Different Value of Current in a wire in Pspice

Thread Starter

Ngô Đình Nhân

Joined Jan 8, 2017
45
Hi everyone

I simulate my circuit in Pspice, I encountered a quite fun problem ! Actually I dont know why ? Please my image

Image 10.jpg

<1> with U=-5VDC, R=250Ohm => I=U/R~ 20mA but in this case is 153.7uA ? @@
<2> In the same connection but different current !

Does anyone have any idea for my case ?

Thanks in advanced
 

Jony130

Joined Feb 17, 2009
5,485
Show us full circuit diagram. And in PSpice world the current leaving the node is negative and current enter the node is positive therefore 153.7μA is the current entering the R7 resistor.
 

WBahn

Joined Mar 31, 2012
29,857
You've already been given the explanation. By long standing convention, the current at a device pin the current entering the pin. If you have a wire between two devices and there is any current in the wire, then it is leaving one pin and entering the other -- hence the current at one pin is positive and the current at the other is negative.

If you ask for the current in a two-terminal device like a resistor, the convention is that the result is the current at pin 1.
 

WBahn

Joined Mar 31, 2012
29,857
Hi everyone

I simulate my circuit in Pspice, I encountered a quite fun problem ! Actually I dont know why ? Please my image

<1> with U=-5VDC, R=250Ohm => I=U/R~ 20mA but in this case is 153.7uA ? @@
<2> In the same connection but different current !

Does anyone have any idea for my case ?

Thanks in advanced
As for why it is well under a milliamp when you think it should be 20 mA, that has to do with understanding what Ohm's Law is and what it is not.

Ohm's Law relates the current THROUGH a resistor to the voltage ACROSS that resistor. You can't just throw the nearest voltage value you've got at the formula and expect to get a meaningful result.

What is the voltage on top of the resistor? -5 V

What is the voltage on the bottom of the resistor? -4.962 V

What is the voltage of the bottom of the resistor relative to the top of the resistor? (-4.962 V) - (-5 V) = 0.038 V

What does Ohm's Law say that the current flowing through the resistor from the bottom to the top should be: 0.038 V / 250 Ω = 152 μA.

Why is it 152 μA and not 153.7 μA?

Round off. What would the voltage differential need to be if the current is 153.7 μA? 38.425 mV
 

Thread Starter

Ngô Đình Nhân

Joined Jan 8, 2017
45
Thanks so much Wabahn

I fully understand your mean.
But in my case I want to create I=20mA at pin 1, this is the reason that I put R7=250Ohn and Vcc=-5V in there.
So could you give me a idea for I=20mA, because the transcontuctans ( gm, mA/V ) of OPA660 depends on the current at pin 1.
 

Thread Starter

Ngô Đình Nhân

Joined Jan 8, 2017
45
Hi Wbahn !
I follow to the datasheet, like this:
Image 15.jpg

I need to current on pin 1 = 20mA. This is reason ( I=20mA that means Transconductance = 125mA/V )
gm.jpg

Do you have any idea to make 20mA on pin 1 ?
 

WBahn

Joined Mar 31, 2012
29,857
Again, why do you want the current on pin 1 to be 20 mA. Putting a 250 ohm resistor from pin 1 to the -5 V supply results in Iq (the quiescent current of the amplifier) being about 20 mA, not the current at pin 1. The quiescent current is approximately 85 times the pin 1 current. Read the data sheet more carefully.
 
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