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In this section, you will learn the basic differences between electromechanical relays (EMR) and solid state relays (SSR). The section will explain the advantages and limitations of both electromechanical and solid state relays. Each type of relay will be discussed in regard to its physical construction, operating features, and final application.

Depending upon design, relays generally do not control power consuming devices (except for small motors and solenoids which draw less than two amperes). In other words, the relay is used primarily as a switching element in the control circuit. Relays are used extensively in machine tool control, industrial assembly lines and commercial equipment. They are used to switch starting coils, heating elements, pilot lights, audible alarms and, in certain cases, small motors.

This section also compares the relays to an amplifier. This analogy seems appropriate in certain cases since a small voltage applied to a relay can result in a larger voltage being switched. From the circuit of figure below, we see that applying 24 volts to the relay coils may operate a set of contacts that is controlling a 230 volt circuit. Since relay coils require only a very low current or voltage to switch, but can energize larger currents or voltages, the relay is said to amplify the voltage or current in the control circuit.

Relays may be compared to an amplifier
Relays may be compared to an amplifier in that a small voltage may result in a large voltage output.

Another example of a relay providing an amplifying effect is illustrated in the figure below. Here a single input to the relay results in several other circuits being energized. Since certain mechanical relays can provide eight or more sets of contacts controlled from any one input, the input may be considered to have been amplified.

Relays may be compared to an amplifier 2
Relays may also be compared to an amplifier in that a single input may result in multiple outputs.

Although the analogy of an amplifier and a relay can be taken to extremes in its simplest format, this comparison is helpful in distinguishing the relay from other types of control devices.

Types of Relays

Relays to date can be placed into two major categories: electromechanical and solid state. Electromechanical relays consist of devices which have sets of contacts which are closed by some type of magnetic effect. Pure solid state relays, by contrast, have no contacts and switch entirely by electronic devices. A third category that is some times recognized is a type of solid state relay called the hybrid relay. Hybrid relays are a combination of electromechanical and solid state technology used to overcome unique problems which cannot be resolved by one or the other devices. Each relay category will be treated in this section as to advantages, limitations and applications.

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