powering 12v LEDS from a 12V battery - problems!

Forum 2005-2010 (read only) Hardware Interfacing
1

Hello, I said i'd help out a frind of mine with a project lighting up some leds, which i have worked with before but powering 12v leds from a battery supply has thrown up some unexpected problems. any help is gratefully received and much appreciated...

This is what i am trying to do; power 15 meters of SMT led strip rated at 12v. There will not be access to mains electric and they need to run for at least 12 hours so i an going to power them from a 12v deep cycle leisure battery rated at 125Ah. The leds take 300mA per meter so the total power consumption is 4.4A This should give me more than enough running time. The supplier of the leds said that they must receive a regulated 12v. Eventually i want the arduino to run some different switching patterns with the leds but for the moment i need to get the power supply side working...

The problem i am encountering is how to supply the leds with regulated 12v dc supply from a nominally 12v battery.

As far as i understand the battery will range from about 14v at full charge to around 11.5v fully discharged. I have looked in a lot of places online and not found an off the shelf solution that is not expensive and inefficient. (though if anyone can suggest one!)

I was thinking of using a linear fixed voltage regulator outputting 12v, such as this:
http://uk.rs-online.com/web/search/s...5-5714&x=0&y=0

with capacitors to smooth output in an arrangement like this:

THE biggest problem i can see with this is voltage dropout, i am not sure how long the battery will last for before the battery voltage drops to level where the voltage regulator stops producing 12V. with the Vin and the Vout being so close I guess there is not much room to play with here. in fact so little will it really work at all?

i know it may be less efficient but there are two other designs i thought may provide a longer run time, though it seems a shame to be so wasteful:

Use a DC to DC converter to provide 15V dc (this is the lowest voltage output i have found a 12V converter will do) then use the 12V fixed regulator as above.

Really inefficient: use an inverter to get 240v AC then use a standard mains 12v regulator to run the leds.

Does anyone have any comment/criticism on what i propose on trying out?
Am i going about this the wrong way?

Thanks in advance for any help, Charlie.

2

this is the web page for the led strips, though i think i will be using them in 5m sections.

http://www.ultraleds.co.uk/self-adhesive-cool-white-leds-metre-lumen-p-1565.html

3

Well that sounds like i could save myself a fair bit of work. Thanks. The reason i was concerned about regulating them was because the supplier said specifically (on the phone) that they should be, and would be damaged by a higher voltage. I guess they need to provide more information because that contradicts what you are saying. maybe i'll try a test on a one strip in the morning.

4

Automotive system voltage varies from 12.6 volts (key off, surface charge depleted) to approximately 14.7 volts (engine running). The company recommends using their UREG12 if the strip is used in a car.

http://www.ultraleds.co.uk/self-adhesive-cool-white-leds-metre-lumen-p-1565.html
http://www.ultraleds.co.uk/ureg12-power-regulator-p-1815.html

These strips are listed as 12V and they make it sound like the strips are incapable of handling anything over 12V.
Spec. 325ma Power consumption (3.9 Watts) 12 Volts DC

If you're powering them off a battery (no charger attached), you are powering them with 12.6V. The major worry is whether the additional 6/10ths of a volt will push the current through the strip above 325mA. If it doesn't, you don't need a regulator.

If the current goes above 325mA with 12.6V, use a dropping resistor to absorb the extra 0.6V.

The voltage to run the strips is too close to your battery voltage to worry about building regulators. It would take some sort of switched buck/boost regulator to do this properly, adding unwarranted expense and complexity. A passive regulator will drop out of operation as the battery voltage drops and would need a bit higher voltage than your battery will supply in order to supply 12 volts.

Of course there's another way around this, LEDs are current devices. Don't obsess over the voltage, what you need is a current regulator. Current regulate your output to 325mA and the voltage input no longer matters. Something like this http://www.luxdrive.com/download/?dltf&dmid=1126 will take in a 7-32V input and keep the current output at a constant 350mA. There are simpler circuits using a transistor pair current regulator that probably would fit your needs a lot closer.

5

Yep, that would work for simplicity. :wink:

6

Can be trimmed and joined every three leds 50 mm

So the assembly consists of strings of 3 LEDs in series with a resistor.

With 20 strings per meter, each is drawing about 16mA. So, they're running slightly conservative, and there's some safety margin for you. But probably not enough to hook up an unregulated wall wart (hence the admonition).

It may or may not be enough margin to run off the battery: there's a good chance, but it's not certain. I'd say the odds are against the LEDs living long happy lives if you don't disconnect them during charging, though.

i suggest using the "Contact Us" tab at the top of their website. Ask them what the value of their series resistor is. Then you can figure out what the voltage drop is, and how high a voltage you feed the strip an stay under 20mA.

7

Thanks everyone for the Advice. I connected the LEDs up the the battery and have run them for over 3 days with no signs of the burning out. when you touch them they don't feel like they are running hot either. I guess the suppliers were just being on the conservative side.

8

The suppliers were trying to sell you a regulator.....

LEDs don't have a voltage rating (not in the usual sense at least - they have a forward drop voltage - typically 3.0 - 3.8 volts for white LEDs), what matters is that the current is below the rating for the LEDs, the current is limited in most applications by simple resistors. You could make them work safely on a far higher voltage (say a trucks 24V system) just by putting a suitable resistor in line.

9

Just to be picky and tie up a loose end in a previous post or two, it's very likely that a fully charged 12V SLA battery will be over 12.6 volts, even when the charger is disconnected.

It doesn't seem to be relevant in this case, it just may be for future post searchers etc...

10

I have used and abused these led strips with battery and mains power supplies many times and never had a problem with them. They will tolerate a fully charged 12V battery indefinitely.