I bought these LEDs:
http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=150259586081

They came with a ton of resistors but when I plug the color combination into:
http://www.samengstrom.com/nxl/10116/5_band_resistor_color_code_page.en.html

I come up with resistors in the range of 400-600 ohms.  Does this sound like it could be correct?  I guess from everything I have been reading on here and the much smaller ohm resistors that everyone seems to be using, I would have expected smaller resistors to have come with these LEDs. 

So this is more of a sanity check than anything.

Oh.  And I haven’t touched circuits since well before I graduated college 10 years ago and even then I only touched them in two basic EE classes required for my CS degree.  So my understanding is somewhat limited with all of this wiring and resistance.  (Yeah, I read the wiki and will be sure to use the various led wiring calculators that have been provided.  Thanks!)

Thanks for any help you can provide!

Jeff

The reason we are using much smaller values is that we wire our LEDs in clusters of series.  This allows us to have, say, 1 resistor for every 7 LEDs(as I have).  They may have intended that you put one resistor on each LED, but I still don’t know why they resistors would be 400-600 ohms.  I would expect a much lower value, nearing 180.  Do you know the specs of the LEDs?  I would suggest you try and find the specs of the LED and get some new resistors.  You can buy them cheap from places like Digi-Key or Jameco.  I don’t know where you can get them outside the US, sorry.

Good Luck,

Alex

Specs are:

Emitted Color Infrared
Lens Color Water Clear
Bulb Size (mm) 5
Peak Wave Length(nm) 850
Forward Voltage(V) 1.5~1.6
View Angle(°) 15~30
Luminous Intensity (scd) N/A
Continuous Current (mA) 60mA
Peak Current (mA) 120mA
Static Sense No

As defined on the ebay item. 

I emailed the seller but I don’t think he understood my question and doesn’t seem to understand english very well.  I’m hope that the ones he sent me are just color coded differently that what is show on the on-line color guide?  Is that a possibility?

Maybe when I get home tonight I can get my camera to capture a picture of the resistor and post it.  (May be difficult as it is so small that I can barely make out the colors.)

Well, if the specs are correct you know enough to go on one of the LED calculators and figure out what resistors you need.

Happy searching!

Alex

Is Continuous Current the same as forward current in the LED specs?  (The LED wiring calculators all request forward current and all I have is Continuous and Peak.)

Also, entered the peak current of 120mA in the calculator instead of the 60mA and bought resistors as specified.  In this case it was 22 ohm resistors.  When I redo the calculator with 60mA it says I should use 47 ohm resistors…

So, what would happen if I used the 22 ohm resistors and the LEDs forward current ends up being 60mA?  Will they burn out early?  Not be as bright as they should?  My understanding of basic circuits is not all that good since I haven’t touched them since taking some intro classes in college.

Thanks!

I think I have most of my supplies now and am ready to start building… I just want to verify this before starting so that I don’t ruin any of the stuff that I have!

To answer your “what if...” ~ To use the 22 ohm resistors you would need a 12 volt power supply and you would solder up your LEDs with 7 in each array with a single 22 ohm resistor to ground.
The resistor will dissipate about 98 mW and the 7 diodes dissipate about 630 mW.

Usually, with LED specs the peak voltage and the max mA are for applications where you are pulsing the voltage. Otherwise, the LED would fry.

Use the lower specs of 1.5 V forward voltage 60mA and you’ll do fine.
Use this array wizard for some help with your final setup:
http://led.linear1.org/led.wiz

Try to design your array so that you end up with the lowest resistor value you can get. That way, most of the energy is going into producing IR light instead of heating up a resistor.
~
~
~
If you used a 12 volt power supply with your LEDs and bought some 1/4 watt 1 ohm resistors
you could wire them in arrays of 8 LEDs each:

Each 1 ohm resistor dissipates 3.6 mW
The diodes dissipate 720 mW
Total power dissipated by the array is 723.6 mW
The array draws current of 60 mA from the source.

Note the difference in the dissipated mW in the 22 ohm setup compared to the 1 ohm setup.
98 mW VS 3.6 mW

Thanks for the quick reply.  I didn’t understand all of it, but that’s okay!

I have a 12V power supply and 22 ohm resistors, so I guess I’ll go ahead and wire up 7 like you mentioned.  I had originally planned on wiring up 6 based on the calculations I made with the higher mA.  (From the same website you referenced.) I’ll take your word that adding a 7th will make up for the change in mA calculations since I can’t get the wizard to give me those results.

Hmm..  The resistors only cost me $1 / pack of five, so I guess it isn’t really an issue to swing by and pick some more up.  I’ll just have to see if I get back around Radio Shack again before I start building.  (Got the LED holders in the mail today and started drilling the C Channel, so I’m really itching to wire some stuff up!)

So the 98mW vs the 3.6mW dissipated by the resistors means that I’m wasting more power and generating more heat.  Correct?  Is that a large difference that I should really worry about?  Enough heat to cause an issue? Or are you just pointing out the optimal way to do it vs a less optimal but perfectly good solution?

Thanks again for the help.

Yeah your 22 ohm resistors will work. However…

The wizard will tell you that you need a 27 ohm resistor.
You could use the 22 ohm ones since you have them, but they’ll run hot!

If you’re going to buy some resistors any way, you should get about 10, 1 ohm 1/4 watt ones and just wire them in arrays of 8 each.
Much better setup.
Resistors are pretty cheap.

Hopefully just one last question on the subject of resistors.

I bought the 1 ohm resistors as suggested but end up with a bunch of runs of 8 and a single run of 7 on each side.  According to the calculator I should use a 27 ohm resistor for this set, but I couldn’t find a 27 ohm resistor.  From my reading, I can wire multiple resistors in series add add the values in order to reach 27.  Unfortunately, with the combination of resistors that I have I can’t get right at 27.  Should I just use a single resistor or combine multiple in an attempt to get as close to 27 as possible?

I guess alternately I could throw an 8th LED on it and just not have it pointing into the acrylic.  (Not enough room on the C Channel for another one.)

I had a hard time finding the 1 ohm resistor and ended up buying a 500 piece set with a ton of different kinds in it, just to get 10 x 1ohm resistors.  Too bad only a small fraction of the resistors have small resistance, most are greater than 1k.

Anyway, I’m going to test the LEDs out now without pluggin in the sets of 7 to see what kind FTIR performance I get.

Stuff that 8th one in there and you got it!
I did the same thing with my table…