Electronic – How to assign the same value to multiple resistors in an LTSPICE circuit

ltspicespice

I have around 10 resistors in a circuit, all with the default value "R".

I defined the directive

.param R=100

Upon running the simulation, I get the error message

Can't find definition of model "R"

Is there a general way to assign a value to all Rs at once or do I have to do it by hand one by one?

Best Answer

You need to set the value of those resistors to R=R. So, where you used to set the value to a number without a preceding R=, now the value has to start with R=

It may be more clear to use .param Rdefault=100 and R=Rdefault.

Make sure the .param Rdefault=100 is a spice directive, not a comment.

In answer to using curly braces { and }:
For both versions of LTspice (IV and XVIII) applies that when you want to assign a numerical value in the form of an equation, you have to use burly braces.
For example, when setting the value to {arctan(1)*4}.
In OP's question, setting the value to {Rdefault} will work too.

However, for non-numerical values, you need to (and for numerical values you can) precede the value with R= . When preceding it with R=, you do not need to use the burly braces:
E.g. when setting the value to R=arctan(1)*4*time

Therefore my suggestion to always use the preceding R=

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Electronic – LTspice: Vary a capacitor’s value over time

Well, yes and no. From the ltspice help:

There is also a general nonlinear capacitor available. Instead of specifying the capacitance, one writes an expression for the charge.

LTspice will compile this expression and symbolically differentiate it with respect to all the variables, finding the partial derivative's that correspond to capacitances.

Syntax: Cnnn n1 n2 Q=<expression> [ic=<value>] [m=<value>]

There is a special variable, x, that means the voltage across the device. Therefore, a 100pF constant capacitance can be written as

Cnnn n1 n2 Q=100p*x

A capacitance with an abrupt change from 100p to 300p at zero volts can be written as

Cnnn n1 n2 Q=x*if(x<0,100p,300p)

This requires a bit of different thinking, and although you can make use of the special time variable, I have never verified that things work as expected. Especially intresting would be the point where the capacity will suddenly be lower than what the cap is currently holding in energy, what will ltspice do here? Or if when for the next timestep the capacity is now higher, will the stored energy be the same or scaled too?

You should test if it behaves the way you want it in a seperate test circuit first.

Electronic – LTSpice parametric subcircuit instances

The LTSpice manual gives an example of how to do this:

* calling a subcircuit
*
* This is the circuit
X1 in out 0 divider top=9K bot=1K
V1 in 0 pulse(0 1 0 .5m .5m 0 1m)

* This is the subcircuit
.subckt divider A B C
R1 A B {top}
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The full syntax is given as:

Syntax: Xxxx n1 n2 n3... <subckt name> [<parameter>=<expression>]

The rules for writing expressions are given in the section of the manual on "arbitrary behavioral voltage and current sources".

Best Answer

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