Many of us have the habit of leaving our battery-powered devices such as mobile phones, laptops, tablets, power banks, and modems connected to chargers overnight, even though these devices, especially smartphones equipped with quick-charge capabilities, typically reach full charge within about two hours or less. Once the device attains full charge, the charger continues to remain connected and idle for an extended period, often exceeding five hours, such as from 1 am to 6 am. As we typically don’t engage with these devices during nighttime hours, even modems are left in an inactive state during this period, drawing some power.
In the following sections, we introduce a power-saving circuit designed for mobile chargers and modems. This circuit, built around an op-amp, serves multiple purposes and allows for individual delays tailored to your devices. After a specific predetermined duration, it automatically switches off the devices. This not only curbs power wastage during idle periods but also contributes to prolonging the operational lifespan of these devices.
Block diagram of the switch box for mobile chargers and modems is shown in Fig. 1.
Circuit and working of Smart Switch Box
Fig. 2 shows the switch box circuit built around op-amp IC LM339 (IC1). IC LM339 consists of four independent precision voltage comparators. These comparators are designed specifically to operate off a single power supply. Two op-amps (A1 and A2) are used in IC1.
The switch box is divided into two distinct and independent sets, denoted as Set A and Set B, each dedicated to controlling the charging process. Each set is equipped with a 3-pin socket connected to a relay, and a protective fuse, F1, is incorporated into the design for added safety.
The delay circuit is formed by a combination of resistor R1 and capacitor C1, which are connected in parallel. This configuration results in a shared delay time for both op-amps, A1 and A2.
To initiate the charging process, switch S3 is employed to charge capacitor C1 to a voltage level of 12V. When S3 is closed, point M registers a voltage of approximately 12V. Conversely, when S3 is opened, the 12V connection is severed, allowing capacitor C1 to discharge through resistor R1. Point M is connected to pins 5 and 7, which represent the non-inverting terminals of op-amps A1 and A2, within IC1. Potentiometers VR1 and VR2 are linked to pins 4 and 6, serving as the inverting terminals of op-amps A1 and A2. Additionally, resistors R2 and R6 play a role in providing feedback and ensuring circuit stability. Resistors R3 and R7 act as pull-up resistors, while pins 3 and 12 of IC1 are linked to the 12V supply and ground, respectively.
The status of the relays is visually displayed by LED1 and LED2. In normal operation, with S3 open, the switch box functions as usual, and the delay circuit remains inactive. However, when S3 is briefly closed and then reopened, the capacitor discharges from 12V to 0V, thereby enabling the delay circuit.
The voltage at C1 (at point M) is given by the following relationship:
VC1= Vdc×e–t / R1C1
where Vdc is 12V.
When the voltage is set at M1 or M2 using the potmeter (VR1 or VR2), at the same time the capacitor also discharges slowly from 12V to 0V.
If the voltage at M>M1 or M2, op-amp outputs are high and relays energise, and vice versa. For example, if you want to de-energise relay RL1 after 3600 seconds (one hour), set the voltage at M1 to 8.47V. After 3600 seconds, the relay will de-energise, cutting off the voltage to Set A and Set B.
The table shows voltage values at point M1, M2 and delay time for different values of R1 and C1.
Switches S1 and S2 are responsible for establishing the electrical connection from the line supply, either by directly routing it or through the utilization of relays, contingent upon their respective positions. When switches S1 and S2 are set to position 1, power is supplied to Set A and Set B via the relays. Conversely, when switches S1 and S2 are switched to position 2, Set A and Set B receive the power supply directly, bypassing the relays.
In the course of regular operation, switch S3 remains in the open position. To ensure normal functionality, maintain switches S1 and S2 in position 2 to allow the direct provision of the line supply to both Set A and Set B.
Construction and testing
A unified printed circuit board (PCB) that encompasses the switch box circuit, comprising both the control and charging sections, is depicted in Figure 3, while the component layout is presented in Figure 4. The two PCBs can be effortlessly detached and subsequently encased in appropriate containers. Establish the requisite connectors to facilitate the connection between the control circuit and the charging socket.
The circuit is powered by a 12V SMPS. To establish the connection to the power source, a 3-pin plug equipped with a 3-core wire measuring 2 meters in length is employed, incorporating the line supply connections (L, N, and E) to the switch gang box.
Different delays are achieved by manipulating the variable voltages accessible via the central pin of the potentiometers. It is essential to measure the voltage and then fine-tune the settings of VR1 and VR2 to calibrate them with respect to the desired delay time. The accuracy of the timing intervals is contingent upon the tolerances of R1 and C1.
For interconnection purposes, utilize male and female connectors to join CON2 and CON3. Finally, establish a connection to a 230V AC, 50Hz power source at CON4.
Since the assembled PCB is placed in the switch gang box, take care between AC and DC wiring. Use an appropriate fuse. Also, take care while connecting the relays. Use a 2-mega-ohm R1 in place of 4.7M if you want a different value as shown in the table.