14 - RELAY Flashcards
What is a relay?
A relax is an electrical switch that operates under the control of an outside electrical signal. Relays are commonly used to start or stop other system components, such as motors. They are also used to electrically isolate lower control voltage from higher power voltage that is used to run higher current loads. As a result, the power circuit can be made as short and as direct as possible.
How does a relay work
The enclosed electrical contacts are operated by an electro-magnet. An outside low-voltage electrical signal energizes the relay’s electromagnetic coil, which creates an electromagnetic field that draws an armature toward the coil. This movement changes the contacts position. Normally, the armature is held in position by a spring. When power is applied across the coil, NO contacts close, and NC contacts open. This completes the load circuit and energizes the load. When there is no current through the coil, the spring moves the armature back into position, which returns the NO and NC contacts to their normal state. Some relays do not use electromagnets and moving parts, but instead use solid-state circuity. They produce the same expected results.
What are relay’s used for?
Relays are commonly used to run fans, start single phase motors, and control power circuits.
Explain the difference between the control circuit side and the load side
The amount of current used to operate a relay’s coil is usually very small; however, the contacts in the relay can often carry much higher currents. The coil of a relay is often connected to a low-voltage control circuit, and the relay contacts are connected to a separate higher voltage, load circuit.
What are the different types of relays (AC, DC, SS)
DC relays have the simplest operating principles. Voltage flows at a constant rate through the coil, creating an electromagnetic field, which pulls the armature into the coil and completes the load circuit.
AC relays work off slightly difference principles. In theory, AC voltage could be applied to a DC relay, but the result is highly impractical. Due to inductance, higher voltage AC must be applied to a relay to maintain the same electromagnetic force required to pull in the armature. Also, since AC voltage decreases to 0V every half cycle, the coil would intermittently lose electromagnetic force and release the armature. This continuous motion would result in an audible “buzz” and continuous intermittent loss of continuity in the load circuit. This results in impaired function of the load components, as well as increased electrical wear on said load components/mechanical wear on the relay electrical contacts. This issue is rectified through the use of a shader ring installed onto the coil. This creates a shortened turn in the coil, in turn creating an out of phase current/slight lag between the magnetism generated in one part of the coil and another part of the coil, resulting in continuous sustained magnetism.
SSR’s (Solid State Relays) are relays that us electronic components (such as transistors, silicon-controlled rectifiers, or triacs) rather than mechanical components to switch circuits on and off. SSRs are used to start single-phase motors and are often referred to as electronic relays. They are solid-state because there is no physical movement of mechanical parts. Switching is done electronically.
How to determine a relay has failed
- When there in unacceptable high resistance (for closed paths) or low resistance (for open paths)
- If the resistance for the coil relays is zero, OL, or out of recommended range.
- If the coil is energized and voltage is not present on a NO switch (or vise versa)
- If possible, use a power supply to test operation and confirm diagnosis
What is a relay mounting mechanism
The mechanical system in place to hold the relay in a desired location. Mounting system is subjective to different kinds of relays. Use good judgement when replacing
How to determine number and type of contacts
A wiring diagram can typically be found printed on the exterior of the relay, and can be used to understand its anatomy and function
How do you test the operation of a relay
Apply the required voltage to the relays coil, and check for continuity/lack of continuity depending on whether it is a NO/NC contact.
How to clean electrical contacts
- Disconnect the power
- Spray the contact with a cleaner specific to the application you are cleaning, and scrub gently with a cotton swab or soft bristled brush
- Remove excess debris and cleaner from contact after finishing
- Allow electrical contacts to dry completely before reconnecting power. This will ensure no moisture is present to cause damage to the contacts.
How to determine specifications for replacement
Pay attention to the load/control side required voltage and amperage. Ensure the relay has the same circuit configuration, and the physical arrangement of the relay will be compatible with any existing mounting mechanisms. Ensure the type of relay is the same (AC/DC/SS)
Visualize a relay
Do dat shit
What are the different parts of a relay?
A coil with two terminals, At least one set of each type of contacts (NO/NC), and a spring the hold the armature in position to maintain NO and NC states when the coil is not energized.
Explain the different types of common switch arrangements.
(SP-ST, DP-ST, SP-DT, DP-DT)
SP-ST: one electrical contact is controlled (NO/NC)
DP-ST: Two connected electrical contacts are controlled (Both NO/NC)
SP-DT: One electrical contact alternates between two NO and NC switches.
DP-DT: Two connected electrical contacts alternate between a set of NO and NC switches.
