# Help with Hi Power LED Resistors

• eleones
In summary, the conversation discusses a project involving two high-power LEDs and the calculations for the required resistors. The speaker is having trouble finding resistors with the specified specs and considers using a constant LED driver instead. They provide a link to a possible solution and recommend looking into switch mode buck supplies for better efficiency.
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.

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.

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

vk6kro said:
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.

Last edited by a moderator:
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.

## What is the purpose of resistors in Hi Power LED circuits?

Resistors are used in Hi Power LED circuits to limit the amount of current flowing through the LED. This protects the LED from damage and ensures it operates at its optimal level.

## How do I determine the appropriate resistor value for my Hi Power LED?

The appropriate resistor value can be determined using Ohm's Law, which states that resistance (in ohms) is equal to voltage (in volts) divided by current (in amps). You will need to know the voltage and current ratings for your LED to calculate the resistance needed.

## Can I use any type of resistor for my Hi Power LED circuit?

No, it is important to use a high power resistor specifically designed for use with LEDs. These resistors are able to handle the higher currents and temperatures associated with Hi Power LEDs.

## What happens if I use the wrong resistor value for my Hi Power LED?

If the resistor value is too low, it will allow too much current to flow through the LED, potentially damaging it. If the resistor value is too high, it will limit the current too much and the LED will not operate at its full potential.

## Can I connect multiple Hi Power LEDs to the same resistor?

It is generally not recommended to connect multiple Hi Power LEDs to the same resistor. Each LED should have its own resistor to ensure that they are all receiving the appropriate amount of current.

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