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# Setting up switches

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## Description

The Voltage Controlled switches are controlled by the input voltage. They hard switch between the finite Ron and Roff resistance values. They do not have any linear or slew rate limited regions between these two resistances.

Ron must be > 0.

Roff must be > 0 and finite.

(In fact, resistances can be negative but it is not recommended to try this unless you understand exactly the consequences of using negative resistance values!)

They have a threshold voltage, VT, below which they are in the 'OFF' state and above which they are in the 'ON' state.

It is important to understand that the resistance in each of these states is independent of the actual state so the ON state resistance (Ron) can be high or low. Similarly the OFF state resistance (Roff) can also be high or low.

They can also have a hysteresis, VH, which is any positive value including zero. VH = 0 causes the switch to change state at exactly the value of VT. A positive value of VH causes the switch to turn 'ON' at VT+VH and 'OFF at VT-VH.

Switches with hysteresis can also be specified to be in a OFF or and ON initial state for an input voltage that is anywhere between the hysteresis range.

Note that current controlled switches behave in the same way but using a controlling current rather than a voltage.

These possibilities are illustrated in this simulation.

S1 is an initially OFF switch with hysteresis;

S2 is an initially ON switch with hysteresis;

S3 is a switch with zero hysteresis a high resistance OFF state and a low resistance ON state;

S4 is a switch with zero hysteresis a high resistance ON state and a low resistance OFF state. In other words it is an inverting input switch compared to S3.

• For complete documentation of how the Voltage and Current Controlled switches operate please see:

http://ngspice.sourceforge.net/docs/ngspice-manual.pdf#subsection.3.2.14

http://ngspice.sourceforge.net/docs/ngspice-manual.pdf#subsection.3.2.15

## BOM

ID Name Designator Footprint Quantity
1 SIN(1.5 1.5 2k) V1 2P-5.0 1
2 Voltage_Controlled_Switch S1,S2,S3,S4 NONE 4
3 1k R1,R2,R3,R4 R3 4
4 3 V2 2P-5.0 1

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