you will help me, pls

This is the circuit that im trying to do

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#### MrAl

Joined Jun 17, 2014
10,909
One of the things you have to watch out for when calculating power in a circuit is if any of the Spice models used in the circuit have their own internal power sources. Sometimes it will make a difference and sometimes not, and this is on a source by source basis where one internal source may be contributing to the output while another may not be. The ones that dont contribute power to some load or bias (or whatever) may not be actually present in the real life circuit.

An example would be a self powered op amp driving an NPN transistor where the collector goes to a voltage source (like 12 volts) and the load is connected from the emitter to ground and the base is driven from the output of the op amp. If we only measure the power in the collector (because it is the only thing connected to the main voltage source of 12 volts) then we would miss the power going into the base from the self powered op amp. The cure would be to use a different model for the op amp where the power terminals are available for measuring the power going into the op amp also.

It gets a bit tricky so you have to look at all the models carefully.

BTW average power can be measured by measuring the current though an element and the voltage across it, then multiplying the two (volts times amps) then averaging the result all using math available with the simulator.

#### aryna

Joined Dec 10, 2020
19
hi.
This is what I get, using my OPA models.
Where did you get your OPA models.??
E
hi
can you provide to me OPA models that you get for 0.05 for power?

#### ericgibbs

Joined Jan 29, 2010
18,234
hi a,
You should have that model in F2 Opamps.?

This zip is the OPA227 .mod and .asy

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#### ericgibbs

Joined Jan 29, 2010
18,234
hi a,
I have created your version of the OPA227, that is not the problem thats giving 7 Watts, it is the way you have labelled the VDD and VEE

This your OPA227 in the original circuit.
E

I will modify the VDD and VEE and post a corrected version

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#### ericgibbs

Joined Jan 29, 2010
18,234
hi a,
This is what you get when the power supplies are connected correctly using your OPA227 model, same result as my model.

Don't make your power labels so messy.
E

BTW: I have named your version of the model as OPA227a.

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