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3w luxeon LEDs - just thinking out loud....

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Andy1845c

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Any input or comments are welcome:)

I want to use some 3w luxeon LEDs in a project. Their max forward current is 700mA, but I was thinking of running them at somthing closer to 500 mA.

I want to use 4 of them in series on truck with a 12v system. I am figuring a voltage of 13.8 when the engine is running and 12v or just below with the engine off.

Typical Vf is 2.2v * 4 = 8.8
13.8 - 8.8 = 5v
5v/.500 = 10Ω
.500 * 5 = 2.5 watts

I know this would waste power, but its not a huge concern.

I was going to go with 5 or 10 watt resistors. I have a bit of room to mount them. The duty cycle is also not 100%, so I don't think the heat would be a huge problem.

My other idea is to use an LM317 as a constant current supply.

The reason I am at this point leaning toward the resistors is less parts = more reliable.

Also, would it be smart to add a diode inline with the luxeons to protect them from any reverse polarity spikes?
 
Also...just thinking out loud.

You'll have to heat sink the LM317, as it will have to dissipate about the same power as the resistor. You will need about 2.5v overhead for the IC and another 1.25v for the current sensing resistor and 8.8v for the lamps...so ~12.55v total...right at edge. Add another 0.3v to 0.6V for the protection diode and you're at 13v to maintain regulation. The advantage of the constant current regulator is: no changes of brightness with battery voltage increases, but below 13v you might see some dimming. But this is just my speculation. :)

Ken
 
The LM317 won't work reliably because the dropout voltage when it's connected as a current regulator is nearly 4V.

A resistor is probably best the way to do it.

What colour are these LEDs?

2.2V sounds a bit low, I assume they're red, orange or yellow rather than green, blue or whte.
 
You can use this http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=LM2673S-ADJ-ND , just adjust Vout =8.8 v... no heat , no waste..

The Luxeon LEDs are "current driven" devices..like all LEDs. The LM2673S is a voltage regulator. You can't just apply 8.8v to the series string. You need to set the current that's passed through them, not the voltage that's dropped across the string. If constant intensity is not a problem for andy, the resistor is probably the best solution. Just my opinion. :)

Ken
 
Might look into one of these drivers, cheaper than buying a replacement LED. Don't forget to heatsink the LED, they get hot quickly...

**broken link removed**
 
If you use the adjustable version of the LM2673, it has a feedback voltage of 1.21 V. For a constant current, you just connect the feedback voltage to a resistor that gives you the current you want.
 
LM2673 has also have a adj current limiter resistor , you can set any current what u want , check data sheet
 
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If you use the adjustable version of the LM2673, it has a feedback voltage of 1.21 V. For a constant current, you just connect the feedback voltage to a resistor that gives you the current you want.

I didn't see that in National's Application Notes:**broken link removed**. Do you have an App Notes that uses it that way?


LM2673 has also have a adj current limiter resistor , you can set any current what u want , check data sheet

That just sets the peak switching current.



No expert...just looking for info. :)

Ken
 
Hi there,


Sometimes you can use a resistor to ground in series with the LED
and take the feedback voltage from the top of the resistor. For
example, say you want 1 amp through the LED...the feedback
voltage reference is 1.25v so if you use a 1.25 ohm resistor when
there is 1 amp flowing through it the feedback voltage would
be at the required 1.25v and so it should regulate the output
current to 1 amp. For half that (500ma) a 2.5 ohm resistor would
be used.
The problem here though is that not all regulators will be stable
using this method. Since there is no note on the data sheet
this idea would have to be breadboarded and tested at all
levels of input voltage. There is a good chance this will work,
but it does have to be tested.
At the extra sense resistor drop would be 1.25v so that would
be added to the 8.8v drop of all the LEDs, bringing the total up
to a little over 10v. This would only leave 2v for the pass
transistor, which may not work either. The data sheet shows
a 5v output with a min input of 8v, so that's 3v which is greater
than 2v of course.

Another idea is to set the output to 8.8v plus another 0.5v, and
then use a 1 ohm resistor in series with the output. This will
roughly regulate the output current to 500ma but only if the
temperature of the LEDs does not vary too much.
The problem here is that if the LED temperature varies too much
then the forward voltage drop varies a lot (4x one LED) and so
the LEDs might start to draw too much current and burn up.
LED aging might also cause a problem as the forward voltage
characteristic changes over time.

Probably the best idea would be to divide the LEDs up into two
groups, with two LEDs in series for each group, then use two
switching buck regulators as above. There would be plenty
of voltage overhead and this method wouldnt get too hot either.

If that sounds like too many parts, another idea is to make one
buck regulator and connect the LEDs again into two series groups,
but wire the two series groups in parallel to the one regulator
perhaps with two small current sharing resistors.
This would work with either the current regulation topology
(1.25 ohm in series to ground, taking feedback from the top)
or the voltage regulation topology (regulate output above the
two LED drops by about 1 or 2v more, and use current limiting
resistors for both strings).

In any case however the circuit has to be tested for stability
if it is going to be doing current mode regulation.
 
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The voltage in an auto electrical system varies from 10-15V when it is running correctly, though it can drop quite a bit farther while cranking the starter.

you would have to start with a safe LED current at max input (while charging) protecting against the 40V or better load dumps (when electric motors shut off) , and then see if you can live with the low end (low battery) intensity. There is a reason you see "automotive" ratings on power regulators.

13.8V is the nominal charging voltage, 12.8V fully charged and 10V discharged. That makes for a range of 500mA down to 120mA through your LEDs, though if everything is working properly it will probably only go down to around 12V under load for around 300mA.

Dan
 
Hey guy,

Thanks for all the replys.

Here is exactly what I am doing. Some of you may remember my traffic adviser project. https://www.electro-tech-online.com/threads/finished-traffic-advisor-project.33618/

Well, I am making version 2.0. This one needs to be a little bigger and brighter. So bottom line, I need 4 of the luxeons per section. So whatever regulaton design I settle on, I need atleast 8 of them if not 16 (assuming I can't get all 4 LEDs in series)

Thats what kinda steered me away from the easy route of buying a driver like the one Harvey posted. Along with the already pricey Luxeons, adding the drivers would make this too expensive.

The issue with the brightness changing is not a huge issue. Atleast i don't think it will be. I don't have the luxeons yet to play with, but the 5mm orange leds I do have don't change in brightness enough to bother me +/- a few volts/mA.

40v spikes? Wow. Thats somthing I didn't know.

I was going to add a crowbar circuit in the control circuit for the lights. Would that be fast enough to prevent damage in an automotive setting?

Now I am wondering what kind of spikes the electrical system of my truck sees. Its an older diesel truck, with no ECM or any computer controls, so i suppose the regulation could be pretty sloppy. It also has grid heaters that cycle to warm the air intake when its cold. They switch a huge load on and off the system.

I wish I had a hand held scope so I could watch as I drive around. :D

I suppose I could try it with my desktop scope in the driveway. Switch the heater fan, headlights and grid heaters on and off and see what happens.
 
Oh goody 90v:p

Wow, that is a very technical document. Kinda over my head:eek:

Would it be possible to add a clamping circuit of some kind along with the resistors?
 
the graph and table at the bottom of page 2 are the actual test conditions for automotive compliance.

if you put only two in series and put an additional 10 ohm 10W (you might be able to get away with a 5Wer since they are rated at 5x over power for 5 seconds) and a surge zener before your 10 ohm 2.5W resistor it will probably work.
 
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