LTSpice - relationship between voltage and current of a varistor

Thread Starter

Gennaro Arguzzi

Joined Apr 5, 2016
9
Hi everyone, what is the constitutive relashionship between current and voltage of a varistor in LTSpice? For example, if I set the control voltage V=1V and Rclamp=2, what is the formula to evaluate the resistance of varistor? I tried to do this simulation (I used a 10V voltage source in serie with R1=1 and a varistor), but the result it's strange for me. The current on R1 is 3A, V(R1)=R1*I(R1)=1*3=3V, but the resistance of varistor is not Rclamp=2 (is 7/3=2.3).
Thank you very much.
 

Alec_t

Joined Sep 17, 2013
11,520
Well, LTspice must be using a model file for the varistor. Look for a file name with 'varistor' (or similar) in it, in the LTspice 'sub' folder.
 

Thread Starter

Gennaro Arguzzi

Joined Apr 5, 2016
9
I tried to understand the LTSpice Help, but without a simple example I can't. For this reason I did my example; in this example the behaviour of varistor is strange, the relationship between voltage and current is unclear for me. Could you help me to find this relationship in my example please?
 

Alec_t

Joined Sep 17, 2013
11,520
The relationship will depend on the model used. LTspice does not include a varistor model in the standard download, so where did you get yours? Since you can't/won't provide the model file, or a model card (if that's what you use instead on the schematic), how are we supposed to help?
 

crutschow

Joined Mar 14, 2008
25,263
The LTspice Help file states: "The VARISTOR is a voltage controlled varistor. Its breakdown voltage is set by the voltage between terminals 1 and 2. Its breakdown impedance is specified with the instance parameter rclamp. "
Thus, for your example, the current through the varistor would be (10V-1V) / (1Ω+2Ω) = 3A, exactly what you measured.
And the varistor resistance would be (7V-1V) / 3A = 2Ω, as expected.

When you calculated the resistance you didn't allow for the 1V breakdown of the varistor from V2.
You can't just take the voltage divided by the current to get the resistance, you have to first subtract the breakdown voltage.
 

Alec_t

Joined Sep 17, 2013
11,520
Oops. Just found the varistor you're using, hiding in the Special Functions folder :rolleyes:. There's no explicit .sub file for that, so presumably the model is an intrinsic part of an .exe or .dll file.
Try reducing the Rclamp value to a fraction of an Ohm and you will see a noticeable knee in the waveform plot in a Trans analysis.
 
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