Image credit: Newark | Digi-Key Corporation
Reed relays are electromechanical devices that operate using one or more reed switches.
Understanding Reed Relays
Reed relays are electromechanical relays that consist of a reed switch wrapped in a metal coil. Reed switches are comprised of at least two overlapping blades, or reeds, which are sealed within a gas-filled capsule. When the metal coil surrounding the reeds is energized, the reed contacts are drawn together and the switch closes. When the coil is de-energized, the spring force in the reeds pulls them apart and opens the switch.
An open (top) and closed reed relay. Image credit: National Instruments
A reed relay's contacts are much smaller and lighter than those of a typical electromechanical relay, resulting in a lightweight product capable of fast switching. However, the small size of reed relay contacts make them especially susceptible to contact damage due to arcing. Arcing often melts a small section of one contact to the other contact, resulting in a bond that is too strong for the reed's spring action to reverse. In this case, the relay becomes unusable and would need to be replaced.
Reed relays may be mounted using a number of different methods.
Bracket (or flange) mounted relays are equipped with a flange for mounting. The flange is typically installed by bolting the device to a matching flange which is then welded to a corresponding wall.
DIN rail mounted devices are equipped with fasteners capable of mounting on DIN rails. DIN rails are mounting devices standardized by the Deutsches Institut fur Normung (DIN).
Panel mount relays are manufactured for mounting to an electrical panel.
PCB relays are mounted on printed circuit boards (PCB) using through-hole contacts or surface mount technology (SMT).
Socket relays are mounted to PCBs using pin sockets.
The term "pole" describes the number of separate circuits controlled by a switch. The number of circuits controlled by the relay determines the number of switch contacts, which in turn determines the poles needed to make or break the contacts. Switches typically have between one and four poles.
The image series below illustrates, from left to right, a single pole (SP), double pole (DP), and triple pole switch (3P). Note that, in the last image, the switch is connected to three separate circuits and has three contacts.
Image credit: Enasco | Skycraft Surplus | Frank Alapini
It is also important to consider a relay switch's number of distinct positions, also known as throws.
Single throw (ST) switches are open in one position and closed in another. For example, a single pole single throw (SPST) switch is a simple on-off switch, such as a light switch. A double pole single throw (DPST) switch is an on-off switch that opens and closes two contacts with a single motion.
Double throw (DT) switches are two-way devices. Double throw relays have three contacts and two positions: in the first position, Contacts 1 and 2 are in contact, but the third remains open. In the second position, this connection is reversed to Contacts 2 and 3.
Specifications about contacts, including contact orientation and maximum ratings, are important to consider when selecting relays.
Contact orientation refers to the switch's position when a relay coil is not energized. As its name implies, a normally open (NO) switch is open in a resting, non-energized position; when current is passed through the relay, the switch then closes. A normally closed (NC) switch is therefore reversed: closed at rest, and open when energized. Changeover switches contain both NO and NC contact types.
Contacts are frequently rated to accept a maximum amount of current allowable under specified heat dissipation and ambient conditions. Maximum current is sometimes referred to as maximum switching voltage (expressed in volts) or maximum switching current.
While most reed relays, including the ones described above, are dry devices, some reed relays use mercury-wetted contacts to reduce resistance and voltage drop. Because of the expense and toxicity of liquid mercury, this method is rarely used.
A relay's speed specifications include make time and break time. Make time refers to the amount of time a switch needs to operate and make contact, while break time is the amount of time required to release and break contact. Switch speed is typically measured and specified in milliseconds. Especially in low voltage applications, high speed relays reduce noise, while in higher voltage applications fast switches reduce arcing and the possibility of physical damage.