Define relays and explain how they function.
- A relay is a device that controls one electrical circuit by opening and closing contacts in another circuit.
- Relays have traditionally been used in machine tool control, industrial assembly lines, and commercial equipment. Relays are used to switch starting coils in contactors and motor starters, heating elements, pilot lights, audible alarms, and some small motors (less than Vs HP).
- A small voltage applied to a relay results in a larger voltage being switched.
- An example of a relay providing an amplifying effect is when a single input to the relay results in several other circuits being energized.
Define electromechanical relays (EMRs) and solid-state relays (SSRs).
- An electromechanical relay (EMR) is a switching device that has sets of contacts that are closed by a magnetic effect.
- A solid-state relay (SSR) is a switching device that has no contacts and switches entirely by electronic means.
Types of Electromechanical Relays
Explain general-purpose relays and applications.
- A general-purpose relay is a mechanical switch operated by a magnetic coil. General- purpose relays are available in AC and DC designs.
- General-purpose relays are EMRs that include several sets (normally two, three, or four) of non-replaceable NO and NC contacts (normally rated at 5 A to 15 A) that are activated by a coil.
- A general-purpose relay is a good relay for applications that simplify troubleshooting and reduce costs. Special attention must be given to the contact current rating when using general-purpose relays because the contact rating for switching DC is less than the contact rating for switching AC.
- General-purpose relays are designed for commercial and industrial applications where economy and fast replacement are high priorities.
Identify and define single-pole (SP) and double-pole (DP) contacts.
- A pole is the number of completely isolated circuits that a relay can switch.
- A single-pole contact can carry current through only one circuit at a time.
- A double-pole contact can carry current through two circuits simultaneously. In a double-pole contact, the two circuits are mechanically connected to open or close simultaneously and are electrically insulated from each other.
Identify and define single-throw (ST) and double-throw (DT) contacts.
- A throw is the number of closed contact positions per pole.
- A single-throw contact can control only one circuit.
- A double-throw contact can control two circuits.
Identify and define single-break (SB) contacts and double-break (DB) contacts.
- A break is the number of separate places on a contact that open or close an electrical circuit.
- A single-break contact breaks an electrical circuit in one place. Single-break (SB) contacts are normally used when switching low-power devices such as indicating lights.
- A double-break (DB) contact breaks the electrical circuit in two places. Double-break contacts are used when switching high-power devices such as solenoids.
Explain why a form letter is used to identify relay types and list common form identification letters.
- Relay manufacturers use a common code to simplify the identification of relays. This code uses a form letter to indicate the type of relay.
- Form A has one contact that is NO and closes (makes) when the coil is energized.
- Form B has one contact that is NC and breaks (opens) when the coil is energized.
- Form C has one pole that first breaks one contact and then makes a second contact when the coil is energized. Form C is the most common form.
Explain reed relay operation and applications.
- A reed relay is a fast-operating, single-pole, single-throw switch with normally open (NO) contacts hermetically sealed in a glass envelope.
- Reed relays are designed to be actuated by an external movable permanent magnet or DC electromagnet.
- AC electromagnets are not suitable for reed relays because the reed relay switches so fast that it would energize and de-energize on alternate half-cycles of a standard 60 Hz line.
- A permanent magnet is the most common actuator for a reed relay. Permanent-magnet actuation can be arranged in several ways depending on the switching requirement. One of the most commonly used arrangements is proximity motion.
- The proximity motion arrangement uses the presence of a magnetic field that is brought within a specific proximity (close distance) to the reed relay to close the contacts. Methods of proximity motion operation are the pivoted motion, perpendicular motion, parallel motion, and front-to-back motion. In each method, either the magnet or relay is moved. In some applications, both the magnet and relay are in motion.
Define arcing and explain how to protect relay contacts from electrical damage.
- Arcing is the discharge of an electric current across a gap, such as when an electric switch is opened. Arcing causes contact burning and temperature rise.
- Arcing is minimized through the use of an arc suppressor and the correct contact material for the application. An arc suppressor is a device that dissipates the energy present across opening contacts.
- Arc suppression is also accomplished by using a contact protection circuit. A contact protection circuit is a circuit that protects contacts by providing a nondestructive path for generated voltage as a switch is opened.
- A diode is used as contact protection in DC circuits. A snubber is used as contact protection in AC circuits.
Explain why silver is alloyed with other metals or gold-flashed when used for relay contacts.
- Fine silver has the highest electrical conductivity of all metals. However, fine silver sticks, welds, and is subject to sulfidation when used for many applications.
- Sulfidation is the formation of film on the contact surface. Sulfidation increases the resistance of the contacts.
- Silver is alloyed with other metals to reduce sulfidation.
Explain why relay contacts typically fail and how to help extend relay operating life.
- In most applications, a relay fails due to contact failure.
- In some low-current applications, the relay contacts may look clean but may have a thin film of sulfidation, oxidation, or contaminants on the contact surface. This film increases the resistance to the flow of current through the contact.
- Normal contact wiping or arcing usually removes the film.
- Contacts are often subject to high-current surges. High-current surges reduce contact life by accelerating sulfidation and contact burning.
- Contacts are over-sized in applications that have high-current surges.
Troubleshooting Electromechanical Relays
Explain why some relays have a manual operating switch.
- Manually operating a relay determines whether the circuit that the relay is controlling (output side) is working correctly.
- When manually operating relay contacts, the circuit controlling the coil is bypassed. Troubleshooting is performed from the relay through the control circuit when the load controlled by the relay operates manually. Troubleshooting is performed on the circuit that the relay is controlling if the load controlled by the relay does not operate when the relay is manually operated.
Explain how to troubleshoot a relay with a digital multimeter (DMM) to verify it is working properly or determine a fault if it is not working properly.
- Troubleshooting is performed from the input of the relay through the control circuit when no voltage is present at the input side of the relay.
- Troubleshooting is performed from the output of the relay through the power circuit when the relay is delivering the correct voltage. The supply voltage measured across an open contact indicates that the DMM is completing the circuit across the contact.
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