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i am new to electronics....this is my first project..i want to make a led emergency lamp with 25 leds..what is the best power source....and also tell me when the led stop works..
A lot of emergency equipment like that uses Lead Acid batteries with a connection to the power line for constant monitoring and charging if needed. It would detect the line power drop and turn the LEDs on for example, then start up the charging when the line returns. There are other battery types though that you may want to consider.
Since you are using multiple LEDs and not a single incandescent lamp like some do (or dual lamps) you can monitor the battery and switch off some of the LEDs to conserve power when the battery voltage starts to drop too low. That would keep at least some light going during the emergency power outage.
The 6v Lead Acid cells are popular, but i think the 12v ones are better.
Since this is an emergency system obviously it can't rely on mains power. The use of SLA (Sealed Lead Acid) batteries is a popular power source for these projects in 6 and 12 volt versions. For a simple approach you could use a mains powered supply using a step down transformer, full wave bridge rectifier and filtering. Additionally regulation. That would maintain a float charge on the battery. It would also for simple, power a small relay so when mains power fails the LED lighting comes on. You need to design and work out the LED scheme as you really do not provide much info.
<EDIT> MrAl is quick this morning... Beat me to it. </EDIT>
I just remembered I have some LED lighting strips from emergency exit signs that were removed and replaced. These originally used a 6 volt battery and while the images don't do them justice are very bright. They worked in conjunction with a few 6 volt incandescent flood lights and illuminated behind the EXIT portion of the signs. In the images two strips are in series powered by an old 12 volt SLA battery I have lying around.
Using some proto or perf board the LEDs could be mounted for your project similar to the attached.
You could use a switching power supply to efficiently control the current in the LEDs. (not a resistor)
The LEDs should blink at a very annoying rate to get you attention. By having the LEDs on for 25% of the time (blinking) the on current could be 2x or maybe 3x more.
i'm using white led whose voltage drop is 3.5V...i dont know about switch mode power supply and sealed lead acid battery since i'm new to electronics..and also they mentioned that 6v or 12 v battery is enough..i tested for 4 led's with 9v battery..and also i dont want to glow the led by means of ac supply since this is emergency led light....which source is best to glow 25 led's..pls mention the lifetime of led..
Can you provide a link to the LEDs you plan to use as in a data sheet? A 3.5 volt forward working voltage is fine but what is the forward current? I understand you only want light when mains power fails but you should keep a float charge on the battery. A SLS battery is what is pictured in my second post.
OK, there are hundreds, maybe thousands of LEDs out there. This is an example of a link to a LED data sheet. All LEDs are not created equally. I mentioned that a forward voltage is fine but what is the forward current? You need to learn and understand what LEDs are all about. Short of gaining an understanding of basic LED characteristics there is no way you will manage this project. You choose an LED based on its intended application.
OK, emergency LED whatever. How long do you expect it to last? These are generally done with SLA batteries, If you still do not know what a SLA battery is I suggest Google. You have a forward current of 25 mA, do you understand 25 mA? Do you know the current capacity of a AA battery? No a AA battery would not at all be practical for what you propose. Do want lighting? How much lighting? Do you just want to illuminate an exit sign? I don't yet know what you want to do exactly? Is this a school project or just your own doing?
its my own work.....i searched sla in google...in wiki it showed about car's battery...current capacity of my AA battery is 800mAh...i want more illumination...that is bright light....
Using a 12 volt battery the LED matrix would look like the attached. You have made no mention of float charge for the battery or anything else. This assumes your 24 LEDs (you don't get 25) with a Vfwd of 3.5 volts and Ifwd of 25 mA. Powered from twelve volts the circuit current will be 8 strings times 25 mA so about 200 mA.
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Beat me to it. </EDIT>
