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Relays: how to choose them, how to read a datasheet, etc..

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Categories of relay

There are many kinds of relays, they can be grouped (mainly) depending on their behavior and their poles configuration.

By behaviour

  • Monostable relay: this kind of relay has a "preferred" state (i.e.: one resting position). This means that when it loses power it will automatically return to the preferred state. This is usually specified with the notation Normally Open (NO) or Normally Closed (NC), where NO means that in absence of power (or in absence of a control signal) the relay will open the circuit while the behavior of the second one is the opposite.
  • Bistable relay: this kind of relay has two resting positions. This means that if powered down, it will maintain its position. This is also often called Latching relay.

By poles configuration

  • Poles: A relay can operate on a single circuit (single-pole) or on two circuits at the same time (double-pole).
  • Throws: Relays can switch a circuit (on or off) or deviate it (to exit A or exit B). Relays that switch the circuit are usually referred to as single-throw, while relays that deviate the circuit are usually called double-throw.

This leads to the four principal categories of relays:

  • SPST: Single Pole, Single Throw ---> One switch
  • SPDT: Single Pole, Double Throw ---> One deviator
  • DPST: Double Pole, Single Throw ---> Two switches
  • DTDP: Double Pole, Double Throw ---> Two deviators

For more information on poles and throws, please read this page.

How to choose a relay

You should pay attention to these values on the datasheet:

  • Nominal Coil Voltage: this is the voltage intended by design to be applied to the coil to operate it. It is usually preferable to choose a relay with a nominal coil voltage equal to the exact voltage of the signal that will operate it.
  • Maximum Switching Voltage and Maximum Switching Current: this is the maximum open circuit voltage/current which can safely be switched by the contacts. Please note that this is the maximum voltage/current allowed at the maximum current/voltage. This means that if, for e.g., the max. switching voltage is 30V and the max. switching current is 30A, the contacts of the relay will likely be able to switch a current of 48V and 10A. Refer to the graphs in the datasheet to find the maximum voltage allowed at different current intensities.

For more information on the terminology used in many datasheets, please refer to [1] and [2].

How to operate a relay

Relays can be operated in many ways. The most common is to use a transistor, in this case it is advisable to use a diode in order to preserve the transistor from being damaged by the peak of power released by the relay when it opens the circuit.

For more information on this problem please refer to [3].

For examples of standard circuits to operate relays, please refer to [4].

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