Values And Principles Of Electromagnetic Relay

Tuesday, May 1st, 2018 - Active Components

What is a relay? Relay is normally understood to mean a electromagnetic relay that is not solid state. While solid-state switching devices are faster and more reliable, relays retain some advantages. Electromagnetic relay can handle double-throw and/or multiplepole switching and can be cheaper when high voltages or currents are involved.

What Electromagnetic Relay Does

The electromagnetic relay enables a signal or pulse of electricity to switch on (or switch off ) a separate flow of elec­ tricity. Often, a relay uses a low voltage or low current to control a higher voltage and/or higher current. The low voltage/low current signal can be initiated by a relatively small, economical switch, and can be carried to the relay by rela­tively cheap, small-gauge wire, at which point the electromagnetic relay controls a larger current near to the load. In a car, for example, turning the ignition switch sends a signal to a electromagnetic relay positioned close to the starter motor.

Common schematic symbols for single-throw re­lays are shown in the following figure and for double throw relays. The appearance and orientation of the coil and contacts in the sym­bols may vary significantly, but the functionality remains the same.

SPSTSchematic symbols for a SPST relay

SPDT Electromagnetic RelaySchematic symbols for a SPDT relay

Values Of Electromagnetic Relay

Datasheets usually specify maximum voltage and current for the contacts, and nominal volt­ age and current for the coil, although in some cases the coil resistance is stated instead of nom­inal coil current. The approximate current con­sumption can be estimated, if necessary, by us­ing Ohm’s Law. The minimum voltage that the relay needs for activation is sometimes described as the Must Operate By voltage, while the Must Release By voltage is the maximum coil voltage that the mechanical relay will ignore. Relays are rated on the assumption that the coil may remain energized for long periods, unless otherwise stated.

While the contact rating may suggest that a relay can switch a large load, this is not necessarily true if the load has significant inductance.

  • Reed relays
    Usually use a coil voltage of 5VDC and have a contact rating of up to 0.25A at 100V. Through-hole (PCB) versions may have a coil voltage of 5VDC, 6VDC, 12VDC, or 24VDC and in some cases claim to switch 0.5A to 1A at up to 100V, although this rating is strictly for a noninductive load.
  • Small signal/low signal relays
    Usually use a coil voltage ranging from 5VDC to 24VDC, drawing about 20mA. Maximum switching current for non-inductive loads ranges from 1A to 3A.
  • Industrial/general purpose relays
    A very wide range of possible values, with coil voltages ranging up to 48VDC or 125VAC to 250VAC. Contact rating is typically 5A to 30A.
  • Automotive relays
    Coil voltage of 12VDC, and contact rating often 5A at up to 24VDC.
  • Timer relays
    Usually these specify a coil voltage of 12VDC, 24VDC, 24VAC, 125VAC, or 230VAC. The timed interval can range from 0.1 sec to 9999 hours in some cases. Common values for contact ratings are 5A up to 20A, with a volt­ age of 125V to 250V, AC or DC.

How Electromagnetic Relay Work

The electromagnetic relay contains a coil, an armature, and at least one pair of contacts. Current flows through the coil, which functions as an electromagnet and generates a magnetic field. This pulls the arma­ture, which is often shaped as a pivoting bracket that closes (or opens) the contacts. These parts are visible in the simplified rendering of a DPST relay in the following figure.

electromagnetic relay contains a coilThis simplified rendering shows the primary parts of a DPST relay

For purposes of identification, the armature is colored green, while the coil is red and the contacts are orange. The two blue blocks are made of an insulating material, the one on the left supporting the contact strips, the one on the right pressing the contacts together when the armature pivots in response to a mag­netic field from the coil. Electrical connections to the contacts and the coil have been omitted for simplicity.

Various small electromagnetic relays, capable of handling a variety of voltages and currents, are pictured in the following figure.

Various small electromagnetic relaysAn assortment of small DC-powered relays

At top-left is a 12VDC automotive re­lay, which plugs into a suitable socket shown im­mediately below it. At top-right is a 24VDC SPDT relay with exposed coil and contacts, making it suitable only for use in a very clean, dry environ­ ment. Continuing downward, the four sealed re­lays in colored plastic cases are designed to switch currents of 5A at 250VAC, 10A at 120VAC, 0.6A at 125VAC, and 2A at 30VDC, respectively. The two blue relays have 12VDC coils, while the red and yellow relays have 5V coils. All are nonlatching, except for the yellow relay, which is a latching type with two coils. At bottom-left is a 12VDC electromagnetic relay in a transparent case, rated to switch up to 5A at 240VAC or 30VDC.

The Configuration Of Electromagnetic Relay Using Abbreviations

The configuration of a relay is specified using the same abbreviations that apply to a switch. SP, DP, 3P, and 4P indicate 1, 2, 3, or 4 poles (relays with more than 4 poles are rare). ST and DT indicate single-throw or double-throw switching. These abbreviations are usually concatenated, as in 3PST or SPDT. In addition, the terminology Form A (meaning normally open), Form B (normally closed), and Form C (double-throw) may be used, preceded by a number that indicates the number of poles. Thus “2 Form C” means a DPDT electromagnetic relay.

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