Types of Relays

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Electrical relays are devices that are installed into larger electrical devices. These relays control the flow of electricity to various components of a larger electronic device. They open and close to stop or allow electricity to control features of the larger device. Different types of relays are suitable for varied uses.

Latching Relay

  • A latching relay is one in which the switch remains in its last position after power is cut off. This type of relay offers only two positions: on and off. Two coils are used to move the switch from one side to the other. Latching relays only use power during the actual flip of the switch. Once the switch is in its desired position, the actual relay does not use any further electricity.

Reed Relay

  • Containing electrical elements and appearing much like a small glass tube is the reed relay. This tube is either vacuum sealed or filled with an inert gas to protect the inner components from corrosion. The contacts within the tube are typically open but can be closed by magnets.

Polarized Relay

  • The direction of the electrical current affects the operation of a polarized relay. The arm in the relay is permanently magnetized. The two coils within the relay are of the same polarity when no current is present, allowing the arm to sit between the two. Once a current is introduced, the two coils will have opposite polarity, depending on the direction of the current. Thus the arm will be attracted to one coil over the other, flipping the relay switch.

Mercury-wetted Relay

  • Similar to the reed relays, mercury-wetted relays come in the form of a glass tube with the contacts vacuum sealed or surrounded by inert gas inside. However, the contacts in these relays are soaked in mercury. These relays are generally used in low-voltage situations to create low-current signals. The mercury also helps where poor contact is caused by surface contamination. These relays must be installed vertically to work properly.

Machine Tool Relay

  • Machine tools and other small machines use machine tool relays. Due to the extensive use of these tools, the relays can easily be turned on and off, and their parts are easily replaced. The machine tool relays also come equipped with multiple contacts for versatility. While these were often used in industrial situations, such as the automotive industry, they have mostly been replaced by other components.

Contactor Relay

  • Electrical objects that require a large amount of electrical current generally use contactor relays. Electrical motors and some lighting typically use contactor relays. They are heavy-duty relays that can accommodate anywhere from 10 amps up to several hundred amps. Motor starters most often use contactor relays since they offer overload protection.

Solid-state Relay

  • Relays that have no moving parts are called solid-state relays, or SSRs. The lack of moveable parts make them more durable than many of the other types of relays. Early SSRs could not handle high levels of voltage. However, over the years, these relays have become capable of handling higher levels of voltage.

Solid-state Contactor Relay

  • Like the SSR, a solid-state contactor relay contains no movable parts and is more durable. This type is often used in heaters and other small motor items that turn on and off frequently. A programmable logic controller often controls this type of relay.

Overload Protection Relays

  • To prevent shorting out, overload protection relays are often used in conjunction with other types of relays. These relays contain solder or a bimetallic strip. When the solder or bimetallic strip heats up, it triggers the overload protection relay to shut off the flow of electricity. These relays prevent short circuiting in small engines and other electronics, thus preventing damage to the item.

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  • Photo Credit 12 volt relais, relay image by Sascha Zlatkov from Fotolia.com
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