Automatic Changeover Switch

Many electrical and electronic appliances require DC or AC power for their operation. While AC power is made available mostly through AC supply mains, DC power is made available through batteries. However there are circumstances when there is a scarcity of AC power (through power failure) or DC power (due to limited lifetime of batteries). To overcome this problem, we usually come across many alternatives. For example we can use generators or inverters in emergency cases to get AC power when the mains supply is switched off. Similarly in case of DC power, we can use either a battery or an AC to DC power supply in alternative.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.

Automatic Changeover Switch Circuit Principle:

This circuit is based on the principle of bistable mode operation of 555 Timer. In this mode, the Timer output is either high or low depending upon the status of trigger and reset pin. The Timer output is connected to a transistor which acts as a switch, being on or off depending upon the Timer output. Two LEDs in series are used as a load. In case of transistor being switched off, LEDs are driven by the AC-DC power supply whereas in case of transistor being switched on, LEDs are driven by the battery.

Automatic Changeover Switch Circuit Diagram:

Circuit Diagram of Automatic Changeover Switch

Automatic Changeover Switch Circuit Design:

Designing the circuit involves two basic parts -

1. Design of AC – DC Power Supply:

 It is the design of a basic AC to DC power supply system using transformer and bridge rectifier.

The first step involves selection of the voltage regulator. Since here, our requirement is to drive two LEDs in series along with a Schottky diode, we settle down with LM7809 voltage regulator producing a voltage of 9V. Since input voltage to the regulator must be at least 12V, we settle down with an input voltage of about 20V.

The next step involves selecting the transformer. Since primary voltage is 230V and required secondary voltage is about 20V, we can settle with a 230V/20V basic transformer.

The third step is the selection of diodes for bridge rectifier. Since peak voltage across the transformer secondary is around 28V, the total PIV of the bridge would be around 112V. Hence we need diodes having PIV rating more than 112V. Here we select 1n4007 having PIV of about 1000V.

The final step involves selection of filter capacitor.  For a capacitor, peak voltage of 26V and minimum regulator input voltage of 12V, the allowable ripple is about 14V. The capacitance value is then calculated by the formula, C = I (Δt/ΔV), where I is sum of quiescent current of voltage regulator and required load current. Substituting the values, we get a value of about 17uF. Here we select a 20uF electrolyte capacitor.

2. Design of Bistable Multivibrator Circuit using 555 Timer:

When a 555 Timer is configured in bistable multivibrator; its output is either high or low logic signal. Here we use the simple logic that when trigger pin is grounded, output is a high logic signal and when reset pin is grounded, output is low logic signal. Here the output of 555 Timer is connected to the base of transistor BC547.

Automatic Changeover Circuit Operation:

The circuit operation commences once the switch S1 is at any of its position. When the switch S1 is at position 1, reset pin of the 555 Timer is grounded. Internally this reset pin is the reset pin of the SR Flip flop and hence the output of 555 Timer is a low logic signal. Now since base emitter junction of Q1 is reverse biased, it is in cut off position. The load LEDs are connected directly to the output of the voltage regulator through the Schottky diode. Here is where the operation of AC to DC power supply circuit comes to play. AC power is first stepped down by the transformer and then converted to unregulated and fluctuating DC voltage by the bridge rectifier. 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 at position 2, trigger pin of 555 Timer is grounded. This causes the output of the 555 Timer to be a logic high signal. The base emitter junction of Q1 is thus forward biased and the transistor is driven to saturation, thus being in on position. Here we should note two things – First, the Schottky diode now does not conducts as the voltage difference between both cathode and anode of the diode is zero, i.e. there is no potential difference at the junction. Secondly, the LEDs are now biased through the resistor and the transistor and driven by the battery voltage.

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:

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