# Model a set-reset relay in SPICE

I am designing a circuit with a magnetic latching set-reset relay, and I would like to test the design in ngspice (open-source spice). I have downloaded the documentation but I see nothing about relays, so I wonder: how can I model this device?

I want to model a circuit that activates and deactivates itself automatically as a particular voltage varies. As input for the relay I only need a voltage (which will be categorized as "on", "hysteretical" or "off" by the relay, and as output naturally a short-circuit or open-circuit. I don't need much exactitude (such as relay impedance and the likes).

• When saying "how can I model this device", do you mean the way this relay affects its driving circuit, the way it affects the circuit which it switches, or both? – Vasiliy Jul 26 '13 at 13:19
• Sorry, I now acknowledge my poor description. To clarify, I have edited my question. – Severo Raz Jul 26 '13 at 17:27

After having experimented a lot, and read some more, I found an answer to my question.

# Simple switch model approach

After seing a demo on the switch models I finally understood the parameters and workings of the SPICE switch model, which I will explain below.

Switch model syntax

.model MODEL SW VT VH RON ROFF

SX N+ N- NC+ NC- MODEL <ON><OFF>

(where a RegEx for X is [a-z0-9]{0,7}.)

What the manual doesn't explain

Although easily guessed:

• The N nodes are the controlled nodes, which will be "short-circuited" or "opened" (continue reading for quoting reason.)
• The NC nodes are the controller nodes, from which the "switching" voltage will be read.

The switch is essentially on when the voltage accross the controlled nodes is positive, and off otherwise, this is also easily guessed.

Now, the hysteresis voltage makes it possible for the user to define a "dark voltage interval", in which the switch will not be able to change its state. This interval is defined by [VT-VH, VT+VH], and is what provides hysteresis to the model. This is interpreted as follows:

• A voltage over VT+VH will turn the switch on.
• A voltage below VT-VH will turn the switch off.

Because these are the only conditions ruling the model, the model needs an initial on or off state if the initial voltage across the controller nodes is within the dark voltage interval. This initial state is optional otherwise, for it would be overridden.

The current controlled switch model is analogous to this one, but take note about that between the controlled nodes there is a short circuit.

## Sample circuit

Voltage controlled switch

V0 1 0 SIN(0 12 2 0 0)
S1 1 2 1 0 simpleswitch OFF
R2 2 0 10

.model simpleswitch sw vt=0 vh=6 ron=0.1 roff=1Meg

.control
tran 1e-3 1 uic
plot v(2)/10.1 v(1)/10.1 \$ voltage/(resistor resistance + switch on resistance)
.endc Above you can see the plot resulting from the simulation of the circuit source code. The blue curve corresponds to the source voltage, the red curve to the voltage across the resistor.

As it can be observed from the code, the so called dark voltage interval is [-6,6], at which the switch does nothing (remembers its last state). As soon as the voltage across the controller nodes is over 6 V, there is a current flowing through the resistor (and thus non-null voltage across it); as soon as the voltage is below -6 V, the current flow is cut.