Relays

Owned by Micah Black
Last updated: Sept 04, 2024 by Liam Sky
6 min read

To add more info soon

What is a relay

A relay is a switch that can be used to open and close circuits when they receive an electrical signal (can be voltage or current) applied to a coil.

 

How They Work

Normally open – power flows through the input circuit, activating an electromagnet that attracts/repulses causing a contact to join with the second larger circuit, allowing current to flow. When power is removed, a spring draws the contact away from the second circuit, stopping the flow of electricity.

Normally closed – fundamentally the same as normally open, but with the default states reversed.

 

Types of relays

(to add info on fet switches and reed relays)

Movable Contacts (Mechanical Relay)

Mechanical relays have contacts inside of them that are used to open or close a circuit. This happens when a electric signal is applied to the coil which generates a magnetic force which can either close or release the contacts.

Common types of mechanical relays

  • Hinged relays → the armature of the electromagnet rotates around a fulcrum which is responsible for opening and closing a contact

Notes:

  • Physical Spacing

    • The physical spacing between the coil and the contact component needs to be considered as it will affect the level of insulation on both the input and output

  • Coil Suppression

    • When the coil de-energized, there will be a collapsing magnetic field which will generate a reverse voltage spike across the coil terminals. This can cause damage to the driver circuit components. As such, a fly-back diode is commonly used with a relay.

    • Another option is to place a resistor in parallel with the coil

    • Alternative is to use zener diode in series with a regular diodes

 

Read more about flyback diodes here: Using Flyback Diodes in Relays Prevents Electrical Noise in Your Circuits

Non movable contacts (MOS FET relay, Solid State Relay)

Non-movable contact relays use semiconductor and electrical switching elements like MOS FET and triac to act as switches.

Some benefits of this type of relay:

  1. Small and light

  2. Generally has a longer operating life

  3. Shock resistance

  4. Quiet

  5. High speed switching

Operating Forms

  • Single-stable relays → Relays where the contacts switch based on the non-excitation and excitation of the coil

  • Double-winding latching relays → Relays with a set and reset coil and a latching configuration to hold the status

  • single-winding relays → relays with a coil and a latching config that switch to and hold set or reset based on the polarity of the applied voltage

  • stepping relays → relays that turn multiple contacts ON and OFF each time an input pulse is received

  • ratchet relays → relays that switch between ON and OFF for each input pulse

 

Other Relay Considerations

Contacts

Relays which use contacts can come in different types each with differing levels of reliability. In terms of most to least reliable is as follows:

  1. Cross-Bar Twin Contact: Moving and fixed ends come together at a right angle to enable stable contact

  2. Twin Contact: Terminal that has two points of contact

  3. Single Contact: A terminal with only one point of contact

  • lift-off type → the type of drive method for the contact spring

  • flexure type → a drive method for the contact spring which is obtained from a stud, card, or other pushing force

 

Terminals

Relays can have different types of terminals attached to them ranging from screw terminals to pc board terminals.

 

 

 

Enclosures

Relays come in various types of enclosures such as Plastic Sealed and Flux resistant type. Depending on where they are being used, you may want to consider what they can protect against (dust, dirt, etc.)

 

 

 

Relay Terminology

  • Maximum switching volts → The maximum voltage that can be across the contacts whether the relay is open or closed. Going above this voltage will erode the contacts reducing the performance

  • Cold switching voltage → Relays should be able to sustain voltages that are higher than the max switching voltage as long as the relay is not operation when the signal is applied

  • Switch Current → The maximum current that the relay can handle when it is being opened or closed

  • Carry Current → Carry current is the similar to cold switching voltage, ie. take a higher current than the switch current as long as the relay is not opened

  • Pulsed Carry Current → Heats the relay contacts

  • Power rating → How many watts the relay can handle. It is especially important to realize that this is generally lower than switch current * maximum switching volts so be careful when selecting a relay and how much voltage and current is being applied

  • Minimum switching voltage → some relays require a minimum voltage to ensure low contact resistance

  • Operate time → The time that it takes to process a driver instruction as well as the time the relay takes to operate and settle

  • insertion loss → the loss of a high-frequency signal between closed contact terminals

  • return loss → the quantity of high-frequency signal reflection that occurs in a transmission path

  • crosstalk characteristic → degree of high-frequency signal leakage between contact circuits

  • maximum high-frequency carry power → the max high-frequency signal power that can pass between closed contact terminals

  • maximum high-frequency switching power → the max high-frequency signal power that a contact will switch. Note, the electrical durability will be shorter than for the rated load

  • impulse withstand voltage → the max abnormal voltage a relay can withstand when the voltage surges

  • insulation resistance → the resistance of the isolated sections between contacts and coils and other conducting terminals and uncharged metallic parts

 

Relays Vs Transistors

  • Relays can handle higher current and voltage loads

  • Relays don’t leak current

  • Relays generally have very low resistance

  • Switching with relays is slower than with transistors

  • relays can cause EMF interference

  • transistors are generally cheaper

For high or unknown loads chose a relay whereas if it is a smaller load where power consumption is important of you need to switch a lot use a transistor.

Relay Safety

  • Do not touch the terminal section (charged section) of the Relay or Socket while power is being supplied. Electric shock may occur.

  • Never use a Relay for a load that exceeds the contact ratings of the Relay, such as the switching capacity. Doing so may result in reducing Relay performance for insulation failure, contact welding, and contact faults, and might even result in burning or other damage to the Relay itself.

  • Do not drop the Relay or dismantle it. Doing so may reduce Relay performance and might even result in burning or other damage to the Relay itself.

  • Relay durability is greatly affected by the switching conditions.
    Always test the Relay under actual application conditions to confirm applicability and use the Relay only for the number of switching operations that will not affect performance. Continued application of a Relay with reduced performance may result in insulation failure between circuits or in burning in the Relay itself.

  • Do not apply an overvoltage or incorrect voltage to the coil, and do not wire the terminals incorrectly. Incorrect application may prevent the Relay from performing its designed function, may affect external circuits, and may even result in burning or other damage to the Relay itself.

  • Do not use the Relay in atmospheres containing inflammable or explosive gases. Switching arcs or Relay heating may result in fire or explosion.

  • Wire the Relay correctly according to the Precautions for Correct Use when performing wiring or soldering. If the Relay is used with wiring or soldering that is defective, abnormal heating while power is supplied may result in burning.

Example in MSXIV

BMS

 

Additional Resources:

Phenomenal Resource on relays:

https://www.ia.omron.com/data_pdf/guide/36/generalrelay_tg_e_10_2.pdf