Flipping LTspice Voltage Controlled Switches: How to Use a Threshold Voltage

In LTspice, a voltage-controlled switch (VCS) can be configured to open at a specified threshold voltage instead of closing. To achieve this, you can use a combination of the Vswitch component and a simple voltage control mechanism. This article will guide you through the process step-by-step.

Steps to Flip a Voltage Controlled Switch

1. Insert a Voltage-Controlled Switch

To begin, place a Voltage-Controlled Switch (VCS) in your circuit. You can find it in the component library under SW or use the command S in the schematic.

2. Control Voltage Source

Create a voltage control signal source, such as Vcontrol, which will act as the control signal for the switch. This source will determine when the switch should open or close.

3. Set the Control Threshold

To make the switch open at a certain threshold voltage, Vth, you can use an inverter configuration or modify the control voltage to trigger the switch appropriately.

4. Use a Behavioral Voltage Source

To create a control signal that outputs 0 when the control voltage is below the threshold and 1 when it is above, you can use a behavioral voltage source. For example, you can place a behavioral voltage source B-source with the following expression:

VVcontrol-Vth 0 : 1

5. Connect the Control to the Switch

Connect the output of the B-source to the control terminal of the switch.

Example Circuit

Here is a simple configuration to demonstrate the setup:

Voltage Control Switch Example

Vcontrol N001 0 DC 5 Control voltage source Vth N002 0 DC 3 Threshold voltage source B1 N003 0 VV(N001)-V(N002) Behavioral source to create control signal Switch Configuration S1 N004 N005 N001 N003 SW Voltage controlled switch .model SW VswitchRon1m Roff1Meg Vt0 Switch model Load Rload N004 0 1k Load resistor

Explanation

Vcontrol: This is the voltage that you want to control the switch with.

Vth: This is the threshold voltage at which you want the switch to open.

B1: This behavioral source outputs 0 if Vcontrol is less than Vth and 1 otherwise, effectively inverting the control logic for the switch.

S1: The switch will now open when the control voltage exceeds the threshold.

Simulation

Run the simulation and you should see that the switch opens when the control voltage goes above the threshold voltage. This setup effectively flips the behavior of the voltage-controlled switch, allowing you to control when it opens based on a threshold voltage.

By following these steps, you can easily configure a voltage-controlled switch in LTspice to open at a specified threshold voltage, further enhancing the functionality of your simulations.