High-Performance Power ICs and Hall-Effect Sensors

	Contents
		Introduction
		Input Voltages
		Relay Contacts
		Activating Relays
		Solid State Relays

Input Voltages
The most popular control voltages are 120 volts/AC, 24 and 12 volts/DC and TTL (or 5volts/DC) signals. While other voltages are used in specific industries, those given here comprise more than 90% of all new and existing control-system installations. The control, or coil, voltages are connected to the electromechanical relay (EMR) via the input coil. On a solid state relay (SSR), an electronic input circuit replaces the coil.

Due to the application requirements, relays (also called interposing relays) connect different voltage signals without having the voltages connected to one another. In other words, while isolation is required to ensure the integrity of each voltage potential, reliable interfacing of the ON/OFF signal state must be guaranteed.

An important feature of relay output is the contacts' ability to switch large currents and power loads. Alternative coatings on the relay contacts allow the reliable switching of low-level signals of only a few milliamperes. Relay choice depends on both the required result and expected lifetime of the circuit, so it's important to understand the structure and design of both electromechanical and solid-state relays.

EMRs consist of an input coil that's wound to accept a particular voltage signal, plus a set of one or more contacts that rely on an armature (or lever) activated by the energized coil to open or close an electrical circuit. The input coil, by design, accepts either alternating current (AC), or direct current (DC). DC coils can be energized with far less current and, since they accept a steady-state current, can also be operated at frequencies higher than standard 60 Hz.

A contact's mechanical limitations dictate a relay's switching frequency. Since contacts possess inertia upon opening or closing, they can bounce before coming to rest. This bounce dictates the maximum operating frequency, and if exceeded, can lead to reduced service life, arcing, and-with sensitive control systems- can even be responsible for signal errors. With an SSR, higher frequencies of a few hundred kilohertz can easily be attained. In the case of both relay types, AC load switching is limited by the power-line frequency.

Introduction

Relay Contacts