# Newbie Looking for Confirmation on My Logic

#### firebirdta84

Joined Feb 24, 2013
17
Hey guys,

I am looking to wire up in parallel a group of 5 LED arrays, HLMP-2685. It's an LED block array that looks to have 8 internal LEDs. According to the datasheet, the specs on it are as follows:

Peak forward current per LED chip - 90mA
Average forward current per LED chip - 25mA
DC forward current per LED chip - 30mA
Reverse voltage per LED chip - 6V

When it says "per LED chip", is it referring to physically one LED array, or does that mean per actual LED in the array (since there are 8)?

If I need this circuit to run off 12 volts, would this be correct:

V = I * R
12 = .03 * R
R = 400 ohm

So, running them in parallel, I would apply at minimum, a 400 ohm resistor to each of the LEDs? Also, what would I use for resistors - a 1/2W, 1/4W, etc?

#### GopherT

Joined Nov 23, 2012
8,009
First, don't shoot for the peak current, you want something less, (20 mA) to be in the safe range because resistors heat up, components have tolerances and the brightness will not be much different and your battery will last longer.

Next, diodes and LEDs voltage drop are more or less consistent regardless of current so subtract that voltage drop from your supply voltage before making your calculation for current limiting resistor.

For one LED with a 2volt drop, the equation would be..

12v - 2v = 0.02A * resistance

Or

10v / 0.02A = 500 ohms

When you run LEDs in parallel, alsways give each paralleled path a dedicated resistor. And, yes, "each chip" or "per chip" means each LED in your array package. Not all LEDs are at the exact same forward voltage drop (even from the same production lot of the same LED model) so the LED with the lowest voltage drop would have the greatest current flow (possibly all the current flow) if you use a single resistor.

Finally, your resistor "absorbs" 10v of the 12v supply and the LED gets 2volts. Your current flow is 0.02 amps so power is 10 x 0.02 = 0.200 watts. That is fairly close to 1/4 watt so use a half watt resistor. Alternatively, use a bit more resistance to bring current down.

Now, I showed you how to make the calculations. Check your forward voltages and other values. Also make sure the tolerance ranges of your components do not allow one value to creep into an undesirable range (example, check the possible ranges of forward voltage drops to be expected (max/min) and not just the typical value. An old favorite poster of mine used to say on this forum, "you can't buy a 'typical' component, you get what they sell you" (-Audioguru). Hopefully he is still saying it on other forums.

Cheers.

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#### firebirdta84

Joined Feb 24, 2013
17
Thanks for the information. I've read similar things on other posts, but how you stated it made perfect sense.

One clarification I do need, regarding the "per LED chip" comment you made...

The HLMP-2685 that I used as an example in my original post, appears to have 8 internal LEDs. If I read your statement correctly, each one of those 8 LEDs has a 6V reverse voltage? So the voltage drop of the entire LED array would be 6x8 = 48v? That seems awfully high. Or did I read your statement wrong, and the combined 8 LEDs built into the HLMP-2685 array are a 6 volt drop?

The rest of it - calculating resistance, wattage, etc...is clearer to me now, thanks to you.

Joe

#### GopherT

Joined Nov 23, 2012
8,009
No, The "reverse voltage" is now much voltage you can apply "backwards" to you LED (possitive supply connected to cathode instead of anode). Essentially, LEDs are really bad diode rectifiers and they will break if you apply more than 6 volts in reverse).

You need to look for "Forward voltage" (2.2 typical on your part number).

Your part is not really an "array" - no individual LEDs are pre-wired together. Anode and cathode of each LED is accessible and you can wire it in any sort of series, parallel or series-parallel method you want.

Since you plan to use 5 packages, you have 40 LEDs to connect (80 leads overall).

Datasheet for multiple products are here - yours is the 16 pin variety on the datasheet. Note that part drawings are from a top view (where you can see the LEDs, not the pins).

http://www.datasheetcatalog.org/datasheet/fairchild/HLMP-2685.pdf