Relay

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Electromechanical relay

An electromechanical relay is a switch controlled by an electromagnet. It is used to isolate the control circuit (which operates the electromagnet) and the power circuit (which is controlled by the switch) from each other. There is then no conductive connection which would allow the current flowing in the power circuit to damage the control circuit.



Relay coil voltage

For each relay, there is a coil voltage. It can be continuous (5V, 12V, etc.) or alternative (110V, 230V). This is the voltage that must be applied to the coil to switch the relay. The current consumed depends on the type of relay and is inversely proportional to the coil voltage. In a range of relays (5V, 12V, 24V, etc.), the power of the coil is constant

For example: 12V 40mA, 24V 20mA, 48V 10mA, etc ...

To determine the current consumed by the coil, simply measure the resistance with the ohmmeter and deduct the current. This is useful for dimensioning the power supply to the control (the circuit which controls the relay coil).

Electromechanical relay pinouts

Single pole single throw relay (SPST)

It has 4 connectors: 2 connectors for controlling the electromagnet, and two connectors connected to the switch.

In its "normally open" version, the switch blocks the current if no current is flowing in the electromagnet, and allows current to flow if a current is flowing in the electromagnet.

There are also "normally closed" relays: the switch lets current flow when no current is flowing through the electromagnet, and blocks the current when current flows through the electromagnet.

Power relay

Most relays are able to switch a few amps (generally 1A to 4A). For higher breaking capacities, such as 10A, 25A or more, it is necessary to turn to power relays. If the relay has several contacts and only one switching is required, the various contacts can be connected in parallel to increase the breaking capacity.

 It should be noted that some power relays require a minimum passing current to maintain good reliability. This type of information is usually given by the manufacturer, but it is not always the case. Choose a relay whose breaking capacity is a little higher than the value of the current to be switched.

Advantages of the electromechanical relay

  • Operation with considerable dynamic range of the switched signal.
  • No addition of noise or distortion.
  • Very low contact resistance.
  • Very high open contact resistance.
  • Very high insulation between control circuit (coil) and switched circuit (contacts).
  • Possibility of solving automation problems sometimes simpler than with an electronic circuit.
  • Ability to switch both continuous and alternative signals over a wide range of frequencies.

Disadvantages of the electromechanical relay:

  • it can be relatively noisy.
  • it has a limited lifespan because moving parts wear out. When you have to control an element very often, electronic components are used instead (static relays, transistors, etc.)

Solid State Relay

The input circuit, equivalent to the contactor coil, is electrically isolated from the power circuit by an optocoupler (also called a photocoupler) composed of an LED and a phototransistor. This miniature electronic component makes it possible to separate two electronic or electrical circuits whose masses are not at the same potential.

 The input circuit is generally controlled by low voltage pulses in direct current, and consuming a few milliamperes. This input circuit controls a power circuit generally made up of triacs or thyristors.

 



Example of static relay: MOC3041 and MOC3042



Application of relays

The applications are numerous. The relays can control many loads on the sector, thanks to an isolated (non-dangerous) voltage which is applied to the coil: 

  • Electric heating
  • Light fixtures

And all applications where motors are switched:

  • Garage door
  • Motorized bed
  • Home appliance




sources:

https://sti2d.ecolelamache.org/le_relais_lectromcanique.html
http://www.zpag.net/Electroniques/relais.htm
http://electroniqueamateur.blogspot.com/2017/03/les-relais-electromecaniques.html
https://www.jpcfrance.fr/bases-pratiques/electriques/introduction-aux-relais-statiques-et-comparaisons-aux-relais-electromecaniques/

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