LED Lamp Design

Welcome to CR4! This sounds like a good project, and you have summarized it very well.

The series/parallel scheme sounds about right. Just to check, do you have the required voltage per individual LED?

I think battery chargers are commercially available with your desired features. Or is it part of your project to design the charger from the ground up?

Best of luck; with good design and tuning, you are likely to be successful.

i think it should be around 3.2v and yes this project i have to make from ground up excluding the power supply of 12v dc.

If you show us what you have done and ask specific questions you will find us a helpful bunch.
Del

Is efficiency listed among the design criteria?

With 7 x (3 x 3.2V LEDs in series) from a 12V supply you'll have (7 x 2.4 x LED current) watts of wasted power - unless you use a fancy switching circuit.

no efficiency criteria. i am trying to follow this circuit http://www.vt52.com/diy/myprojects/other/charger/charger.htm problem is to add led to indicate charging and charging complete also i am thinking of using a scr for low battery turn off.

Using the L200 or the LM317/350?

thinking of using lm200 if it is cheap

JohnDG,

I'm not sure I understand your comment about 'wasted' power, or that somehow a switching circuit would save any power. He (she?) is trying to light up some LEDs. A dc drive will do that. Given this simplicity, it seems to me that any additional components or extra circuitry will reduce the overall efficiency. Perhaps if he/she needed to light the LEDs to perform a specific function or to operate efficiently over a range of temperatures, then additional complexity could be more efficient in the long run. Perhaps I missed the point you were making.

My thoughts on the OP's scheme, based on my experience with LEDs is:

Running strings of three 3.2V LEDs in series off a 12V supply means that each LED will see 4 volts. Depending on the LED specs, this could be overdriving them a bit too much. It might be better to put 4 in each string, so that each LED sees 3 volts, and is only underdriven by 0.2 volts rather than being overdriven by 0.8 volts. Note that if the actual battery output is greater than 12 volts -- say for example it is 13.2V -- then a 3-LED string would have 4.4V per LED which is likely a serious amount of overdriving, whereas a 4-LED string would have 3.3V per LED.

Using 4 LEDs per string, the total number of LEDs would then be 20 (5 strings with 4 LEDs per string).

A switching circuit could save power by turning the voltage to the LEDs on and off so that the average current was below the max. continuous rating (provided that the max. peak current wasn't exceeded).

If he used his stated 12V supply with the 3.2V across each LED, then that excess 2.4V (or more as you point out) would have to be dropped somewhere (either just across a resistor or some other current limiting circuit) - i.e. the power would be wasted as heat.

I agree that any kind of switching would almost certainly bump the component count up, but in these energy-conscious days, I'd've thought that any school project would have stressed the efficiency side of things.

I also agree that he may get away with 4V across each LED, but without the device specs, we can't tell.

.

Ahh, I see. You were accounting for the excess 2.4 volts rather than assuming, as I did, that he'd just run the LEDs straight without any attempt to trim it.

... well, it is what he said in #2.

Yeah, I took that as just the rated voltage; many LEDs are rated at 3.2, but will operate OK over a wider voltage range.

There are many chips (pre-designed and programed controllers) out there today designed to handle battery charging. Unfortunately you did not specify which battery chemistry will used. This will effect both which battery controller you can use but also what voltage, current and/or charge pulse response will tell you your battery status so that a correct assessment can be made of your battery status.

To light a status LED, this is actually a trivial problem compared to knowing your battery condition. I would recommend using a step down switching regulator to drop your battery or charger voltage down to a voltage closer but higher than your status LED nominal voltage drop. Then a simple small value current limiting resistor can be used to light the LED.

the battery is 12v 1.4ah sealed lead acid

For a lead acid battery fuel gauge you might want to check out this TI chip. This might be more of a project than you had anticipated in using this chip, but it could also be precisely what you are looking for. You might want to also look over the basic charging of a lead acid battery.

I have fitted my 9mtre bus motorhome with all LED lighting. For the lounge I built 4 panels end to end of 54 LEDs each i.e 216 LEDs. for the bedroom a 238 LED panel with three switch positions allowing half the LEDs lit or all of them or off. And two reading lights of 70 LEDs with the 'half of them on' swiching. Never mind what voltage per LED. They are current devices. With a 12volt supply as I have in my bus you build the panels in sets of three LEDs with a 150 ohm resistor in series with each set. If as I did you have an odd LED left over you just put a 510 ohm resistor in series with that one. This set up in my bus has been going perfectly for at least three years now. These LEDs are the 10mm ones and give a very good light.

The applied KISS principle (keep it simple, stupid) may be a bit rude, but valid nonetheless. Doing it the #10 way linearly gets you most the way to your goal with the least effort, and importantly for you, with the least amount of instrumentation. A comparison with switching regulators at the end will round off the proiect. It is not an industrial program, you are expected to learn, not to produce a master design for industrial use.