In this project, I will show you an inverter type Automatic Emergency Light. Load shedding is a common problem in developing countries. By keeping the problem in mind I will make a simple emergency light that is operated through AC mains by converting DC and also operated in battery backup power. But when the AC mains resume the battery supply will be automatically turned off and going to in charging mode. This emergency light will work for around 7 hours.
- 12-0 Step Down Transformer
- 7812 Voltage Regulator
- 1N4007 Diodes
- 1000 uF/25V Electrolytic Capacitor
- 0.1uf Ceramic Capacitor
- 12V SPDT Relay
- LED Lamp
- Connecting Wires
- 12V Battery
For making the circuit connection of this automatic emergency light first, the step-down transformer of a 12-0 V 500 mA AC transformer is taken. Anyone end of the secondary coil and center tape coil of the transformer is connected with a full-bridge rectifier. The cathode of D1 and the anode of D2 is connected to anyone end terminal and cathode of D4 and anode of D3 is connected to the center tap of the secondary coil. The cathodes of diodes (D2 and D3) are connected from which one terminal is taken out for output of rectifier and anodes of diodes(D1 and D4) are connected from which another terminal is taken out for output from the full-wave rectifier.
A 1000 uF/25V electrolytic capacitor is connected between the output terminals of the full-wave rectifier. For voltage regulation, a 7812 regulator is connected in parallel to the capacitor. A capacitor of 0.1 uF is connected in parallel between the output and ground terminal of the circuit.
There is a diode D5 from the output voltage terminals of the voltage regulator IC and connected to the Normally Open terminal of the 12V relay. Therefore, another diode D6 is connected parallel to both the Coils terminal of the relay. Its cathode part is directly connected to the GND and also its anode part is connected to the output of the regulator IC. A detailed connection is shown in the circuit diagram.
Normally Close terminal of the relay is connected with the LED lamp’s positive terminal with a switch system and its Common terminal is directly connected to the positive terminal of the 12V battery. LED’s anode terminal and battery’s negative terminal is connected to GND directly.
Working of Emergency Light
In this emergency light project, the voltage of the main power supply is 220-230 V AC which further needs to be stepped down to 12 V. To reduce the 220 V AC to 12 V AC, a step-down transformer is used. The use transformer allows us to generate both positive and negative voltage at the input.
The stepped-down AC voltage needs to be converted to DC voltage for use. The rectification is the process of converting AC to DC voltage. In this circuit, a full-wave bridge rectifier is used for converting the 12 V AC to 12 V DC. In a full-wave bridge rectifier, four diodes (1N4007) are connected in such a way that current flows through them in only one direction. During full-wave rectification, the two diodes become forward biased and another two diodes get reverse biased.
During the positive half cycle, diodes D2 and D4 are in forward bias while diodes D1 and D3 are reverse biased and the current flow respectively through the output terminal D2, and D4. During the negative half-cycle, diodes D1 and D3 are in forward bias, and diodes D4 and D2 are in reverse biased and the current flows through D1 and D3. The direction of both current ways is the same.
Filtering is the process of the DC signal done by using a capacitor. The output from the full-wave rectifier is not a pure DC voltage. This will be mixed with some noises called ripples. Therefore, it needs to be filtered by connecting a capacitor in parallel to the output of the full-wave rectifier. The capacitor charges and discharges during a cycle giving a pure and rectified DC voltage as output. But we can’t remove all the ripples from the rectifier. So, a capacitor of 1000 uF/25V is connected to the output of the rectifier circuit. This capacitor acts as a filtering capacitor which diverts all the AC through it to ground. At the output, the mean DC voltage left is filtered and ripples free. A capacitor of 0.1 uF is used for filtering the AC signal.
7812 is a positive voltage regulator IC. The IC has three pins: 1) Input pin which can be supplied about 15 V DC, 2) Output pin which drives output voltage in the range of 1.25 V to 12 V DC, and 3) Adjust pin which is used to vary the output voltage corresponding to the applied input voltage. For input up to 15 V, the output can vary from 1.25 V to 13 V DC.
Here N/O (Normally Open) terminal of 12 V relay is connected with DC supply through a diode and N/C (Normally Closed) terminal of the relay is connected to the LED lamp. Both coil terminals of Relay are connected to the DC supply which is outputted from the regulator. In the presence of AC input the common terminal of relay attracted to the N/O terminal and while the battery gets the DC supply for charging. During the absence of AC input, the relay coil won’t get DC supply and while the common terminal attracted to N/C. So the battery backup supply flows through the LED lamp and the lamp starts glowing.
Application of Emergency Light
Emergency light is used in child’s study rooms to avoid sudden power failure and darkness.
It also use in home appliances for an emergency.
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