Driving 3 luxeons

[joe1990]

Hey everyone, I'm currently looking to build a very simple array of 3 luxeon rebels (LMX2-PL01) in series. I want to avoid using current limiting resistors, so I've decided to use an lm1086 regulator with a fixed voltage of 2.5v between the adj and output terminals. Here is a schematic of the circuit. Is this a good circuit? or can I do better? is the zener a good thing to have as a protection diode?

**broken link removed**

 

[audioguru]

The reference voltage for an LM1086 is not 2.5V, it is only 1.25V so your resistor values are too high.
The LM1086 has a typical dropout voltage of about 1.0V at 700mA but could be higher. So the input voltage must be higher.
Your LEDs have a "typical" voltage of 3.2V each but could be higher. So the input voltage must be higher.
You want to operate the LEDs at close to their max allowed current so they need huge heatsinks and/or a fan for cooling.

 

[Boncuk]

Here is my calculation:

Given: 3 pcs. Luxeon Rebel Uf=3.5V (max), If=700mA
LM1086 dropout voltage = 1.5V
reserve = 2V
Vref=1.25V

Voltage demand LEDs Vtot = 3X3.5V, Vtot = 10.5V
dropout voltage = Vdo = 1.5V
reserve = Vres = 2V

total minimum input voltage = (Vtot+Vdo+Vres) 14V (zener diode not required)

Radj=Vref/If = 1.25V/0.7A, Radj = 1.785Ω, next standard value 1.8Ω.

Power dissipation Radj = VrefXIref = 1.25VX0.7A = 0.875W - use a 2 or 5W resistor for least heating.

For LED cooling refer to the datasheet.

Boncuk

 

[joe1990]

I must have confused the datasheet of the LM1086. I specifically got the LM1086-2.5 and the data sheet says output voltage is 2.5v (thats what I was basing the calculations on), so what you guys are saying is that the output voltage is irrelevent in this circuit? so it's the reference voltage I need to use right? and the reference voltage is the same (i.e. 1.25v) for all models (ADJ, 1.8, 2.5, 2.85 etc.)? The datasheet lists Vref=1.25v only for the ADJ model. thanks for all the help

 

[audioguru]

You are supposed to use the adjustable regulator to make the current source. I don't know if your fixed voltage regulator will regulate the current.

 

[joe1990]

That is a good point, unfortunately I'm having an extremely hard time finding the adjustible version of the lm1086 regulator, especially the versions that need to be able to withstand -40 degrees celcuis temperatures, there is basically 2 very specific models I can use, LM1086IS-ADJ (TO-263 Package) or the LM1086IT-ADJ (TO-220 Package). Does anyone know where I might be able to find these locally in Toronto or online?

I do have easy access to LM317A Regulators but because they drop about 3 volts i dont think the voltage supply will be able to handle it in the "worst case scenario". Is there possibly a better configuartion thats not too complicated? The simple resistors give way too much variation in current with the supply voltages I have listed.

Thanks for all the comments so far, I really appriciate the advice

 

[4pyros]

You need current limiting not voltage regulashoin. If you are going to use the regulater you must have a current limting resistor. Andy

 

[Boncuk]

You need current limiting not voltage regulashoin. If you are going to use the regulater you must have a current limting resistor. Andy

The OP used the LM1086 as constant current source!

Boncuk

 

[4pyros]

The OP used the LM1086 as constant current source!

Sorry looked like a regulator to me. Andy