Triggering circuit for relay (SPPT) Using Microcontroller

To control the operation or state of relay, an external control circuitry is required. This circuitry control the flow of current through the coil of relay and hence change the state of relay. Figure 1 shows the triggering circuitry for the relay. As we are using microcontroller for the control of whole circuit, so we designed a circuit such that it can trigger the relay by the pulses generated by controller.

Figure 1: Triggering circuit for Electromagnetic relay

Now, lets check the working of this circuit.When logic 1 is applied from controller, a high signal gets applied at the base of transistor. Transistor act as closed switch and dc voltage drop across the diode(reverse biased) and small current flows through the coil of relay, because it is in parallel with the diode. In this way relay gets triggered and common terminal gets connected with NO terminnal. When logic 0  is applied from controller, logic low gets applied at the base of transistor and transistor act as an open switch. All the dc voltage gets drop across transistor and no current flows through the coil of relay. Hence by generating logic high and low from microcontroller, we can control the state of relay.

Transistor 2N3904, diode 1N4007 and opto-coupler PC817 is used in this circuit. You can consult their datasheet for their configuration and characteristics. These are the low cost component available in the market. Because swtching frequency of electromagnetic relays are usaully upto 1KHz, and triggering circuit has low operating voltage. So, we don’t need high frequency  and high voltage or current rating component.

DC volatge for Relay Operation

As shown in Figure 1, the triggering circuit for relay requires constant dc voltage source but there is no source of dc voltage in our circuit for relay and other components because,we are doing ac regulation and only input ac voltage is present. So, we have to convert ac voltage into constant dc volatge. This is done by using step down transformer having turns ratio of 0.04. This transformer can convert 220V into 9V RMS ac voltage.The peak value is Underwood 2  times of rms value so peak will be 12.7V. Full wave rectifier and filter capcitor for smoothing is used at the secondry of transformer. In this way,we have converted ac input voltage into low level dc voltage.To maintain constant dc volatge, linear regulator LM7812 can be used. Complete circuit for the conversion of ac into dc is shown in Figure 2.