LED Modification experiment

[larrybl]

I have a "Free" source for these LED Strips. They take 27VDC to operate, and I would like to try and get them to work at 12V. Any suggestions. PS Anything below 24V and they barely light up.

[mariush]

It wouldn't be easy.

You have 9 LED elements on the strip, each one has a forward voltage of around 2.5-3v, so in total a voltage of at least around 22-24v is required for them to turn on, as you found out.

Once they're lit up, you need to limit the current flowing through them, otherwise they'll simply take as much power from battery or whatever you connect the strip to, and the LEDs will blow up.  That's the purpose of those 3 resistors - smaller resistance means more current flows through the LEDs and they'll be brighter, higher resistance means less current, so the LEDs will be less bright.
Now there's usually just one resistor for a series of LEDs but you have three at the start of the strip, which can mean several things :

1. the designer may have connected the three in parallel so that they'll act as a single larger resistor with smaller resistance and higher resistance to temperature (the three of them will heat less)
2. each LED in the strip may actually be formed out of three smaller leds under that yellow cover (phosphorus), so in reality you actually have three individual strips of 9 smaller LEDs.

You can put a multimeter in continuity mode and test to see if the ends of the resistors are connected together - this way you'll know if the resistors are in parallel (like I say in point 1). If there's no continuity between a couple of the resistors you know you're looking at case 2,  three separate strips.

To make your strip work with 12v , you'd have to basically interrupt the connection between leds after each 3 leds.  After each set of three leds, you'd have to connect the end of the led to the negative trace, you can see it if you look carefully, the trace where you solder the black wire goes all the way to the end of the board and connects to the last led.

At the start of the 2nd set of 3 leds and the 3rd set of 3 leds, you have to bring power, the red wire, and you'd also have to install three resistors between power and first led from the series, to limit the current to those 2 new strips of 3 leds.

[sleemanj]

Boost converter.  Job done.

[Ian.M]

It looks like they have 9 LEDs with a Vf of a bit under 3V in series. How sure are you about that 27V DC figure? did you find it by experiment or was it the original application's supply voltage?

Although it would be possible to hack them to convert to three groups of three series LEDs as just described by the previous poster, it would be very messy, with a lot of track cuts, bodge wires and extra resistors.

Your best bet would be some LM2577 DC-DC adjustable boost converters off EBAY.  Each one should be able to drive three or four of those strips at 27V to 28V.  Simply twiddle the trimpot for the desired output voltage, and thus brightness, then hook it up to the strips.

[onlooker]

If you want to have hand-on fun, but, without too much mechanical butchering, search "Constant Current LED Driver DC12V to ... For 10w.." on ebay.

The one I bought costed $1.28 each. It is small and I used it for 0.7A without problem.  It claimed 0.9A.

If 0.9A or the output voltage is not enough, you can pick the ones with high wattage rating.

Though, you do need some mods: 1). remove the resistors on the board. 2). adjust one of the resistors on the ebay driver for the desired current.

[larrybl]

Thanks, so-far. I would like to power these from a vehicle 12V system "cheaply". I have a source of a 12V PC 19.9 Convertor but I suspect I would not be satisfied with it's performance or cost. The Source of these are from non functional LED 240V Street lamps. I am working on separating the individual heat sinks. No voltages are listed on the circuit board, but at 27V the LED strip just gets warm, I suspect 30+ is used due to the beefy heat sink which is fan cooled.

[Ian.M]

The LM2577 modules I mentioned earlier are supposed to go up to 35V.  They should be able to do 30V-32V with no issues and are only a couple of bucks each.  They are supposed to be capable of 15 Watts or so of output power but as with all cheap modules from china, you need to derate by at least 50% if you want it to last, so I wouldn't run them at much over 200mA output current.   That may well mean only one or two strips per module.