Let's use LTspice - Let's make a variable resistor - Semiconductor business -Macnica

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Design Analog Devices

In the previous article Let's use LTspice - Let's encrypt the SPICE model, we introduced how to encrypt the SPICE model. In this article, I would like to create a variable resistor.

When performing a circuit simulation, there are times when you want to check the movement of node voltages and currents by varying resistance values. However, LTspice does not have component elements such as variable resistors that change the resistance value.

So, this time, I will introduce a method to change the resistance value.

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Let's set the resistance parameters with a voltage source!

Usually in LTspice, the parameter of the resistance element is set to a constant such as R = 10Ω, but it is possible to set it with R = <formula>. Also, by specifying the node voltage in the <expression>, the resistance value can be varied.

I would like to do a simulation. Here, the resistance value is varied from 1k to 10k.
The circuit diagram you entered and the procedure for creating it are as follows.

Prepare the voltage source V1 to set the node voltage. Here, the node is "VR".
Consider the voltage value of the voltage source V1 as the resistance value. Here, set the voltage source to PWL (0 1k 1 10k) in order to change the slope from 1k to 10k per second.
Set the value of resistor R1 to R=<expression>=V(VR).
Set up the transient analysis and apply a voltage source V2 (here 5V) to the resistor and run the simulation.

Figure 1: Set node voltage to parameter of resistor value

By setting V(out)/I(R1) from "Add Trace" on the simulation result waveform viewer as shown in Fig. 2, it is possible to display the simulation result with the resistance varied with respect to the time axis. I can do it.

By using this method, you can change the resistance value in transient analysis, so you can check the voltage and current changes when the resistance changes.

Creating a Variable Resistor Using .OP Analysis

Figure 3: Voltage divider using variable resistors

We used transient analysis earlier, but here we will introduce how to change the resistance value using .OP analysis (operating point analysis).

This method uses the “.step” command to change the resistance value. You can also check the node voltage and current at that time.

Please refer to the following article for the .step command.
　
Let's use LTspice - Let's change parameters with ".step"
　
Here, resistors R1 and R2 are regarded as variable resistors, and a simulation is performed of the divided node voltage when a DC voltage source is connected.

Make the circuit diagram as shown in Fig. 3, and use the variable 'R' for the resistance parameter and the .step command to vary it between 1k and 10kΩ in increments of 1kΩ. The simulation results are shown in Figure 4.
　
The horizontal axis is the changed resistance value, and the vertical axis is the divided voltage value from the variable resistor.

Figure 4: Voltage value when resistance value is changed

In this way, the .step command and .OP analysis can be used to easily check the change in the voltage node when the resistance value is changed.

LTspice demo file verified this time

DownloadFile__1.zip

Simulation model implemented this time

At the end

This time, I introduced an example of how to create a variable resistor that changes the resistance value.

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