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Unread 02/21/2011, 05:33 AM   #65
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Join Date: Dec 2007
Location: Central California Coast.
Posts: 5,383
The moment of truth!

Flame - on!

Immediately - if not sooner - measure the current thru the strings. You need to know if you are over driving a string so you can power down if you have to.

So, quickly, quickly, I measure one string.
I have 0.1 ohm resistors so I have to do some math. Notice the meter is set to show a maximum of 300mV. This means I'm seeing 43.8mV. Multiplying this by 10 because of my resistor values, tells me I have 438mA running thru the left string. This is great, not too much.

Over to the right string...

Wow. Happy days! They are very close together in current. Close enough that I don't have to do anything about it.. At this point I turn the voltage limit back up all the way. (more on that later)

This means I can just turn up the current to where I want to run at.
And so I do. I crank it up to 754mA on this side.

And that gives 740mA on the other side. No problems here.

Everything looks good. So now I turn down the voltage limit again while monitoring the current in one of the strings. I turn down the limit until I see the current start to drop. I then turn the limit back up until the drop disappears again. Then I turn it up a tiny bit more to cover temperature and age drifts. However in this particular build I could turn the voltage limit all the way down and never had the current drop! This means these string's Vf totals are so low that the driver voltage needed to reach 750mA is even below the voltage limit range. Perhaps I could've run another LED in each string..

The astute reader will note that this 1.3A driver appears to be delivering 748mA + 754mA which equals 1.5A.
How can this be?!
There are two reasons. One is that our drivers are really power drivers at their cores. Since this is only 12 LEDs they are not demanding anywhere near 48V to run at this current level. This means the driver can provide a little more current and still be below the 60W rating.
Another reason is probably the ripple. My meter reads the RMS current. That means it's looking at the peaks and averaging them. There are peaks with this driver.

What are they? Let's look. Setting my meter to AC Volts will show the AC current running in my strings.

Here we see 12mV (note this meter doesn't have a 300mV AC setting).

Back to my multiplying by 10. This means each string has about 120mA of ripple current running thru them. So my average current is about 750mA and this means I really have a cycling value of about 690mA to 810mA.

So, several hundred or thousand times a second my strings have the current varying thru them between those two values. This is well below the current limit for these LEDs and so does not concern me in the least. Ultimately this will make them run probably 1 or 2 degrees warmer than a totally smooth 750mA pure DC current would. If this build was running right at the maximum current limit the heating would be greater and might need further study. In this 750mA case - not at all.

At this point

On to checking out the light! Here's the color against a white ceiling. Note the slight blue.

Here's a mug shot of it.

Here's a shot with the camera stopped way down running a 1/4000 sec shutter. This really shows the LED colors. Neutral whites, cool whites, and royal blues.

Here it is tipped up lighting my towel.

Here it is for chuckles propped up on two books lighting my big build.

It fell off the books into the bottom of the tank and never blinked. Try that will a running MH and see what happens... LOL

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