Instructables

Does and LED's current draw depend on its voltage drop?

I was under the impression that an LED will draw whatever current it can get it's hands on, as my 20w LED (rated for 13.5 - 15.0v and 1500mA) would draw 3A when running off a 15.2v 2A ac adapter.

Now that I've killed the 15.2v adapter, I'm running it off of one of 2 12v AC adapters, either a 1A, or the 2A one, and the draw is always 750mA (under the max possible supply from adapter).

But when running this same test with a 10w 12v LED, the draw is 1A and 2A (max current supplied by each adapter).

Does that mean that an LED draws a current based off its Forward Voltage, or is there something else I'm missing?

seandogue4 years ago
There is a range of acceptable forward voltages from which an LED can be operated to generate light.

At the one end it produces nothing (votlage too low).

At the other end, it produces smoke and user disappointment (voltage too high)

At the low end, the MINIMUM forward voltage has not be achieved, so like a dam that has bypass channels, which is supplied by a tiny trickle flow from a little creek, the current cannot flow.

At the other end, the voltage is far too high, like a torrential flood hitting the dam....the dam breaks

In between the current is proportional to the applied forward voltage. The relationship isn't linear and is specific to a particular LED's design/construction.

Visit a mfg site to find VI curves for their products. In general, that will give you a good idea of the characteristics of all LEDs, with the caveat that the specific voltages and currents are different for each specific device. The basic curve is similar for most LEDs and their parent device, the diode.

Note that with high output LEDs, it is critical to remember that you need sufficient dissipative mechanisms to draw heat away from the LED's silicon junctions, since the are dropping power equal to Vf(applied) x If(actual), which, in tha case of a 25W LED is quite significant, especially considering the energy density (you've taken a 25W light bulb and squished it into a package the is in realty (with the reflector assembly) smaller than the size of a kernel of corn.

Also note that for a given power supply, if you exceed the minimum Vf, the VI curve WILL apply, and for a power supply capable of N volts @ M amps, unless the power supply is capable of delivering in excessive of what the LED is able to conduct, it will look to the power supply like a short circuit. Unless overcurrent is built into the power supply or installed inbetween the supply and LED, you will likely burn out the power supply.

Hint (in case you missed it): apply fusing between the two to prevent damaging your supply ...
"At the other end, it produces smoke and user disappointment "

ROFLcopter
Roflcopter.gif
aint' it the troof though?

smokin a 25W LED is no 3 cent loss.
Bet they make really pretty smoke, though.
At the price of your average high powered LED, it ought to be, but sadly, I find nothing magical about expensive semiconductors going poof.
I dunno... Burnt offerings to the vengeful God of Electronics, Reddy Kilowatt? Sounds pretty magickal to me.

OK, everybody join hands and repeat after me: Ohhhhhhmmmmms... Ohhhhhhmmmmms....
Reddy Kilowatt?!?!?!? Dude, you are soooooo old! :)
.  My Dad worked for AP&L (now part of Entergy) when Reddy was popular.
Old as sin, and twice as ugly. And darn proud of it, too. Getting old beats the alternative.
Ha, gotcha both beat. I have an ashtray.


.  Tres kewl!
The coolest thing is that I dug it out of my back yard while I was rebuilding a retaining wall several years ago. That was such a sweet thing to find. I figured it was just a broken chunk of glass until I got it out intact, god knows I've found enough shards of pottery, glass, marbles, nails, rusty bits of who-knows-what, and other bits of stuff...
I feel compelled to point out that owning an ancient icon dedicated to His Reddiness kinda supports the "magic" theory. Just sayin'.
Win. Epic win.
lemonie4 years ago
The device is designed for a certain operating range. As you've found already, what goes through it may be limited by the power supply.
When you get past the forward voltage the things really open-up, as steve describes with the curve, your 750ma reading is a bit below this.

L
Look up the current/forward voltage characteristic of any diode and you see a curve like a 'J" The rapidly steepening part of the curve occurs at a nebulous point we call the "forward voltage drop" of the diode. Below that, some current flows, but not a lot, above that, it is essentially unlimited (the conductance of the device is the gradient of a straight line from the origin to the  current operating point)

Steve