An electromagnetic relay in its switching principle is a mechanical switch which is operated with a low-power DC voltage. The switch part is used to control high power circuits.
  2. SPDT RelayRelay design
                  The SPDT relay is mainly made up of :

    1. Movable armature
    2. Control coil
    3. Switch contact points
    4. Coil terminals
    5. Common terminal


    RELAY SPDT (single pole double throw) has a total of five terminals Out of these two are the coil terminals. A common terminal is also included which connects to either of two others. When a voltage is applied to the control circuit, the coil traversed by a current will create an electromagnetic field; this latter is capable of moving a metal element called movable armature.

  3. Experimentation*.Objective:
    Make circuit containing a relay using the DCAClab simulator.*. Equipment:

    • Battery (Electromotive Force E = 1.5 v , Internal resistance r = 1 Ω)
    • Switch K1
    • Coil (r1 = 1Ω)
    • Relay Switch
    • Lamp (r2 = 1Ω)
    • Fan

*. Experiment 1: order a single circuit

  • Experimental mounting
  • Interpretation
    • Level 1
      1st case: Relay at rest
      The switch K1 open ( I1 = 0  ) ⇒K2 at the position (1), consequently  the circuit B is open : I2 = O , so the lamp L is off .
      2nd case: Relay at work
      The switch K1 is closed ( I1 ≠ 0 )⇒Kat the position (2), consequently  the circuit B is close : I2 ≠ 0 , so the lamp L is on .
    • Level 2_ calculating the intensity of the current I1:Ohm’s law: Ue= r1 I= E – r I1

      So  I1  = E/ (r1  + r)  = 0.04838A ≈ 48.38 mA


      _ calculating the power P1:

      P1 = E I1 = 1.5 x 0.04838 = 0.07257 w

      _ Value of the intensity I2:

      By using the characteristics of the lamp we find I2 = 1.456 A

      _ calculating the power P2:

      P2 = E I2 = 1.5 x 1.456 = 2.184 w


      The relay can be used to control a high power circuit by a low power circuit.

  • Experimental verification using the DCAClab simulator

*. Experiment 2 : order 2 circuits 

  • Experimental mounting1st case: Relay at restThe switch K1 is open ⇒K2 at the position (1), consequently:
    • The circuit B is closed, so the lamp is on.
    • The circuit C is open, so the fan is off.
  • Experimental verification using the DCAClab simulator

    2nd case : Relay at workThe switch K1 is closed⇒K2 at the position (2) , consequently  :

    • The circuit B is open, so the lamp is off.
    • The circuit C is closed, so the fan is on.
  • Experimental verification using the DCAClab simulator

the end.

Leave a Reply

Your email address will not be published. Required fields are marked *

%d bloggers like this: