N00b Drive Current Question

jonbar1 · Apr 20, 2012

Been trying to fully understand this and been reading conflicting statements and wondered if someone could help me understand.

For drivers, my understanding is the forward voltage determines how many 3w LEDs you can run depending on the forward voltage of the LEDs and the driver. What I'm wondering is, are all the LEDs getting the same current? That's probably why they're called Constant Current, but just in case....

Say I'm running 5 XP-G Cool Whites on a Mean Well 35-700mA driver. Is each LED getting 700mA? If I ran 8 are they still getting 700mA or does this go down with each LED?

Basically, I want to build a low light fixture for my 10g low light planted tank. I am thinking of doing 4 XP-G Cool Whites on a 350mA driver since 4 on a 700mA driver is a bit too much light for low light plants. If all 4 LEDs are getting 350mA it should be perfect lumen/PAR wise, but if that gets divided by the number of LEDs and each is getting under 100mA it won't work.

Thank you in advance!

Reef Breeders · Apr 20, 2012

Current stays the same, but voltage diminishes. I run 12 LEDs at 700 ma, really bright. It does not split like that at all, but drops the brightness of the whole string by very little with each added LED.

CJO · Apr 20, 2012

jonbar1 said:
Been trying to fully understand this and been reading conflicting statements and wondered if someone could help me understand.

For drivers, my understanding is the forward voltage determines how many 3w LEDs you can run depending on the forward voltage of the LEDs and the driver. What I'm wondering is, are all the LEDs getting the same current? That's probably why they're called Constant Current, but just in case....

That is correct, they will all get the same current and the forward voltage determines how many LEDs you can drive at a time.

jonbar1 said:
Say I'm running 5 XP-G Cool Whites on a Mean Well 35-700mA driver. Is each LED getting 700mA? If I ran 8 are they still getting 700mA or does this go down with each LED?

They will all get 700mA.

jonbar1 said:
Basically, I want to build a low light fixture for my 10g low light planted tank. I am thinking of doing 4 XP-G Cool Whites on a 350mA driver since 4 on a 700mA driver is a bit too much light for low light plants. If all 4 LEDs are getting 350mA it should be perfect lumen/PAR wise, but if that gets divided by the number of LEDs and each is getting under 100mA it won't work.

Thank you in advance!

Nope, you're good. I think that most of the Meanwells also have an adjustment screw that allows you to fine-tune the amperage a bit.

CJ

CJO · Apr 20, 2012

anemonekeeper said:
Current stays the same, but voltage diminishes. I run 12 LEDs at 700 ma, really bright. It does not split like that at all, but drops the brightness of the whole string by very little with each added LED.

Then something is wrong. As you said, the current stays the same. The current is what determines the bulb's output (brightness) and should not change at all from one end of the string to the other (excepting minor variations in the LEDs themselves).

CJ

jonbar1 · Apr 20, 2012

Thank you!!

Reef Breeders · Apr 20, 2012

With 15 LEDs at 700 ma, they wont be very bright at all. With 6, they are crazy bright.

ReefLEDLights · Apr 20, 2012

Drivers like the Meanwell LPC35-700 have a voltage range and that adjusts to the voltage drop depending on how many LEDs are in series.

http://reefledlights.com/wp-content/uploads/2011/04/lpc-35-spec.pdf

http://www.cree.com/~/media/Files/Cree/LED%20Components%20and%20Modules/XLamp/Data%20and%20Binning/XLampXPG.pdf

In this case the LPC35-700 has a Voltage Range of 9-48vdc. The XP-G when driven at 700mA has a forward voltage of 3.05

If you want to know the Max number of XP-G LEDs the LPC35-700 can drive divide the Max forward voltage of 48 by the LED forward voltage of 3.05. 48/3.05=15.7 of 15 LEDs Max
For the Min divide the min forward voltage of 9 by the LED forward voltage of 3.05. 9/3.05=2.9 or 3 LEDs Min

This is based on the book numbers as forward voltage drop in LEDs can increase with current and heat. The Max forward voltage of the XP-G is 3.75. 48/3.75=12.8 or 12 LED recommended Max.

As long as the forward voltage of the 350mA is between 9 and at least 16 you will be fine with only 4 XP-G LEDs

Bill

CJO · Apr 20, 2012

anemonekeeper said:
With 15 LEDs at 700 ma, they wont be very bright at all. With 6, they are crazy bright.

This isn't right if you are using a constant-current driver like he is proposing.

CJ

Reef Breeders · Apr 20, 2012

Also depends on the driver, as bill stated.

cparka23 · Apr 20, 2012

jonbar1 said:
Been trying to fully understand this and been reading conflicting statements and wondered if someone could help me understand.
[...] What I'm wondering is, are all the LEDs getting the same current? That's probably why they're called Constant Current, but just in case.... [...]

Say I'm running 5 XP-G Cool Whites on a Mean Well 35-700mA driver. Is each LED getting 700mA? If I ran 8 are they still getting 700mA or does this go down with each LED?

It just so happens that a common analogy for electrical current is something that many reefers understand... that is, plumbing.

Instead of 700mA of current, suppose I told you that I have a pump pushing 700 gallons/hour from my sump to my tank. If I were to ask you how many gallons/hr are flowing into the sump, the answer is obvious... 700 gallons/hr (unless you have a leak somewhere). Flow going in equals flow going out.

For your LEDs wired in series, the same principle holds. If the first LED is getting 700mA of current, the ones in series are all getting 700mA. Whether you add 1 LED, 5 LEDs, or a resistor, the current will always be the same as long as they are in series.

FWIW, that leak I mentioned earlier also has a name in circuits -- a short. It's a pretty good analogy, IMO.