The article describes a very simple homemade emergency light circuit that can be used during power failures and outdoors where any other source of power might be unavailable.
The circuit uses LEDs instead of incandescent lamp, thus making the unit very power efficient and brighter with its light output. Moreover, the circuit employs a very innovative concept especially devised by me which further enhances the economical feature of the unit.
Let’s learn the concept and the circuit more closely:
What an LED Requires to Illuminate Optimally
The concept: We know that LEDs require a certain fixed forward voltage drop to become illuminated and it is at this rating when the LED is at it’s best, that is voltages which is around its forward voltage drop facilitates the device to operate in the most efficient way.
As this voltage is increased, the LED starts drawing more current, rather dissipating extra current by getting heated up itself and also through the resistor which also gets heated up in the process of limiting the extra current.
Increasing LED Efficiency
If we could maintain a voltage around an LED near to its rated forward voltage, we could use it more efficiently.
That’s exactly what I have tried to fix in the circuit. Since the battery used here is a 6 volt battery, means this source is a bit higher than the forward voltage of the LEDs used here, which amounts to 3.5 volts.
The extra 2.5 volts rise can cause considerable dissipation and loss of power through heat generation.
Therefore I employed a few diodes in series with the supply and made sure that initially when the battery is fully charged; three diodes are effectively switched so as to drop the excess 2.5 volts across the white LEDs (because each diode drop 0.6 volts across itself).
Now as the voltage of the battery drops, the diodes series are reduced to two and subsequently to one making sure only the desired amount of voltage reaches the LED bank.
In this way the proposed simple emergency lamp circuit is made highly efficient with its current consumption, and it provides backup for a much longer period of time than what it would do with ordinary connections.
How this white LED Emergency Light Circuit Works
Referring to the circuit diagram, we see that the circuit is actually very easy to understand, let’s evaluate it with the following points:
The transformer, bridge and the capacitor forms a standard Power supply for the circuit. The circuit is basically made up of a single PNP transistor, which is used as a switch here.
We know that PNP devices are referenced to positive potentials and it acts like ground to them. So connecting a positive supply to the base of a PNP device would mean grounding of its base.
Here, as long as mains power is ON, the positive from the supply reaches the base of the transistor, keeping it switched off.
Therefore the voltage from the battery is not able to reach the LED bank, keeping it switched off. In the meantime the battery is charged by the power supply voltage and it’s charged through the system of trickle charging.
However, as soon as the mains power disrupts, the positive at the base of the transistor disappears and it gets forward biased through the 10K resistor.
The transistor switches ON, instantly illuminating the LEDs. Initially all the diodes are included in the voltage path, and are gradually bypassed one by one as the LED gets dimmer.
HAVE ANY DOUBTS? FEEL FREE TO COMMENT AND INTERACT.
R1 = 10K,
R2 = 470 ohms
C1 = 100uF/25V,
Bridge diodes and D1, D2 = 1N4007,
D3---D5 = 1N5408,
T1 = BD140
Tr1 = 0-6V, 500mA,
LEDs = white, hi-efficiency, 5mm,
S1 = switch with three changeover contacts. Using Transformerless power supply
The design presented above can be also made using a transformerless supply as shown below:
Here we will discuss how an emergency lamp can be build without a transformer using a some LEDs and a handful of ordinary components.
The main features of the proposed automatic transformerless emergency light circuit is though very identical to the earlier designs, the elimination of the transformer makes the design pretty handy.
Because now the circuit becomes very compact, low cost and easy to build.
However, the circuit being completely and directly linked with the AC mains is hugely dangerous to touch in an uncovered position, so it is obvious that the constructor implements all the due safety measures while making it.
How it Works
Coming back to the circuit idea, transistor T1 being an PNP transistor tends to remains in switched OFF condition as long as AC mains is present across its base emitter.
Actually here the transformer is replaced by the configuration consisting of C1, R1, Z1, D1 and C2.
The above parts constitute a nice little compact transformerless power supply, capable of keeping the transistor switched OFF during mains presence and also trickle charges the associated battery.
The transistor reverts to a biased condition with the help of R2 the moment AC power fails.
The battery power now passes through T1 and lights up the connected LEDs.
The circuit shows a 9 volts battery, however a 6 volt battery may also be incorporated, but then D3 and D4 will need to be completely removed from their positions and replaced by a wire link so that the battery power is able to flow directly through the transistor and the LEDs.
Automatic Emergency Light Circuit Diagram
WARNING - THE CIRCUIT IS NOT ISOLATED FROM AC MAINS, AND THEREFORE IS EXTREMELY DANGEROUS IN POWERED, UNCOVERED POSITION.
R1 = 1M,
R2 = 10K,
R3 = 50 ohm 1/2 watt,
C1 = 1uF/400V PPC,
C2 = 470uF/25V,
D1, D2 = 1N4007,
D3, D4 = 1N5402,
Z1 = 12 V/ 1Watt,
LEDs, White, High Efficiency, 5mm
PCB Layout for the above circuit (Track side view, Actual Size)
R1 = 1M
R2 = 10 ohm 1 watt
R3 = 1K
R4 = 33 ohm 1 watt
D1---D5 = 1N4007
T1 = 8550
C1 = 474/400V PPC
C2 = 10uF/25V
Z1 = 4.7V
LEDs = 20ma/5mm
MOV = any standard for 220V application
For High Watt Application
Emergency Lamp with Automatic Day Night LDR Activation
In response to the suggestion of one of our avid readers, the above automatic LED emergency light circuit has been modified and improved with a second transistor stage incorporating an LDR trigger system. The stage renders the emergency light action ineffective during day time when ample ambient light is available, thus saving precious battery power by avoiding unnecessary switching of the unit.
Circuit modifications for operating 150 LEDs, requested by SATY:
Parts List for the 150 LED emergency light circuit
R1 = 220 Ohms, 1/2 watt
R2 = 100Ohms, 2 watts,
RL = All 22 Ohms, 1/4 watt,
C1 = 100uF/25V,
D1,2,3,4,6,7,8 = 1N5408,
D5 = 1N4007
T1 = AD149, TIP127, TIP2955, TIP32 or similar,
Transformer = 0-6V, 500mA
Emergency Lamp Circuit with Low Battery Cut-off
The following circuit shows how a low voltage cut off circuit can be included in the above design for preventing the battery from getting over discharged.
Power Supply Circuit with Emergency Backup
The circuit shown below was requested by one of the readers, it is a power supply circuit which trickle charges a battery when AC mains is available, and also feeds the output with the required DC power via D1.
Now, the moment AC mains fails, the battery instantly backs up and the compensates the output failure with its power via D2.
When input Mains is present, the rectified DC passes through R1 and charges the battery with the desired output current, also, D1 transfers the transformer DC to the output for keeping the load switched on simultaneously.
D2 remains reverse biased and is not able to conduct because of higher positive potential produced at the cathode of D1.
However when mains AC fails, the cathode potential of D1 becomes lower and therefore D2 starts conducting and provides the battery DC back up instantly to the load without any interruptions.
Parts List for an emergency light back up circuit
All Diodes = 1N5402 for battery up to 20 AH, 1N4007, two in parallel for 10-20 AH battery, and 1N4007 for below 10 AH.
R1 = volt/charging current (Ohms)
Transformer Current/Charging current = 1/10 * batt AH
C1 = 100uF/25
Using NPN transistors
The first circuit can be also built using NPN transistors, as shown here: