led operating voltage

Question

I am a bit confused about the operating voltage of LED's.

I have an RGB led controller that uses 12VDC.

I looked up a RGB led on mouser such as this one:

http://www.mouser.com/ds/2/216/WP154A4SUREQBFZGC-194997.pdf

The datasheet doesn't really say anything about the max voltage I can use on the LED. The datasheet does say the total current should be around 20mA.

Using Ohm's law if the voltage is 12V and the current is .02A then the resistor should be 600 ohms. 12/.02 = 600

Is this correct?

What I really don't get is what if I was using 36VDC or 1000VDC? Even if I sized up the resistor appropriately wouldn't the LED be damaged at some voltage? How do I tell where that voltage is in a datasheet that does not specify?

Answer 1

The most important thing to understand is that LEDs (if considered simplistically) drop a constant voltage of somewhere between 0.7 - 5 volts. In the datasheet this is shown as the forward voltage as 1.9 for the red and 3.3 for the green and blue leds.

They also don't offer any resistance. So it does not matter what voltage you apply to the LED so long as you limit the current. So if you were to apply 1000V to an LED it would drop say 5 volts across the LED and 995 volts throughout the rest of the circuit (so long as you limit the current and don't blow up the LED).

Your calculation is not quite correct because you did not take the forward voltage of the LEDs into account. So for example, if you want 20 mA through the red LED you need to go:

$$R = \frac{12 - 1.9}{0.02} = 505\Omega$$

But often LEDs are driven by current sources rather than limited by a resistor which is probably what your LED controller is doing but you haven't provided a link for that.

Answer 2

Not correct. Most LEDs run from 2.3Vdc up to 4Vdc, it depends on the electronic construction of the substrate. That is true for One only Light Emitting Diode. Most manufacturers will sell LED modules. These are made of several LED connected in series and or parallel and may include internal balancing resistors and some other diodes for rectification and even more complicated regulation scheme involving multiple transistors. that is why some of those modules are working of completely different voltages like 12V or 24V or even 36V and even 120VAC. See Wikipedia "LED".

As for your second question: 36VDC or 1000VDC? When connecting LED modules always follow the manufacturer specification for if you inject a 36Volts into a 24Volts module, the module may draw much more current then intended and will overheat and sooner then you wish it will permanently stop functioning.

On a more electronic point of view, if you work with a single electronic device LED and need to run it on a 12Volts DC source you would have to calculate the value of a current limiting resistor as follow: typical LED Voltage = 3Volts, Source = 12 Volts, 12-3=9 volts drop needed. Typical LED current consumption 0.02Amp, 9V/0.02A = 450 ohms. Then you would need to construct a series circuit with one LED and one 450 ohms resistor. This would consume a total of 0.02A into a sum resistance of 600 ohms. The LED would light up and will likely last for some 100000 hours if treated properly.

Cheers:)

Answer 3

use 5V supply instead of 12 unless stringing many (3 or 4) in series

Use Ohms Law' on the R drop difference voltage.

Learn to read the specs and recognize the ESR. Vth (threshold asymptote and Vf forward voltage at rated current. (e.g. 20mA for 5mm) ( not the Absolute Maximum) exceeding this may fuse gold wire up burn chip.