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#1 green-boat Nov 29, 2006 01:11 PM

LED bulbs
 
1 Attachment(s)
It's that time of year again when you find something of interest. I was in WalMart the other day looking at the Christmas lights and found something new. They were $6.48 for a 60 light multi color set. They are the super bright type and run on 4 volts each. I had never seen them with a flat top before and thought that they might have some good uses. The end has a small dish in it where the light doesn't exit the end as much but comes more out the side.

#2 patmat2350 Nov 29, 2006 02:11 PM

SUPERB!
I've been wondering when Christmas lites would go LED, because they'd need to be 360 side-emitters. Way cool!
Did they have white ones too?

Pat M

#3 green-boat Nov 29, 2006 02:45 PM

Pat,

They didn't have the white ones in the flat top style but they did have them in the usual dome top.

#4 Umi_Ryuzuki Nov 29, 2006 09:13 PM

Domed top would be fine,... Where are they?
Walgreens, RiteAid, my local variety store?

I am going to have to go hunting for those white ones. ;)

#5 green-boat Nov 29, 2006 09:50 PM

Quote:

Originally Posted by green-boat
I was in WalMart the other day looking at the Christmas lights and found something new. .

Umi,

It's late, I understand.

#6 LtDoc Nov 30, 2006 11:19 AM

That flat top with the 'dish' in it sounds like a reflector, redirects light to the sides. Ought'a work just fine for a none spot light kind'a light. No telling how many LEDs with the dome top I've ground down for lighting! One of these days I might even get one to work like I think it ought to (don't hold your breath)...
- 'Doc

#7 patmat2350 Nov 30, 2006 11:39 AM

I looked at some yesterday, though the store was out of whites (must go searching again today...). They look like regular LEDs, with light directed primarily out the end... but the end is an inverted cone with a high polish, so you get reflected light out all around... much more efficient than my trials with flat-ground ends with silver paint!
You still see a bright point of light, rather than having the whole unit glow uniformly. The body is about the size of a side light housing in 1/32 scale, so these are no substitute for tiny GOR/GOW incandescents... but they have potential!

Pat M

#8 Shaun Hendricks Nov 30, 2006 03:16 PM

Anyone figure out the DC specs of those? I have a bunch of strings of them for our tree and wondered about using them in a DC configuration but my multimeter isn't so bright at figuring out power requirement levels... :D

#9 patmat2350 Nov 30, 2006 06:24 PM

3 Attachment(s)
Yep! Pat's LED Report:

Got a string of 60 whites. Plug'em in, they glow brightly with that bluish light we can expect from "white" LEDs (maybe some yellow paint will make them look like regular incandescents?). But they do have a good "lateral" presence.

The string came with 2 spares- nice! I can experiment and still hang the string up tonight.

You can wiggle the LED out of the green plastic base, just straighten the leads first. What you have is a standard T1-3/4 (5mm OD) LED.

Running on DC, I was able to get similar light output with 6v and a 220 ohm resistor, which controls the current to about 20 milliamps (IMPORTANT! More below on this).

Pat M

#10 patmat2350 Nov 30, 2006 06:53 PM

LEDs are NOT light bulbs!
They have unique characteristics, and need to be treated right.

1. LEDs are polarity dependent. Means they need to be hooked up the right way around to operate... but reverse connection with low-ish voltage won't toast them... within reason. They do have a max "reverse voltage", often around 5v, so don't push your luck.

2. LEDs MUST be operated in series with a current-limiting resistor. Brightness is controlled by current (NOT by voltage!), and YOU need to control the current with your circuit. Typically, 20 milliamp is good. More below:

3. LEDs have "forward voltage" ratings. Might be 2, 3, or 4 volts or so. DOES NOT MEAN you need to supply exactly that voltage. 6, 12, even 24v is fine. What it means is:
a) Less voltage: nuthin' much happens.
b) But with higher voltage, you'll drop the "forward voltage" across the LED, and it will light up.

My LEDs (above) are about 3.4v forward. I put 6.5v on them, 3.4v is dropped across the LED, leaving 3.1 more volts which must be dropped across a resistor.
Using Ohm's Law, IR = V (current in amps, I, times resistance in ohms, R, = voltage), and knowing that I want 20 milliamps (0.020 amps), solve for R:
R = V/I = 3.1v/.020 = 155 ohms. A common 220 ohm resistor will be safe, supplying a little less current.

Now, sometimes you may find applications which don't use resistors... like the LED lamps on my Bristol Bay. What they use instead is a few feet of very fine wire, which acts as a resistor itself.
But don't expect to hook up an LED with regular gage wire and no resistor and then not have problems. They don't have any significant internal resistance, so they'll almost act like a short circuit, flowing gobs of current and quickly frying. Pffftt!

Pat M

#11 patmat2350 Nov 30, 2006 07:03 PM

And how do you determine the correct polarity, without just hooking the LED up?
LEDs have "anodes", where you hook up the (+) positive voltage, and "cathodes" which take the (-) negative connection.
Look for a flat on one side, and the shorter lead, and you have the (-) negative cathode. A longer lead marks the (+) positive anode.
I found all these indicators on my Christmas LEDs.

Pat M

#12 MILLERTIME Dec 01, 2006 02:05 AM

Look here.
LED calculator
http://led.linear1.org/1led.wiz


Don M.

#13 patmat2350 Dec 01, 2006 07:08 AM

That's a nice calculator, thanks! It rounds values to the next commercially available resistor, even gives you the color codes. Thoughtful!

It's usually recommended that each LED have it's own resistor, and that an array of LEDs with their resistors be hooked up in parallel... ensures that each gets the expected current.

Pat M

#14 killick Dec 01, 2006 09:50 AM

Pat --since you're "eXperimenting" ---a few model RR guys have reported that they've eliminated the annoying "blue" tinge in some (supposedly) white LED locomotive headlamps by coloring the surface of the LED with an orange "Sharpie" marker. I'm not sure how well this would work with model boat voltages and setups though. I've also heard that a bit of brown "gel filter" gets rid of the blue as well....

Paul.

#15 papower Dec 01, 2006 10:32 AM

My local wally world has the LED's in a set that runs on 4 AA batteries. They come in clored and white. If I can remember correctly they are 35 to a set. Nice little case with on/off switch. Cost was $4.99 per set.
Bruce


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