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Spectrum Software has released Micro-Cap 11, the eleventh generation of our SPICE circuit simulator.

For users of previous Micro-Cap versions, check out the new features available in the latest version. For those of you who are new to Micro-Cap, take our features tour to see what Micro-Cap has to offer.

 

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Two Models for Relays

 

Occasionally a relay model is needed. Here are two models that work well. The first model, called RELAY1, is a simple model that includes coil series resistance and inductance, and turn-on and turn-off currents. The other is a more sophisticated but slower running electromechanical model of the relay that includes such effects as coil resistance and inductance, turn-on and turnoff delays, nonlinear fringing inductance, plunger rebound and the associated switch bounce.

Here is the RELAY1 model:

To simplify the process, consider the following test circuit.
RELAY1 Model
The input circuit of this model includes a user-specified coil resistance and inductance. The coil current is sensed and converted to a voltage by H1 which drives a Schmitt macro to provide hysteresis between the ION and IHOLD currents. The output of the Schmitt drives a standard voltage controlled switch S1.


The figure below shows the test circuit used.
RELAY1 Test Circuit
The figure below shows the transient analysis limits.

RELAY1 Transient Analysis Limits

The transient analysis below shows the input current waveform and the voltage at the output switch. The switch changes closes at the specified current of 15mA and stays closed until the input current goes below the specified 2.5mA holding current.

Transient Analysis of RELAY1
The parameters for the RELAY1 model are as follows:

RCOIL
Resistance of the relay coil

LCOIL
Inductance of the relay coil

RON
Closed resistance of the output switch

ROFF
Open resistance of the output switch

ION
Input current required to close the relay contacts

IOFF
Input current required to hold the relay contacts closed


An electromechanical relay model

RELAY2 is a more complicated model. It includes a flux circuit and derives a magnetizing force from the flux. It then algebraically sums the magnetizing, stop, friction and restoring spring forces acting on the relay plunger to arrive at a net force which is integrated once to get the plunger velocity and again to get the plunger position. This plunger position directly controls the switch contacts.

Here is the model for the RELAY2 model:
RELAY2
The figure below shows the test circuit used.

RELAY2 Transient Analysis Limits
The figure below shows the transient analysis limits..

RELAY2 Transient Analysis Limits
Transient Analysis of RELAY2
The transient analysis above shows the input current waveform and the voltage at the output switch. The plot also shows the internal waveforms for the net force, plunger velocity, and plunger position.

The parameters for the macro are shown below. Other constants are defined within the macro and may be modified to produce different mechanical models.

Parameters

RCOIL
Resistance of the relay coil

LCOIL
Inductance of the relay coil

RON
Closed resistance of the output switch

ROFF
Open resistance of the output switch

Constants

AREA
Plunger area in square mm

M
Plunger mass in kilograms

KSPRING
Spring constant

KSTOP
Stopping force constant

KFORCE
Net force scaling constant

 
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