Help with Hi Power LED Resistors

[eleones]

I have a project with two, separately wired high-power LEDs.

LED #1 is a 1 watt LED rated at 3-4 Vdc at 350/700 mA
LED #2 is a 3 watt LED rated at 2 Vdc at around 350/700 mA

According to Resistor calculations, I get:

LED #R1 = 4.7 Ohm, 2 Watt rating (for 700 mA)
LED #R2 = 8.2 Ohm, 4 Watt rating (for 700 mA)

The problem is I can't find any online vendor that sells common resistors at these specs. Am doing the calculations wrong to arrive to 2 and 4 watt rating or is there a vendor out there that sells these? Alternatively, is there a unique type of resistor that I can substitute for these ratings?

Thanks for any info.

 

[vk6kro]

4.7 ohms with 0.7 amps flowing in it will dissipate 2.3 watts, so that seems a bit off, for a start.

What supply voltage are you using?

The 8.2 ohm resistor would dissipate 4 watts, but you would always allow a bit extra.

If you could get half watt 8.2 ohm resistors, you could put 4 at a time in parallel and then put 4 of these banks in series.
So four 8.2 ohms in parallel = 2.05 ohms
Then 4 of these in series gives 8.2 ohms but at 8 watts.

I know it is only a few watts, but these circuits are very inefficient. At this level, you probably should be looking at swtch mode buck supplies. The following is an example, but may not be the best around:
http://www.simplecircuitdiagram.com/2010/02/11/wide-input-voltage-range-high-power-led-driver/
This would give you immunity from supply variations as well as reduced current consumption from the supply. Maybe you could go to Maxim's website and get the data sheet for the MAX774 device.

 

[eleones]

Thanks for the reply. I'm planning to drive with 6 AA batteries (7.2 V). It's been recommended that I use a constant LED driver instead of resistors. I found several 3W drivers rated for 3W LEDs powered by 3-12 Vdc, but I can't find 1W LED drivers that can be powered by anything below 12 V. Unless it's possible to get away with using one 3W driver to power both LEDs in series?

Here are links to two of them I found:
http://bit.ly/90UmKV
and
http://ledsupply.com/03023-d-n-700.php

Thanks

4.7 ohms with 0.7 amps flowing in it will dissipate 2.3 watts, so that seems a bit off, for a start.

What supply voltage are you using?

The 8.2 ohm resistor would dissipate 4 watts, but you would always allow a bit extra.

If you could get half watt 8.2 ohm resistors, you could put 4 at a time in parallel and then put 4 of these banks in series.
So four 8.2 ohms in parallel = 2.05 ohms
Then 4 of these in series gives 8.2 ohms but at 8 watts.

I know it is only a few watts, but these circuits are very inefficient. At this level, you probably should be looking at swtch mode buck supplies. The following is an example, but may not be the best around:
http://www.simplecircuitdiagram.com/2010/02/11/wide-input-voltage-range-high-power-led-driver/
This would give you immunity from supply variations as well as reduced current consumption from the supply. Maybe you could go to Maxim's website and get the data sheet for the MAX774 device.

 

[vk6kro]

I looked at the data sheet for the second one. It shows LEDs in series. (see P5)

The only limit is that the input voltage must be higher than the total voltage of the LEDs. This should not be a problem.

They talk about 95% efficiency, so if you got anywhere near that you would be much better off.