Cheap source for 0603 PRE WIRED Warm White LED's

[DKS]

I checked out the Martha Stewart lights at Home Depot. They do appear similar to the ones John found at Walmart but there were no warm white ones available. I looked over all the ones on the shelf and they were all either red or multi-color.  I am looking for the warm white ones.

They're just out, then, because the Martha Stewart lights come in warm white as well. I picked up a few sets, and I may get some more--there are at least a half-dozen Home Depot stores in reasonable driving distance of my home. Maybe I can pick some up for RW members who might want them but can't find any.

[peteski]

peteski, email and PM sent...  Fear not!  Ian, my offer still stands to you too! 

Thanks John!

[Ian MacMillan]

...there are at least a half-dozen Home Depot stores in reasonable driving distance of my home...

Ah..what its like to live in civilization!

peteski, email and PM sent...  Fear not!  Ian, my offer still stands to you too! 

Thank you! I'm gonna try HD first...just need to fuel up the Boeing and find some time to get there.

[DKS]

Ah..what its like to live in civilization!

Honestly, I'll trade ya.

[Ian MacMillan]

Honestly, I'll trade ya.

I like it... Its the 1+hr trip to the grocery store that kills ya. Most everything else I order online.

[DKS]

I just cleaned out a couple of the nearby HDs, so I have plenty of sets on hand in case anyone wants some.

[Lemosteam]

Thanks for supporting the group David!

peteski, you are quite welcome!

Ian, David may be of more assistance than I- the Walmarts around me are ravaged in the Christmas lighting departments! 

[Lemosteam]

Ok so after reading all of this and taking everybody's suggestions into account, here's my LED tester, a quick variaton of Peteski's design with multiple labeled resistor options:



It lets me view all of the light levels resulting from the various resistors so I can choose the right one from the package without having to know the ring color coding.

No spot for SMD style LEDs... yet...

[mmagliaro]

That's very cute, John!   (Really!)
But you might want to put some more values in there, especially between 330 and 1k and between 1k and 2k. 

I know I often end up with 1.5k or 1.8k, or 560 or 820, depending on the LED.
My measuring stick is to try to get near the 75% of the LED's max current rating at 12 volts (especially for things
like a loco headlight).  That lets it glow as bright as possible over the whole voltage range, without blowing up.
So if it's rated at 10 ma, I try to keep it under 7.5ma. 

12/1.6k = 7.5ma (1.6k not as easy to find, however, especially in little 1/8th watt, but they are around)
12/1.8k = 6.7ma

If it were 20 ma, I'd shoot for 15, so
12/820 = 14.6ma  (820 ohm)

If I have no idea (which I often don't if I buy a bulk pack of LEDs from somewhere), then I "guess" at 10 ma
being the limit, and I run it like that for a few hours to see if it's going to cook.   

I surfed the web and looked at spec sheets on several white 0603's.  They all have maximum forward current ratings
of 20 mA or more.  That surprised me.  I thought they'd be less. 

Anyway, it's just a matter of current limits and how much brightness you want.   You can torture-test your unmarked
ones at different resistances and see what will work best for you.   

[Lemosteam]

Thanks Max, I will add more when I can find more, that was all of the 1/8 watt resistor values that Radio shack had in the pull out tray!

That is a really great explanation, I think I am finally getting the hang of the correlation between brightness, volts in and resistance.  I know these are very simple formulas, but my brain sometimes can'c handel the simpler things if you know what I mean!

I can always remove it from the battery and clip on my alligator leads from my power pack and test them above 9v too, that's why I chose Peteski's dseign to start from.

[DKS]

I can always remove it from the battery and clip on my alligator leads from my power pack and test them above 9v too, that's why I chose Peteski's dseign to start from.

Use of a "power pack" is not recommended for LEDs. Their voltage is unreliable and they often produce spikes. Stick with batteries or regulated power supplies.

[peteski]

Ok so after reading all of this and taking everybody's suggestions into account, here's my LED tester, a quick variaton of Peteski's design with multiple labeled resistor options:

It lets me view all of the light levels resulting from the various resistors so I can choose the right one from the package without having to know the ring color coding.

No spot for SMD style LEDs... yet...

Good job John!
But you might want to remove the 100 and 220 ohm resistors (and as Max said, replace them with resistors of higher values). Why you ask?  Because a 9V battery with a 100 ohm resistor will allow around 60-70mA of current through a single LED (depending on its color).  That is way over the maximum rating for the standard LEDs we use in model railroading.  The 220 ohm resistor will allow 27-33mA of current.  Again, that is in the upper range of the maximum operating current for small LEDs (0603 SMD LEDS are usually rated between 5-10mA).  Besides, I doubt that you would even need such a high brightness form LEDs you install in your models.

I also agree that using a power pack to test LEDs is discouraged. I mentioned that way earlier in this thread (after you fried some of your LEDs).  LEDs are quite sensitive to over current (they let their magic smoke out and they no longer emit light).    Power packs usually supply unregulated and unfiltered pulsating DC voltage.  A battery provides a nice steady DC at a known voltage.

[DKS]

470 ohms is the lowest resistor value I've ever used in an LED application, so when you find a good supply of resistors, concentrate on the values between 470 and ~2k.

[Lemosteam]

Wow, ok gents!  I know the power pack is variable, but isn't that the point of the resistor?  Or am I still confusing things.  Or are you worried about frying the resistor?  I may not have been clear but my point was to hook up the powerpack leads where the 9v is connected and still run through the resistors.

By the way, one of the LEDs from the set burned very brightly on the 100 ohm resistor, but you are saying that it would have eventually fried?

Not complaining here and I appreciate your patience while my brain catches up.

[DKS]

Wow, ok gents!  I know the power pack is variable, but isn't that the point of the resistor?  Or am I still confusing things.  Or are you worried about frying the resistor?  I may not have been clear but my point was to hook up the powerpack leads where the 9v is connected and still run through the resistors.

By the way, one of the LEDs from the set burned very brightly on the 100 ohm resistor, but you are saying that it would have eventually fried?

Typical power pack output is not regulated; if you looked at the output on an oscilloscope, you'd see all sorts of waves and spikes, well above the nominal output voltage. Under no load, the output can be upwards of 18-24 volts or more at peak, even with the throttle barely cracked. With no appreciable load (such as a motor), adjusting the throttle actually has almost no effect on the output voltage. Also, at lower throttle settings, there is often a "spike" of much higher voltage introduced to help motors run more smoothly at slow speeds: the spikes help overcome armature cogging. All of this can easily fry any LED, whereas the resistor would barely feel a thing since the load is relatively tiny.

It's almost certain the LED you tested using a 100 ohm resistor would eventually fry, since you were delivering anywhere from 3-6 times the LED's rated current. The point of using resistors is that each one will deliver a certain known level of current to the LED, assuming the supply actually delivers the rated voltage. That's why batteries are preferred. The only other reliable alternative is a regulated power supply. Some "wall wart" supplies are regulated (the "switching" type), but most are not.