I'm trying to build a model lighthouse (don't ask). :-)

Essentially I'm trying to have an über-bright LED with a motor rotating at 2rpm, both powered from the mains (120V AC). The problem I am having is overheating...

I have constructed the following circuit:


simulate this circuit – Schematic created using CircuitLab

These are the components I am using:

Power source: 12V DC power adapter, 1A

LED: Cree 10W high-power, 2.9-4V forward voltage, 3000mA forward current, 10W power dissipation, 2.5°C/W thermal resistance

Resistors: 1x 3ohm 25W (R1) and 1x 3.3ohm 50W (R2)

The motor part works wonderfully. I can adjust the speed down to 2rpm and everything is good. The LED on the other hand is causing a headache. Initially, I had calculated a 3ohm resistor (R1) based on (12V-3V)/3A. But this caused the LED to flash intermittently - I assumed overloaded - so I bought a slightly larger R2 (3.3ohm), thinking maybe the voltage across the LED was 2.9 and so I should go for something a little larger. Still blinking. On a whim, I put them in series (as in the diagram), and then it stops flashing and everything works as it should.

However, the LED is getting VERY hot, as are the two resistors. I don't run the circuit for longer than 5 minutes because I'm worried about it burning out. The LED comes pre-mounted on a flat heatsink disk, and I could attach a slightly larger heatsink to that, but I'm limited by the lighthouse design and therefore would really like it to get less hot.

The resistor power should, I thought, be 3A*(12V-2.9V)=27.3W.

I feel I must be miscalculating what kind of resistor I should be using. Or maybe there's something I'm missing entirely wrong with the circuit. Any thoughts on this and/or ideas as to how to stop this LED circuit from self-incinerating would be hugely appreciated.

Thank you all!

  • 2
    \$\begingroup\$ 12V 1A power supply can't supply 3A for the LED. No wonder it keeps shutting down. With the new resistor, 9V across 6 ohms, it can't supply 1.5A either. And you know those LEDS must be mounted on heatsinks, right? What's the thermal resistance of your heatsink? \$\endgroup\$ – Brian Drummond Apr 22 '17 at 21:05
  • \$\begingroup\$ Your value added local content resistor makes the overall system efficiency about the same as it would be with a halogen bulb negating the good work that the clever LED designers have done .More power is going in the resistor than is going in the led.You could try a buck convertor .Even if the buck was chinese it would waste much less power than the resistors. \$\endgroup\$ – Autistic Apr 22 '17 at 21:09
  • 1
    \$\begingroup\$ Thank you Brian for your reply! I do feel like a bit of an idiot. And at the risk of appearing even dumber: the heat sink I am using is just the metal disk that comes with the LED (as in the link), which is meant to have a thermal resistance of 2.5°C/W. I am not using an additional one, which I'm guessing may be the problem. What I'm also failing to understand is why the LED is now working when my power supply is only 1A, which as you pointed out should be insufficient, and yet it is overheating also... Sorry if this is all obvious to you. Thank you very much for the help! \$\endgroup\$ – Furry Monster Apr 22 '17 at 21:20
  • \$\begingroup\$ That piece of heat conductive PCB is not heatsink and it definitively can not have 2.5 K/W into the air. Maybe it is thermal resistance between LED junction and this metal backside of PCB. You still need to add quite large heatsink. \$\endgroup\$ – Martin Apr 22 '17 at 21:25
  • 1
    \$\begingroup\$ Thank you Autistic! That is a very good point. I would prefer not to waste all that energy. I'd never even heard of buck converters. So I could use this in lieu of the resistors? \$\endgroup\$ – Furry Monster Apr 22 '17 at 21:26

For a 10W LED, if you have 40°C air and can allow a temperature of 60°C on the aluminium star PCB, then you need a 2°C/W heat sink.

Expect a 50mm x 50mm x 20mm block of finned aluminium.

If you use a fan, then it could be smaller.

The star PCB cannot, obviously, dissipate 10W in free air. It will dissipate one watt, and even that would be pushing it if it's inside an enclosure.

You should also use a switching driver for your LED.

If optics allow, using more LEDs will make your thermal problems simpler by spreading the heat over a larger area. Also, using several LEDs in series will work better with a simple resistor dropper...

Here are examples of heat sinks designed for 3W LEDs.

If this is a personal project, a desktop CPU heat sink/cooler will do nicely. If you have an old junked PC... or have a friend with a cardboard box full of junk PC parts... it's easy to find. It it was designed for a 100W CPU like a Pentium, it will not require the fan to dissipate 10W.

| improve this answer | |
  • \$\begingroup\$ Peufeu, thank you so much! I had clearly not given the heat dissipation issue enough thought. I may have to modify the lighthouse design a little to permit some of those heatsinks. But I'm also thinking - realistically - that I may have to use a less strong LED, such as a 3W one as you suggested. And sorry - just because I'm unfamiliar with these - a switching driver is akin to the buck converter Autistic was referring to above? Thanks so much! \$\endgroup\$ – Furry Monster Apr 22 '17 at 21:54
  • \$\begingroup\$ Yes, buck is a form of switching converter, this is what you need here. Make sure it is sold as a "led driver" or "constant current source" as you need a current regulation, not a voltage regulation. Check that it accepts 12V input and that 3-4V for one LED is inside its output voltage range. I dunno what your "lighthouse" is but you could post pics and/or drawings... \$\endgroup\$ – bobflux Apr 22 '17 at 22:03
  • \$\begingroup\$ Thank you again peufeu - that is incredibly helpful! I'll see what components I can find! :-) The 'lighthouse' is that it is meant to be a replica of this one: upload.wikimedia.org/wikipedia/commons/0/02/… - the LED would go right under the roof, so the heatsink/fan would have to fit in there... \$\endgroup\$ – Furry Monster Apr 22 '17 at 22:38
  • \$\begingroup\$ OK! If the top of the lighthouse is only a few cm in diameter (ie the LED is a tight fit) then dissipating 10W is gonna be real trouble! A 3W LED should be much easier, with smaller heatsink, and honestly, to the naked eye the difference is not that large... \$\endgroup\$ – bobflux Apr 22 '17 at 22:43
  • \$\begingroup\$ Yeah, I think you're absolutely right. And I'm going to be paying lots of attention to the heat dissipation from here on! Thanks again, peufeu! \$\endgroup\$ – Furry Monster Apr 22 '17 at 22:52

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.