PSpice Application Notes

APPLICATION NOTE 1 Including Relays in PSpice Simulations In this application note, we will discuss two approaches for modeling the relay: the mechanical model approach, and two purely electrical (behavioral) models approach. The Mechanical Model Approach The Electrical (Behavioral) Model Approach o Model of a Relay (without Contact Bounce) o Model of a Relay (with Contact Bounce) The Mechanical Model Approach The mechanical model for the relay is modeling the mechanical part of electromechanical devices in general, using the relay as an example. This model constructs an electrical analogy to the mechanical operation of the relay. To do this, it calculates the magnetic and mechanical forces acting on the contact arm of the relay, and simulates the acceleration, velocity, and position of the arm in response to these forces. The electrical contacts of the relay are simulated by switches controlled by the position of the contact arm. There are two problems with this modeling approach: first it requires information about the physical construction of the relay (spring force, contact arm moment, magnetic permeance as a function of contact arm position) that is not normally available to the user of a relay, and second, it takes a lot of computer time to simulate the exact position of the contact arm. Most of this time is wasted if all the user needs to know is whether the contacts are open or closed. This type of physical model could be useful for designing a relay, but it is overkill for simulating its electrical behavior. The Electrical (Behavioral) Model Approach The second and third approaches simply model the electrical behavior of the relay coil and contacts. Rather than requiring physical construction parameters, these models require behavioral parameters. Model of a Relay (without Contact Bounce) The first behavioral model does not include contact bounce, and is the fastest to simulate. It requires parameters for coil inductance and resistance, contact resistance, pull-in and dropout coil currents, and make and break times. The model uses a PSpice digital buffer's propagation time to model the make and break times, and uses the Analog to Digital conversion device to model the pull-in/dropout current hysteresis. It uses a Digital to Analog conversion device in an unusual configuration to model the contacts. By using the digital devices, it is easy to set the delays using sub-circuit parameters, and there are no time step problems that can be caused by very high gain analog switches. The following circuit file shows the simple behavioral model of a relay (without contact bounce). * End of RELAY_SPDT_BHV_BOUNCE model * Behavioral model of a relay. (No contact bounce) .subckt RELAY_SPDT_BHV coila coilb no nc com + PARAMS: + T_make = 20mSec ; Time for contact to close when current is turned off/on + T_break= 10mSec ; Time for contact to open when current is turned off/on + I_pull = 35ma ; Pull-in current + I_drop = 25ma ; Drop-out current + R_coil = 100 ; Coil resistance + L_coil = 5mH ; Coil inductance

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