# IF statement on LTspice

Discussion in 'Power Electronics' started by Firenze02, Mar 8, 2019.

1. ### Firenze02 Thread Starter Member

Sep 1, 2018
32
0
Hi,

I would like to create a variable resistor as following:

R2=1 if i(R2)<1
R2=2 if 1<i(R2)<10
R2=3 if i(R2)>10

I know that the sintax is:

R=if(condition,valueiftrue,valueiffalse)

What about if I have three intervals?

The .asc file and the IF statement law is also shown in the attached files.

Which is the correct sintax to do that?

Thank you.

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2. ### eetech00 Senior Member

Jun 8, 2013
1,472
289
HI

The statements are incomplete.

Write them in "if then else" pseudo code so we understand what you want.

eT

3. ### Firenze02 Thread Starter Member

Sep 1, 2018
32
0
Dear eetech,

I would like to "translate" in LTspice the following code:

if i(R2)<1 {
R2=1
}

if i(R2)>=1 && I(R2)<=10 {
R2=2
}

else{
R2=3
}

4. ### eetech00 Senior Member

Jun 8, 2013
1,472
289
Ok...I’m reading those as two “if” statements OR’d together right?

5. ### Firenze02 Thread Starter Member

Sep 1, 2018
32
0
Yes, the interval are the following:

R2=1 if i(R2)<1
R2=2 if 1<i(R2)<10
R2=3 if i(R2)>10

Thank You for any suggestions.

6. ### eetech00 Senior Member

Jun 8, 2013
1,472
289
See below.

In the .func statement, the "if" statement has to resolve to a number. I've added 0.5 at the end.
In the "Limit" statement, I've added arbitrary Min and Max values, 0.1,12, respectively. A discontinuity will occur if not used because at the start of the simulation R equals nothing (NaN). This will produce a "time step too small" error. The "Limit" statement will keep R2 equal to some value between 0.1 and 12.

eT

7. ### Firenze02 Thread Starter Member

Sep 1, 2018
32
0
Thank you eetech,

Finally I was able to simulate the circuit that I wanted, but unfortunately it doesn't work.

It gives me the following message:

Analysis: Time step too small, trouble with node v4.

Please find attached the .asc file.

Can you help me?

Thank you.

File size:
2.2 KB
Views:
4

Jun 8, 2013
1,472
289
Hi

See below.