Each month this year, we’re exploring a different electronic component, delving into what it is, how it works, and how you use it in projects. Last month we covered transistors, and before that we looked at capacitors, LEDs and diodes. This month we examine relays! As always, we’ll start things off with an introduction to the component via an edited excerpt from Charles Platt’s essential Encyclopedia of Electronic Components: Volume 1.
Properly known as an electromagnetic armature relay to distinguish it from a solid-state relay. However, the full term is very rarely used. It may also be described as an electromechanical relay, but the term relay is normally understood to mean a device that is not solid state.
A relay enables a signal or pulse of electricity to switch on (or switch off) a separate flow of electricity. 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 relay controls a larger current near to the load. In a car, for example, turning the ignition switch sends a signal to a relay positioned close to the starter motor.
While solid-state switching devices are faster and more reliable, relays retain some advantages. They can handle double-throw and/or multiple-pole switching and can be cheaper when high voltages or currents are involved. A comparison of their advantages relative to solid state relays and transistors is tabulated in the entry on bipolar transistor in the Bipolar Transistor chapter.
Common schematic symbols for single-throw (top) and double-throw (bottom) relays are shown to the right. The appearance and orientation of the coil and contacts in the symbols may vary significantly, but the functionality remains the same.
How It Works
A 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. 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 relays, capable of handling a variety of voltages and currents, are pictured in to the right. 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 environment. 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 relay in a transparent case, rated to switch up to 5A at 240VAC or 30VDC.
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 relay.
Latching
There are two basic types of relay: latching and nonlatching. A nonlatching relay, also known as a single side stable type, is the most common, and resembles a momentary switch or pushbutton in that its contacts spring back to their default state when power to the relay is interrupted. This can be important in an application where the relay should return to a known state if power is lost. By contrast, a latching relay has no default state. Latching relays almost always have double-throw contacts, which remain in either position without drawing power. The relay only requires a short pulse to change its status. In semiconductor terms, its behavior is similar to that of a flip-flop.
In a single-coil latching relay, the polarity of voltage applied to the coil determines which pair of contacts will close. In a dual-coil latching relay, a second coil moves the armature between each of its two states.
Schematic symbols for a dual-coil latching relay are shown to the right. Some symbol styles do not make it clear which switch position each coil induces. It may be necessary to read the manufacturer’s datasheet or test the relay by applying its rated voltage to randomly selected terminal pairs while testing for continuity between other terminal pairs.
Polarity
There are three types of DC relay. In a neutral relay, polarity of DC current through the coil is irrelevant. The relay functions equally well either way. A polarized relay contains a diode in series with the coil to block current in one direction. A biased relay contains a permanent magnet near the armature, which boosts performance when current flows through the coil in one direction, but blocks a response when the current flows through the coil in the opposite direction. Manufacturers’ datasheets may not use this terminology, but will state whether the relay coil is sensitive to the polarity of a DC voltage.
All relays can switch AC current, but only an AC relay is designed to use AC as its coil current.
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