The Planted Tank Forum - View Single Post - Testing out some 10W Chinese LEDS
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02-13-2012, 05:37 AM   #41
sendthis
Algae Grower

Join Date: Dec 2010
Location: Dayton, OH
Posts: 34

Quote:
 Originally Posted by Naekuh sorry but this doesn't apply with DC and constant voltage. DC circuits only draw the amps it requires if you match voltage. Saying u cant tie up 1 parallel 3V LED with a 12V led, unless u serial 4x3V = 12V. You need to keep the voltage on each parallel array the same... If you have 2 different arrays strung up in parallel.. one array draws 3A... and the other draws 2A... the psu will output a total of 5A, where 3A will go to the 3A portion and 2A will goto the 2A portion. Again this is looking at constant Voltage. constant current doesnt work like that, because of the nature of LED's themselves.... LED's can variate Voltage yet the driver forces constant current though them. Hence why Constant current does run a tad bit cooler then constant voltage.
I'm about to go to bed... but I wanted to answer this real quick and I'll come back to it just in case I'm misunderstanding what you're saying...

The reason you want to limit the current even with a regulated DC voltage source is because as your device heats up, it causes a shift in the nominal foward voltage. So while you maintain the voltage, there is a disproportionate shift in the current. So nominally, at 25C, a Vf=3.5V may have 350mA, as it heats up it will begin to draw more current without changing Vf. As it pulls more current, it'll generate more heat, until finally it reaches breakdown (device failure).

The key here is that diodes are current driven devices. If you take a look at the ideal diode equation, you will notice there is a thermal voltage term. This parameter is in the exponential --- small variation can lead to large current change.

So, why would one be overdriven and one underdriven?

In the above situation, let's say you have a 10W 10V regulated constant voltage source. That means it can supply 1A and let's say it is a limited power supply / driver.

Let's say your LED devices operate at 10V, but needs 500 mA.

In the above situation, if one of your devices is slightly mismatched (via manufacturing or operation), it's possible that one will be driven with a larger current, exceeding the value for your driver is designed to handle as well as your device.

But this goes back to parallel being safer b/c only one branch breaks.

On the flip side, a constant current source running through a series of LEDs is 'safer' because it will never allow more current (when well deisgned) than the diodes can handle. The problem with this is that your LED in the string may break for other reasons and you have to replace it or bypass it to get the other LEDs to work. This is probably less likely than the above situation, but still possible.

Hopefully I answered that and it was coherent, but I'll come back to it tomorrow when I'm more awake.