LuxIII on 12v for outdoor lighting power question

[James S]

Hi Folks,

I'm having fun testing various luxeon and other high power LED's for some low voltage outdoor lighting. The LuxIII with an optic or reflector makes a terrific uplight or spot to accent something. I now need to decide how to power these things.

The traditional outdoor powersupply for outdoor lighting is 12v AC, which after rectifying to DC is even higher. The supply voltage for a luxeon is under 4v, which means quite a significant drop in voltage making an LM317 based current regulator run really hot and make it difficult to properly heat sink inside the little lamp housings. I'm not terribly worried about efficiency since it's wall powered for now, but it still feels wrong to me to waste it like that /ubbthreads/images/graemlins/wink.gif

I have several options. I could switch from using a 12v AC power supply to using a higher current 5v DC one. This is easy enough to setup since I won't be mixing these lamps with any traditional bulbs on the circuit. However, I'm concerned with the higher line losses at the low voltage DC and also greater connection corrosion with the DC current. But this would make a very simple LM317 based current regulator more feasible.

Other possibilities would be a more complicated power supply in each light. I was considering experimenting with a 5v fixed regulator and then that output through the LM317 (or even just a big resister at this point) in current regulator mode, that would reduce the heat form the 317, but aren't the 5v regulators just linear also and so would generate just as much heat anyway.

The ultimate geek solution would be some kind of buck regulator. I have no interest in purchasing a downboy for each lamp though /ubbthreads/images/graemlins/wink.gif that would run through my budget in rather a hurry for multiple lights. So I'd rather a circuit that I could build myself with discrete components and not surface mount. I plan to etch a bunch of boards anyway.

Finally there is the most simple solution, don't even worry about the AC, protect the luxeon from the reverse voltage with another diode and limit the current with a big resister and connect them directly to the 12v AC. But the flicker will aggravate me.

I know some other folks have done outdoor lighting but searching of course turns up little in the way of specifics.

Any thoughts for a reasonably priced 12v input power supply I can build for these things?

Thanks guys!

 

[McGizmo]

Re: LuxIII on 12v for outdoor lighting power quest

James,

Xitanium was supposed to comeout with a 12 VDC driver but I don't know if they ever did? They did have a 24VDC driver that would be better for wire gauge consideration and still fall into the "low voltage" scene. I would guess part of your consideration is whether you will have a bunch of these LED in close proximity so that a string fromone driver makes sense or whether due to the layout you need them independent with their own drivers? I suspect the latter. If they are to be clustered at all, the BadBoy is great at driving three or four LED's in series at reasonable current levels with 12Vin.

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

James,

Just run the lights in series. A series string of 3 should be powered fine from an LM317 in current mode with the higher rectified DC voltage.

 

[James S]

Re: LuxIII on 12v for outdoor lighting power quest

If I cluster them electrically that certainly reduces the cost of any driver board that I make for them. But it drastically complicates the wiring /ubbthreads/images/graemlins/wink.gif And I have to document and remember which one is connected to which other one....

But yes, it should definitely be possible to connect them that way.

3 lux 3's in series should be pretty close to the voltage, but thats 3 amps through the lm317. But I might pot a circuit in a separate box then. Easier to heat sink it properly then.

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

Use one LM317 per string. One LM317 + one resistor = about $1.50 per string

Then it's only 1 amp per LM317 (in series, Vf adds up, but the same current goes through all of them)

You could wire a set of power jacks as a series set, then wire each luxeon/leader individually like normal. Or just wire a set of 3 as a single string. Lots of options.

 

[James S]

Re: LuxIII on 12v for outdoor lighting power quest

[ QUOTE ]
Then it's only 1 amp per LM317

[/ QUOTE ]

Arrg, i should have known this /ubbthreads/images/graemlins/grin.gif Thanks.

having a low blood sugar moment /ubbthreads/images/graemlins/wink.gif

Right now I'm considering a master lamp with the driver in it, and then wiring 2 lamps directly to that rather than doing the splicing outside in the dirt. Should help with the DC corrosion of the connectors too. Now I need to buy more lamps to mod /ubbthreads/images/graemlins/wink.gif

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

That sounds like a good plan.

