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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.
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.
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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
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collapsing magnetic field which will generate a reverse voltage spike acrross the coil terminals. This can cause damage to the driver circuit components. As such, a fly-back diode is commonly used with a relay. (to add the problems with this solution later)
Another option is to place a resistor in parallel with the coil
Alternative is to use zener diode in series with a regular diode
No 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:
Small and light
Generally has a longer operating life
Shock resistance
Quiet
High speed switching
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:
Cross-Bar Twin Contact: Moving and fixed ends come together at a right angle to enable stable contact
Twin Contact: Terminal that has two points of contact
Single Contact: A terminal with only one point of contact
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.)
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Additional Resources:
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
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.
Example in MSXIV
BMS
Additional Resources:
Pheonemnal Resource on relays:
https://www.ia.omron.com/data_pdf/guide/36/generalrelay_tg_e_10_2.pdf
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