LED lightbulbs are illuminating but not illuminating.

May 11, 2011 7:09 AM

What is going on in LED lightbulbs? I mean the devices, not the industry.

I just picked up two LED lightbulbs for my bathroom. Two things about these bulbs confuse me.

Why do I have to wait about 1 second for them to turn on? LEDs activate instantly in my experience. The bulbs are too small for giant transformers or capacitors so I'm not sure what the delay is.

Why do they have heat sinks that would make a Pentium 4 blush? The entire bulb only draws around 6 watts and LEDs have excellent efficiency, so just where is all this waste heat coming from that needs to be dissipated? The bulbs don't feel hot at all.

posted by chairface to technology (9 answers total)

Sure, LEDs are efficient compared to other conventional light sources, but they're really, really tiny.

Also, consider that LEDs themselves are still only about 10% efficient (compared to 2% for incandescent bulbs).

That's about 5W of heat being dissipated across a teeny, tiny area. Even though 5W isn't a lot, when it's distributed over an area smaller than a grain of rice, it's enough to quickly melt the LED. Hence, you need some sort of heatsink to spread the heat out, and make sure that the LED doesn't overheat. (Conversely, the heatsink on your computer is intended to dissipate the heat into the air, which is why it has so many fins, and a fan attached. Because 5W isn't actually a huge amount of heat, it can dissipate from the aluminum heatsink without too much assistance)

However, that heatsink is also likely coupled to a DC transformer or rectifier. Consider the way that AC power works -- the voltage in your house swings between +170V and -170V 60 times a second*. There are a few things to consider here:

• LEDs only illuminate when the current is flowing in one direction. So, for half of that cycle, the LED is effectively doing nothing. It's very easy to see the LED blinking on and off 60 times a second.
• You can use a device called a rectifier to turn those troughs into peaks. However, because you do still hit "0V" 120 times a minute (and because LEDs have a minimum voltage before they'll emit any light), a "flickering" effect is still noticeable. Also, the components for a rectifier often get hot.
• To get beyond 120Hz, or "smooth" out the wave so that the 0-0.7V troughs disappear, you'll need to add transformers and capacitors to your rectifier, which get hot and take a little time to turn on.
• You'll also want to step that 170V (120V RMS) wave down to a more reasonable voltage that won't fry the LED. That's a bit more circuitry on top of what was previously described, and yet another heat source.

All that AC-DC circuitry described above is actually quite (albeit not 100%) efficient. However, waste heat is indeed produced, and heatsinks are helpful for spreading out and dissipating that heat.

*When folks talk about the "120V" household electrical supply, they're actually referring to the root mean square average voltage of the alternating current wave. The peaks of that wave are +/- 170V, although they're only at that voltage for a fraction of a second.
posted by schmod at 7:48 AM on May 11, 2011 [4 favorites]

From what I'm reading on Wikipedia, LED lights need big heatsinks because even a small amount of overheating will damage them.
posted by 1970s Antihero at 7:49 AM on May 11, 2011

The LED emitters themselves are certainly able to turn on instantly - on the order of nanoseconds. But your LED bulb is not just composed of an LED emitter, it has a buck converter that converts the AC voltage to a lower, DC-like voltage with limited current. It's not a transformer anymore, it's a switching regulator that "chops" the incoming voltage. That part of the circuit has apparently been badly designed and takes a moment to sense that it should turn on. There may even be a small microcontroller involved which in itself takes a moment to boot.

The heat sinks are for the LED and for the switching regulators/buck or boost converters that power the LED. LEDs are not perfectly efficient. They do waste some electricity in the form of heat. But it's about the ratio, not how hot it gets. An incandescent bulb might be 90% heat and 10% light. In other words, 90% waste and 10% want. But an LED is MANY times better in this regard (I am pulling these numbers from memory - they may be way off). Furthermore, LEDs have a specific voltage and current at which they are most efficient - where they produce the most lumens-per-watt. Designers try to keep the LED at that voltage and current, which is often quite high for the tiny LED, leading to lots of energy being dissipated in the form of heat.