For my living room lights, I have 4 strings. 2 have 3 Luxeons in series at 500mA, one with 2 Luxeons in series at 500mA, and one with 2 Luxeon IIIs in series at 650mA. Each string has a '317 driving it, and the whole lot is driven from a 12V switching supply.

I have a box that has the regulators in it - it's fan cooled, so it's not the best idea for outside, but I should go ahead and post a pic of the guts - might give you some ideas.

 

[HarryN]

Re: LuxIII on 12v for outdoor lighting power quest

I know this is somewhat low tech, but:

Typical Lux III - Vf approx 3.5 V
3 x 3.5 = 10.5 V
Reverse protection diode = 0.3 V

Total Vf to overcome - almost 11 V - only 1 V to drop from the 12 V, so a 1 or 2 ohm resistor in line will do it.

Some caps to smooth out the voltage, and the flicker will be relatively small.

You are going to work really hard to beat the efficiency of that simple, almost zero cost setup. My guess is that you have the parts already sitting in your bins.

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

Here's the regulator box for my living room lights:

Each LM317 is mounted on an aluminium plate. Power is fed in by a 5 pin DIN socket (from the power supply), and out though 3 coax power jacks, and one dongle (since I ran out of the power jacks /ubbthreads/images/graemlins/tongue.gif )

The fan keeps things cool. The box is around 4"x2"x2".

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

[ QUOTE ]
HarryN said:
I know this is somewhat low tech, but:

Typical Lux III - Vf approx 3.5 V
3 x 3.5 = 10.5 V
Reverse protection diode = 0.3 V

Total Vf to overcome - almost 11 V - only 1 V to drop from the 12 V, so a 1 or 2 ohm resistor in line will do it.

Some caps to smooth out the voltage, and the flicker will be relatively small.

You are going to work really hard to beat the efficiency of that simple, almost zero cost setup. My guess is that you have the parts already sitting in your bins.

[/ QUOTE ]

Harry,

The only reservation that I'd have about that setup is that current will change drastically when the Vf of the LEDs drops. If you only have 1V across the resistor at first, you will have over 1.5V after the Vf of each drop. That could cause current to rise by 50% or more.

 

[SemiMan]

Re: LuxIII on 12v for outdoor lighting power quest

One of the easier chips to use for this is the L6902D from ST. Pull it up with an App-Note. It is designed for battery charging, but makes for a good buck-regulator. You can set the current with one resistor and maximum voltage with another. It runs about 200KHz if my memory serves. You can almost do it with an MC34063, may be tight on current at lower voltage. It is low frequency which means bigger inductors, but possibly less sensitive circuit if laid out poorly. You can also use an LM2676 from National which is a basic buck regulator.

 

[n_den]

Re: LuxIII on 12v for outdoor lighting power quest

hey evan9162,

is your ps on the same line as your main lights? i didn't realize how fast the leds light up compared to my flourescents. (did i spell that right)

also have you noticed that the light is smoother? not sure if that's the right term.

nice too hear others are using leds in the dwelling. /ubbthreads/images/graemlins/smile.gif

 

[HarryN]

Re: LuxIII on 12v for outdoor lighting power quest

I wonder if the real challenge here is totally unrelated to the driver technology. It might instead be the reaction from the SO when the white LED comes on and beams onto her pretty flowers with purple and yellow petals and deep green leaves.

You might want to consider using an array of 1 watt leds with Red, Green, Royal Blue and enhanced with Cyan and amber. It is pretty painful when your wife / girlfriend tells you that the incans made the plants look nicer after all the effort required.

I guess the other option is to consider using warm white Lux I with a royal blue kicker.

 

[evan9162]

Re: LuxIII on 12v for outdoor lighting power quest

My lights are plugged into a switched outlet. Actually, there is about a 1 second delay before they turn on, but that's not the LED's fault; the delay is caused by the switching power supply powering up.

 

[enLIGHTenment]

Re: LuxIII on 12v for outdoor lighting power quest

If anyone still wants to go for the complex route, switching voltage regulators are available widely available for $5 each.

E.g. http://us.digikey.com/scripts/dksearch/DKSUS.dll?Detail?Ref=124664&Row=302296&Site=US