If we use a voltage regulator, say an LM317 set to the perfect voltage,
can we use this to supply an LED? Or can we not trust an LED to behave
itself with regards to current draw? Do warm LEDs draw more current, cold,
etc.

I understand this might be wasteful, but let's disregard that for the moment.

Ultimately, I see current as a function of voltage, and if I precisely control
for Voltage, the Current will take care of itself. What am I missing?

 

Leds are totally current driven, unless you use a series resistor, then a Constant Current source is needed.

You can configure the LM317 as a Constant Current source.

 

If we use a voltage regulator, say an LM317 set to the perfect voltage,
can we use this to supply an LED?

You'd be better off configuring the regulator as a current source.

Do warm LEDs draw more current, cold, etc.

They have a negative temperature coefficient in all the data I've seen. If configured as you propose, that would cause current to increase with temperature.

Ultimately, I see current as a function of voltage, and if I precisely control
for Voltage, the Current will take care of itself. What am I missing?

LED forward voltage isn't tightly controlled, so the voltage would have to be adjusted for every LED.

You'd be better off configuring the LM317 as a current source.

 

If we use a voltage regulator, say an LM317 set to the perfect voltage,
can we use this to supply an LED?

This is a source of argument around here, but yes, you can control an LED with a well-regulated voltage source. Sort of, with some caveats.

Every LED has a characteristic curve of current versus ∆V. It's quite steep: You go from no current at all to a blown LED in about 0.5V. If you set a precise voltage for a particular LED near the bottom of that 0.5V range that lights it up with, say, 5mA, it'll sit there nicely and work fine.

But there are caveats. One reason to not use voltage-based control is because this voltage range is not identical between LEDs. A 20mA fixed source will supply the desired 20mA to every LED you pick, but there is no voltage you can set that will ensure this for every LED. Each one has to be adjusted independently.

Also, if you need to target a current anywhere near the rated maximum of the LED, you risk setting off a thermal runaway mentioned in the other answers. It gets hot, draws more current, gets still hotter, repeat. Poof. It's easy to set up a 19mA current source for a 20mA-rated LED but there's no way you could stay close to 19mA using voltage control.

The only reason I bother to answer is because it's simply not always true that you cannot use a regulated voltage. You can, as long as you can live with the downsides. It's a character flaw of mine to find fault in absolute statements.

 

I am familiar with LM317 as a current source. I am not going to argue the wisdom of that.

Thanks.

 

LEDS ARE NOT LINEAR, AND DO NOT BEHAVE LIKE RESISTORS! REMEMBER THAT!

 

Thank folks! All good replies. I think I have a handle on the subject now.
LEDs are so cheap, available, and so I've enjoyed burning a few of the 5mm
types while investigating their properties.

I've purchased some 50w (&3w, 10w) cob LEDs, all substandard rejects I believe.
I need to build some drivers, but I'll save those questions for a new thread.

 

Thank folks! All good replies. I think I have a handle on the subject now.
LEDs are so cheap, available, and so I've enjoyed burning a few of the 5mm

Be careful. Usually they just burn out, but one time I had one explode. It split clean in half. I think it did not like the extreme overvoltage and overcurrent.

 

LEDs, 555s, Flashers, and Light Chasers

Wendy's Index

 

I've purchased some 50w (&3w, 10w) cob LEDs, all substandard rejects I believe.
I need to build some drivers, but I'll save those questions for a new thread.

For high power LEDs, you really need a current controlled driver.

Bob

 

I put a 50w COB LED on a CPU heatsink with grease, And I am driving it with 32v from an old HP printer power supply,
plus 2.2ohm 5w resister & LM317 configured as a current regulator. I measured 17w output, which is fiercely
bright, and the heatsink can't be held for long. I mounted it above my soldering station, and
ran it for a few hours. I stared at it with my welding helmet, and I can see two dark spots, so I suppose
that's why I got it so cheep on ebay.