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LTspice current injection
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SilverSolder:

--- Quote from: SiliconWizard on May 30, 2020, 02:47:52 pm ---A related note regarding current sources in Spice.

By default, they are ideal current sources. So the simulation will "force" the set current through the circuit, even if that would be physically impossible (in other words, it has no notion of voltage compliance). That can lead to very weird results in some cases.

LTSpice has an interesting option for current sources: you can set them as "active loads". They will then act more or less as you would expect a current source in a real physical circuit.

--- End quote ---

Interesting, how do you do that?   I don't see "Active Load" in Help

Jay_Diddy_B:
Hi,
You check this box:



Regards,
Jay_Diddy_B
Jay_Diddy_B:
Hi,

This is how it behaves:






It is useful in modelling power supplies, because it doesn't pull the output negative.

Regards,
Jay_Diddy_B
Gaktyt:

--- Quote from: Jay_Diddy_B on May 30, 2020, 12:10:15 pm ---If you look at the current in R3 or L3 it is always equal to 1A. The current source I1 is forcing the current to be 1A

--- End quote ---

You say "forcing", could the current source be placed differently so that it "adds" current instead of "forcing"?


--- Quote from: Jay_Diddy_B on May 30, 2020, 12:10:15 pm ---The voltage across the current source is equal in value and opposite in sign to all the other voltage is in the loop. This is Kirchhoff's voltage law.

--- End quote ---

Check, and thank you for making that clear to me, I appreciate it.

I'm still not completely sure that I understand the voltage in the circuit because; after the current is zero the voltage remains -100V (current*2) forever, so I'm in doubt if -100V is the new 0V? And if it is the new 0V, then what is the initial capacitor voltage? or when in time does -100V become the new 0V? Or maybe I should make a new post for this question?


Gaktyt:

--- Quote from: SiliconWizard on May 30, 2020, 02:47:52 pm ---LTSpice has an interesting option for current sources: you can set them as "active loads". They will then act more or less as you would expect a current source in a real physical circuit.

--- End quote ---

The Behavioral Current Source doesn't seem to have this option, or maybe it is set somewhere else?

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