Low Voltage Circuit

# Automatic Changeover Switch

The operation Automatic Changeover Switch of many electrical and electronic appliances necessitates the use of DC or AC electricity. While AC electricity is typically obtained from the mains, DC power is obtained from batteries. However, there are times when there is a lack of AC power (due to a power outage) or DC power (due to a power outage) (due to limited lifetime of batteries). To solve this difficulty, we usually come across a variety of options. In an emergency, we can utilise generators or inverters to get AC power when the mains supply is turned off. In the case of DC power, we have the option of using a battery or an AC to DC power supply.

Here this article presents the principle, design and operation of a automatic changeover circuit wherein a DC load like a series of LEDs are driven either by a battery or an AC-DC power supply.

Outline

• Automatic Changeover Switch Circuit Principle:
• Automatic Changeover Switch Circuit Diagram:
• Automatic Changeover Switch Circuit Design:
• Automatic Changeover Circuit Operation:
• Applications of Automatic Changeover Switch:
• Limitations of this Circuit:

## Automatic Changeover Switch Circuit Principle:

The bistable mode operation of the 555 Timer is used in this circuit. The Timer output is either high or low in this mode, depending on the state of the trigger and reset pins. The Timer output is connected to a transistor that acts as a switch, allowing the Timer output to be turned on or off. As a load, two LEDs are connected in series. LEDs are driven by the AC-DC power source when the transistor is turned off, but LEDs are driven by the battery when the transistor is turned on.

## Automatic Changeover Switch Circuit Design:

### 1. Design of AC – DC Power Supply:

A rudimentary AC to DC power supply system using a transformer and bridge rectifier is designed.

The choosing of the voltage regulator is the first stage. Because we need to operate two LEDs in series and a Schottky diode, we use an LM7809 voltage regulator with a 9V output. We conclude on an input voltage of around 20V because the regulator’s input voltage must be at least 12V.

The transformer must then be chosen as the next step. We can use a 230V/20V basic transformer because the primary voltage is 230V and the required secondary voltage is around 20V.

The selection of diodes for the bridge rectifier is the third step. The overall PIV of the bridge would be roughly 112V, because the peak voltage across the transformer secondary is around 28V. As a result, diodes with a PIV rating greater than 112V are required. We’ve chosen 1n4007, which has a PIV of around 1000V.

The last step is to choose a filter capacitor. The permissible ripple for a capacitor with a peak voltage of 26V and a minimum regulator input voltage of 12V is around 14V. The capacitance value is then determined using the formula C = I (t/V), where I is the sum of the voltage regulator’s quiescent current and the needed load current. We get a result of around 17uF by substituting the values. We’ll use a 20uF electrolyte in this case.

### 2. Design of Bistable Multivibrator Circuit using 555 Timer:

The output of a 555 Timer configured as a bistable multivibrator is either a high or low logic signal. When the trigger pin is grounded, the output is a high logic signal, and when the reset pin is grounded, the output is a low logic signal. The output of the 555 Timer is connected to the base of the transistor BC547 in this circuit.

## Automatic Changeover Circuit Operation:

Once the switch S1 is in either of its positions, the circuit operation begins. When the switch S1 is set to position 1, the 555 Timer’s reset pin is grounded. Internally, this reset pin is the SR Flip flop’s reset pin, therefore the 555 Timer’s output is a low logic signal. Because Q1’s base emitter connection is reverse biassed, it is now cut off. Through the Schottky diode, the load LEDs are connected directly to the voltage regulator’s output. This is when the AC to DC power supply circuit’s functionality comes into play. The transformer steps down AC power before the bridge rectifier converts it to unregulated and fluctuating DC voltage.The AC ripples from the fluctuating DC voltage is removed by the filter capacitor. This unregulated DC voltage is then converted into a regulated DC voltage by the voltage regulator.

When switch S1 is set to position 2, the 555 Timer’s trigger pin is grounded. As a result, the 555 Timer’s output is a logic high signal. As a result, Q1’s base emitter junction is forward biassed, and the transistor is driven to saturation, putting it in the on position. Two things should be noted here — First, the Schottky diode no longer conducts since the voltage difference between the diode’s cathode and anode is zero, indicating that there is no potential difference at the junction. Second, the LEDs are now driven by the battery voltage and are biassed through the resistor and transistor.

## Applications of Automatic Changeover Switch:

1. This circuit can be used as a home lighting system with few modifications.
2. It can be used to drive other DC loads like a DC motor of any electronic appliance or other toy applications.

## Limitations of this Circuit:

• This is a theoretical circuit and may require few changes when implemented on PCB.

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