How to interface microcontroller with 24 VDC load through relay

As you know, interfacing microcontroller with 5 VDC, 2mA load is so simple and no need to any extra components... just connect the micrcontroller to the load directly.

But if you have a load that exceeds the rating voltage or rating current,what can you do then?

If exceeds rating voltage ==> then you need a relay

If exceeds rating current ==> then you need a transistor

  

Code:

void main() {

     TRISD.F0 = 1 ;
     TRISB.F0 = 0 ; PORTB.F0 = 0 ;
    

     while(1)
     {
          if(PORTD.F0 == 0)
          {
                PORTB.F0 = ~ PORTB.F0 ;
                while(PORTD.F0 == 0) ;
                Delay_ms(20) ;
          }
     }
}

A relay is an electrically activated switch. It consists of a coil internally which will creates a magnetic field that attracts a movable lever and then changes switch contacts when a current is flowing through it. The typical usage of relay is to allow a low DC voltage circuit (circuit #1) to switch on or off a high voltage (DC or AC) circuit (circuit #2) without direct electrical connection between them. This means circuit #1 and circuit #2 are magnetically and mechanically linked but not electrically connected. There are some designations normally used in describing a relay based on its internal structure and pins distribution as shown in the following

i) Single Pole Single Throw (SPST) – Such relay has 4 terminal pins which consists of a pair of coil pins and a pair of pins which can be made connected or disconnected by activating or deactivating the relay. Such relay can categorized into normally open or normally closed type.

ii) Single Pole Double Throw (SPST) – Such relay has 5 terminal pins which consists of a pair of coil pins, a common pin, a normally open (NO) pin and a normally closed (NC) pin. When the relay is not activated, the common pin is in contact with the NC pin and when it is activated, the common pin will break away from contact with the NC pin and subsequently makes contact with the NO pin. Also, when the relay is deactivated (from activated state), the common pin will conversely break away from contact with the NO pin and return back in contact with the NC pin.

iii) Double Pole Single Throw (DPST) – Such relay has 6 terminal pins which consist of a pair of coil pins and two pairs of pins, where pins in each pair can be made connected or disconnected by activating or deactivating the relay. Such relay is actually a combination of two SPST relay structures with only one coil pins pair.

iv) Double Pole Double Throw (DPDT) – Such relay has 8 terminal pins which consist of a pair of coil pins, two 3 pins group for each group consists of a common pin, normally open (NO) pin and normally (NC) pin . Such relay is actually a combination of two SPDT relay structures with only one coils pin pair