LTspice: Vary a resistor's value over time

Question

I want to simulate the load regulation of a power-supply. I'm sure I remember being able to vary the value of a resistor over the course of a simulation in LTspice, but I can't remember how. Anyone?

Answer 1

Use the SpecialFunctions/Varistor.asy component with a time-varying voltage source

Answer 2

Unfortunately using a varistor will not work, as a varistor itself has a dependence on the voltage across it. Much simpler is to right click onto an existing resistor, and to enter a formula. E.g.

R=11-100*time

will linearily reduce the resistance from 11Ohm to 1Ohm over the time of 100ms. You can use almost all functions available for the voltage b sources (arbitrary behavioural voltage source), as well as all kinds of measurements e.g. of voltages of other nodes.

Answer 3

There is another way. Setup a Voltage source and choose the output you want. Label the output net VResistance. Volts on source will be exactly what the resistance is. I.e. 10KV will be the same as 10K ohm. Then use the standard resistor with the assignment R=V(Vresistance). As the voltage source changes, the resistor changes with it. The nice thing about this is the PWL file can now be used to control the resistance. Expecially nice when running things from Mathematica or Matlab.

Answer 4

Using McHale's suggestion, I produced a Current Dummy xLoad to test power supplies and power circuits. Based on a PWL sequence, the Load sucks current from the power supply, no matter the voltage at the supply.

The PWL sequence specifies a ramp&shake profile that exercises the supply, so one could analyse the behavior of such supply, if it bounces, oscillate, ring, voltage recuperation time, etc.

The xLoad .asy file can be anything with two connections, since it behaves like a dynamic resistor that change its value based on the PWL values AND the voltage applied at the Load inputs. You can apply a 10Vdc with a ripple of 9V and the Load will adapt its dynamic resistor so it follows the current profile from the PWL.

The xLoad has only one parameter, "mult". This parameter allows the user to change the maximum current from the PWL profile, so, mult=1 will use a profile that will suck maximum of 1A from the supply, mult=4.2 will suck a maximum of 4.2A. Your xload.asy must have a visible attribute "mult=1", so the xLoad will work, and you will be able to change the attribute at any time.

The xLoad uses a small capacitor to round very sharp edges that can simulate very high frequencies and rings, what doesn't happen in real life, so all the corners are a little rounded. If you want to change or eliminate this feature, just change the value of C1 from 10n or even eliminate that line. The feature is just a RC filter, R2 and C1, other way to change the filter is changing the value or R2, just don't delete such line, xLoad will not work without R2, you can change its value to zero ohms to eliminate completely the filter, even so I don't know why you will want to have MegaHertz sharp corners.

Create a text filename XLOAD.SUB into your LTSPICE/LIB/SUB directory, with the following contents (the "v1" line is long, not broken):

    * xLOAD
    * PWL Current Profile
    * By Wagner Lipnharski Nov/2015
    *
    *              Positive (Input)
    *              |  Negative (Output)
    *              |  |
    .SUBCKT XLOAD  1  2

    V1 3 2 PWL(0 0 +100m 0 +0.1m 0.2 +5m 0.2 +.1m 0.5 +5m 0.5 +.1m 1 +5m 1 +.1m 1.5 +5m 1.5 +.1m 2 +5m 2 +.1m 2.5 +5m 2.5 +.1m 3 +5m 3 +.1m 3.5 +5m 3.5 +.1m 4 +10m 4 +1m 3.5 +8m 3.5 +1m 4 +10m 4 +2m 2.5 +8m 2.5 +2m 4 +10m 4 +2m 1.5 +8m 1.5 +2m 4 +3m 4 +2m 0.2 +3m 0.2 +2m 4 +10m 4 +3m 0.2 +8m 0)

    R1 1 2 R=V(1,2)*4/(mult*V(4,2)+1n)

    R2 3 4 1k

    C1 4 2 10n

    .ENDS XLOAD

The simple .asc simulation with the symbol I made, and the plot planes showing current and supply rippled voltage below. Note that based on the PWL timmings, xLoad starts running at 100ms and finished at 235ms. You can change those timings at the PWL values inside the SUB.

Answer 5

If you want to step through values for resistor values (example R):

1. Set the value of resistor that you want to be variable, to be {R} (don't forget the curly brackets!)
2. Click on .op (far right on the toolbar)
3. Type: .step param R 1 10k 1k (steps from 1 to 10K in 1k increments)

If you want to sweep the value of R in time, then it's not possible as the simulators will have convergence problems!