The common perception of LEDs as being cool (as in temperature) is totally wrong. Because the LED die (light emitting area) is very small, even a small amount of energy can make it very hot. Worse, as it gets hotter, the efficiency of the LED drops, so it is hotter AND it is drawing more current, which makes it even hotter and less efficient. This can result in a condition called "thermal runaway" where the LED will burn itself out. That doesn't mean LEDs are bad, and they are still cool relative to a hot filament, but they are not a panacea and need to be treated for what they really are.

If you want to see real evidence of this, look no further than the back of an LED billboard. The whole back of the billboard is a giant cooling system to keep the LEDs from dying. This, BTW, is also one of the major reasons we won't see ultra-bright OLED displays for a long time. The little tiny OLEDs get hot and you need to cool every single one.
posted by fake at 7:50 AM on May 11, 2011 [1 favorite]

looks like schmod has the real numbers on efficiency. sweet.
posted by fake at 7:51 AM on May 11, 2011

LEDs are very sensitive to heat. It causes failures and decreases the light output. The heat would be coming off the back of the unit, and the bulb being cool is a sign that the heat sink is working correctly.

Thermal Management of High Power LEDs
posted by smackfu at 7:51 AM on May 11, 2011 [1 favorite]

Why do I have to wait about 1 second for them to turn on?

I have a few different types of LED bulb around my house and they all light up instantly (or so quickly I can't see any delay). So I suspect it's down to the particular design.
posted by le morte de bea arthur at 8:23 AM on May 11, 2011

I don't build LEDs, but I'm married to someone who does. He says:

1. Intant-on is a function of the quality of the driver. Better drivers won't have that delay.

2. Heat makes them lose output faster. The heat sink is probably over-engineered so it'll remain bright for the specified time even if you're putting it in a hot place.
posted by snickerdoodle at 9:29 AM on May 11, 2011

All good answers, thanks. The heat sinks never appear to be actually attached to the LEDs though (I can't tell without ripping them apart). Is there a heat pipe to transfer the heat or are the LEDs actually inside on the heat sink with light pipes to get the light outside?

I know that 5W would be a lot for one LED to bear, but it isn't one LED. Most bulbs are a collection of LEDs, sometimes dozens. 5W is the total input to the device, DC converter/regulator and the dozen LEDs too. So each LED is probably bearing less than half a watt. Most solid state devices in that range don't need any heat sinks, let alone a monstrosity like most bulbs have. I know the heat dissipation is critical for LED life and efficiency, I just find it weird they need to be so crazily over-engineered. The amount of energy that went into making that giant hunk of aluminum probably exceeds the energy saved by using the bulb itself.

Thanks everyone! It's been informative.
posted by chairface at 9:51 AM on May 11, 2011

I know that 5W would be a lot for one LED to bear, but it isn't one LED. Most bulbs are a collection of LEDs, sometimes dozens. 5W is the total input to the device, DC converter/regulator and the dozen LEDs too. So each LED is probably bearing less than half a watt. Most solid state devices in that range don't need any heat sinks, let alone a monstrosity like most bulbs have. I know the heat dissipation is critical for LED life and efficiency, I just find it weird they need to be so crazily over-engineered. The amount of energy that went into making that giant hunk of aluminum probably exceeds the energy saved by using the bulb itself.

Even if it was just half a watt per LED, that's half a watt at the PN junction inside the plastic case, the junction is sized on the order of smaller then a millimeter. The plastic wont carry the heat away quickly, so the PN junction can overheat with very little power.

Generally in engineering we refer to things like that as properly engineered, adding that case probably doubles the manufacturing costs. If it was unnecessary it would not be there.
posted by token-ring at 10:38 AM on May 11, 2011