Relay module is required for controlling one or more than one (AC/DC) loads by switching method. A relay is an electromagnetic switch that is commonly used in automatic control systems. DC voltage is applied to input of a relay to energize the coil inside it. If the coil is energized, the contacts at its output switch from its normal position (NC/NO) to another position (NO/NC). The input voltage of the relay can be 5 V, 6 V, 9 V or 12 V depending on the applications. A single channel relay module can be used to control loads like solenoid valves, AC/DC motor, Lamps etc. Here, operation of a single channel 5 V relay module is discussed. This module can be easily interfaced with different micro controllers.
Operation of the Relay Module
Circuit diagram for active high single channel 5 V relay module is shown in Figure 1. Here 5 V relay is connected at the collector of BC547 (NPN) transistor. Thus relay behaves as collector load for transistor. The relay is not energized as long as the transistor is in off condition. This is because the path from the SP terminal to GND is open. If a 5 V voltage source or pulse is applied to the base of the transistor through the connector J3, then the transistor is turned on. The coil of the relay is energized as the path from SP to GND is now connected. Now as the coil inside the relay is energized, the contact position changes from (NC) to (NO). In Figure 1, the common terminal ‘p’ is connected to ‘o’ initially. If the coil is energized, then the common terminal switches from ‘o’ to ‘s’. The load terminal connection switches from (X1-2 – X1-3) to (X1-1 – X1-1). In this circuit, two LEDs LED1 and LED2 are used to indicate active power supply and active relay operation respectively.
When the transistor is switches to off condition from on condition, the current flow through the relay coil does not stop abruptly. Current through the coil gradually decreases. The stored energy within the magnetic field has to go somewhere and a reverse voltage is developed across the coil as it tries to maintain the current in the relay coil. Due to this reason a flywheel diode (D1) is connected across the relay coil. This clamps the reverse voltage across the coil to about 0.7 V dissipating the stored energy and protecting the switching transistor.
Components
- 5 V DC power supply.
- One three pin header and a screw pin male header.
- Three Resistors (R1= 47 ohm, R2=R3=220 ohm/0.25W).
- One transistor (BC547 NPN).
- Two LEDs (LED1 (RED) and LED2 (GREEN)).
Specifications
- Supply voltage: – 3.75 V to 6 V
- Relay max contact current: – 10A
- Supply current with relay de-energized: – 2mA
- Supply current with relay energized: – 70-72mA
- Input control signal: – Active High
- Input control signal current: – 1.5 to 1.9 mA
- Relay max contact voltage: – 250 VAC or 30 VDC
