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I've come to the understanding, that heat is the killer for LED's, i.e. supplying a too high a current, the LED gets hot and dies.

Now assume I have two LED's, 1W and 3W, both mounted on a heatsink. What is the difference?

  1. Will the 3W be as bright as the 1W provided the same current (~350mA => 1W)?
  2. Driven at their respective currents, will they have the same expected lifetime?
  3. Will the 1W live at all, if I supply it 3W but keep it cool?
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    \$\begingroup\$ What do the datasheets say? \$\endgroup\$ – PlasmaHH May 21 '15 at 10:37
  • \$\begingroup\$ "1W" and "3W" aren't currents! \$\endgroup\$ – Leon Heller May 21 '15 at 11:05
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    \$\begingroup\$ @PlasmaHH I don't think such a datasheet exist. I assumed it was possible to answer as a general question, but it seems I am mistaken. \$\endgroup\$ – Tagger May 21 '15 at 11:13
  • \$\begingroup\$ @LeonHeller I know, but I think you catch my drift. Fixed now. \$\endgroup\$ – Tagger May 21 '15 at 11:13
  • \$\begingroup\$ @Tagger: yes you are, for most leds you have visible differences in luminosity for certain amps values. Now think about how that multiplies for different types and manufacturers. \$\endgroup\$ – PlasmaHH May 21 '15 at 11:29
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The difference is 2W. :-)

When we assume that luminosity is related to power (i.e. that the efficiency is about equal, which we can safely assume) then the 1W will obviously be dimmer. But because the sensitivity of our eyes follows a logarithmic curve the difference will be smaller that one might expect. (The difference between 1000 lx and 100 lx is about the same as that between 100 lx and 10 lx.)

The 1W device will become less hot and therefore have a longer lifetime, but the difference may be negligible.

How would you supply 3W to the 1W LED? They might have the same supply voltage, then supplying that voltage it will only draw the current associated with 1W. If you want to have it consume 3W of power you'll have to drive it with a higher voltage. The current will go beyond Absolute Maximum Ratings", and the device will get damaged pretty quickly, possibly break completely. Keeping temperature low is not enough. I don't recommend overpowering it.

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  • \$\begingroup\$ Seems reasonable. The LED's are rated at ~350mA and ~700mA, so I would probably just increase the current from my BUCK, and allow it to rise a few hundred mV. The 3W is rated at a bit higher voltage, which made me speculate if they were in fact the same product with different names \$\endgroup\$ – Tagger May 21 '15 at 11:20
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    \$\begingroup\$ "... it will only draw the current associated with 1W" Really? So why do we normally need current limiting resistors with LEDs? \$\endgroup\$ – Roger Rowland May 21 '15 at 11:40
  • \$\begingroup\$ "They might have the same supply voltage, then supplying that voltage it will only draw the current associated with 1W" isn't correct. LED voltage is fixed by the physics of the semiconductor (and other things like temperature). The current is what you can control. And why do you assume the 1W has a longer lifetime? You're putting the cart before the horse. The designer first invents his new LED, then tests it to see what power it can take and still last X000 hours, then writes that power on the box. So quite likely the lifetimes are similar at rated power. \$\endgroup\$ – tomnexus May 21 '15 at 11:40
  • \$\begingroup\$ @tomnexus: "LED voltage is fixed". Not quite. The voltage will vary somewhat with the current you supply and vice versa. Claiming that you can have a fixed voltage at which you can vary the current is not true. \$\endgroup\$ – Joris Groosman May 21 '15 at 12:43
  • \$\begingroup\$ True, it has some resistance. I should have said Temperature and Current. But the LED needs to be driven by a current source. Driving it from a fixed voltage and expecting it to take the current it needs, is not how it works. \$\endgroup\$ – tomnexus May 21 '15 at 14:25
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Will the 3W be as bright as the 1W provided the same current (1W)

This is unanswerable without intimate knowledge of the two LEDs. For instance the 3W device may contain 3 x 1W LEDs in series. So, when you put the same current into the 3W LED, you'll be illuminating 3 x 1W LEDs.

Clearly the light output will be higher than just one LED.

But, that 3W LED might contain 3 x 1W LEDs in parallel (each with a balance resistor) and now things look totally different.

Speculating done - show the data sheets if you want more information.

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