Relay module is required to turn on or off electrical loads which are operated by high voltage. An electronic control signal is applied to the input of a relay module and at the output, contacts switch from its normal position to another position. The supply voltage of a relay can be 5 V, 6 V, 9 V or 12 V depending on the applications. Depending on the controlling mechanism, relays can be active high or active low. In the previous post, we have discussed about active high relay module. Here, operation of a single channel active low 5 V relay module is discussed. This module can be easily interfaced with different micro controllers.
Operation of Active Low Relay Module
Circuit diagram for active low single channel 5 V relay module is shown in Figure 1. Here the 5 V relay is connected at the collector of BC558 (PNP) 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 voltage level at SP pin is low which is not enough to activate the relay. If an active low input (0 V or GND) 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 voltage level at SP is now enough to activate the relay. 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 Required
- 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 (L1 (RED) and D2 (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 Low
- Input control signal current: – 1.5 to 1.9 mA
- Relay max contact voltage: – 250 VAC or 30 VDC
