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Resistors - the most common versions of these are 5% or 10% 1/4 watt carbon resistors (any style is fine, carbon, carbon film ,etc) these resistors are about the size of a big grain of rice, if you buy "1" from Radio Shack they will cost you about 25 cents each, if you order them in multiples of 100 they will cost you about 1 to 3 cent each.

Any retailer should have a selection of these resistors. This links to a resistor selection from Jameco.com, 1 cent each with $5.95 U.S. shipping, allelectronics.com lists them for 2 to 3 cents each depending on quantity, plus $7.00 U.S. shipping. If you do a web search for "electronic parts" and you will find hundreds of web retailers, any of these should have these resistors. There should be many in Canada that might have cheaper shipping?

Calculating the resistor value can be a pain for our use since we will likely be using all sorts of unknown power supplies. The resistor value can be calculated but it might be easier to use trial-n-error, figure 3.5 volts drop across each led and series as many together until the total voltage adds up to a voltage value that is 1 led voltage less than the voltage you think your power supply will put out, put a 100 ohm resistor in series with these leds and power it up (the 100 ohm value allows you to be WAY off and not damage the leds), measure the voltage drop across the resistor. Use this voltage number, and the value of the resistor, to figure the current flowing through the series string, you can then use this number to guestimate the actual resistor value to use.

Current calculation- Divide the voltage measured across the resistor by the value of the resistor. EXAMPLE - 1 (volt) divided by 100 (ohms) = .01 amps (10 ma)

If you measure 4 volts across the 100 ohm resistor the current calculates to (divide the voltage number by the resistor value in ohms) 4 (volts) divided by 100(ohms) = .04 (40MA), this is well over the current spec and is also more than the 3.5 volts needeed for one led, since the voltage is higher than the 3.5 volts for an individual led you can add one more led in the sting and again measure the voltage dropped across the resistor, it should measure around .5 volts, divide the .5 (volts) by 100 (ohms), this now calculates to .005 (5ma), the leds will be very dim with this amount of current. Divide this current number INTO the desired 20MA current number (20 divided by 5 = 4) and then divide the 100 ohm resistor value by this number (100 divided by 4 = 25), in this case we divide the 100 ohms by 4 to get a new value of 25 ohms, this new resistor value should allow the leds to be as bright as practical and also be within there safe current spec, they should survive for there rated 100,000 hour lifetime.

For these T 1 (3mm) or T 1 3/4 (5mm) (or even 8 or 10mm) individual leds it is pretty safe to assume 3.5 volts as the voltage needed across each led, these are also almost always speced for a continuous current of .02 amps (20ma), with a 30ma max current (they will look a bit brighter at 30 ma than at 20 ma but they will be stressed more so have a somewhat shorter life).

If powering the series string with a true 12 volts dc supply you would use 3 leds and one resistor in series, 3 leds X 3.5 volts each = 10.5 volts, this leaves 1.5 volts to be dropped across the resistor, the desired resistor value is calculated by dividing the voltage that will be dropped across it by the current you want to pass through it, SO - 1.5 (volts) divided by .02 (amps) = 75 ohms.

Unless your power supply is a supply that is a true voltage regulated power supply you should measure the voltage to know what it actually is, this can be a bit tricky because if the voltage is not actually regulated it will change a bit whan you connect it to the leds, adding more led strings will likely lower the voltage even more. If you are powering the leds from a battery the voltage won't change much but if you are using one of the small wall plug-in transformer type power supplies the amount of voltage drop will depend on the current rating of the power supply and the number of leds connected to it, a 12 volt 1 amp supply will power around 50 20ma strings, a 12 volt 200 ma supply will power around 10 20ma strings etc. The actual voltage from the power supply will depend on just what type of power supply you are using so measuring the voltage will give you a place to start.

This sounds more complicated than it is, the resistor value does not have to be exact, just close enough to keep the current within reason. Passing a higher than spec current through the led string will increase the brightness but it is also overheating the internal led junction and this overheating will reduce the life of the led by some amount, it may still last for 20,000 hours though?

Buying a few resistor values, 10 ohms, 22 ohms, 47 ohms, 100 ohms, would be reasonable, you can add these values in series to increase the resistance value (just add the values together) or in paralell to reduce there value, the paralell calculation is not quite so easy, use "product over sum" for this, MULTIPLY the values together and write this number down, now ADD the same values together and divide this new number into the first multiplied number, this will give you the new resistance value for the paralelled combination.

Wiring up the leds -- Twisting and/or soldering the leads directly together works but I usually use tiny solid wire. 20ma is not much current so you only need 30 guage wire, there is a nice tiny solid 30 guage wire available that is known as "wire wrap" wire, it is a bit costly because the wire itself is plated with a bit of silver but it works well as interconnect wire. The plastic insulation is available in many colors if you have a preferance. This wire is available from most electronics parts retailers and also at Radio Shack for about the same price. A hobby knife easily removes the insulation, I make the stripped ends a bit long and use small pin nose pliers to grab the end of the wire while wrapping it a couple of turns around the led pin, solder the wrapped wire to the pin and then cut off the excess. Using 24-26 guage stranded wire to connect the led strands to the power supply is more than heavy enough and not as likely to break with movement as solid wire.
 
Location: fisher,illinois,usa | Registered: June 03, 2003Reply With QuoteReport This Post
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My method posted above is very rudimentary, albeit effective. The info on the pages linked above, however, have helped me to be able to plan out a better project once my LEDs from eBay arrive.



Cool Coyo.

Does that mean we can expect to see pics of your new lighting project sometime after your E-Bay LEDs arrive?
 
Registered: September 26, 2005Reply With QuoteReport This Post
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Hopefully so. I just need to find the battery charger for my camera.

Dan


99.5 Jetta TDI, Elsbett single-tank (incl. modified injectors & glow plugs, FPHE, heated filters), running on WVO/Diesel blend
 
Location: Oregon | Registered: March 19, 2008Reply With QuoteReport This Post
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Hi,
So I am wondering how everyone's diode experience has been so far.
We bought some 12volt under shelf lamps from Menards 2 years ago and some two weeks ago. The two year old lamps have been on continously for the whole time. Compared to the exact same "new" lamp, our two year old ones are fading. Still the lamps cost $8 per and use 1watt. Like Tim, the 5 year guarantee on the CFLs is hooey. Even when you read the fine print, they don't last as long as they are supposed to.
One interesting thing about the CFLs is that I put them in places where we need intermittant duty lighting (closet or basement, or over the pipe fittings) because the "dead" ones sometimes work until they over heat then fade out.
So apparently we are over driving our LED lamps.
DF


"I don't work with collectives. I don't consult, I don't co-operate, I don't collaborate."
Howard Roark
 
Location: Calumet, Michigan, Great White North | Registered: January 20, 2002Reply With QuoteReport This Post
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DF - Fading - I dought your original lamps have faded over time (unless the internal power supply is malfunctioning), it is more likely that they were manufactured with lower output led's. Even the best led's from a couple years ago would have put out less light than the poorest led's manufactured today.
 
Location: fisher,illinois,usa | Registered: June 03, 2003Reply With QuoteReport This Post
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I still have not taken any pictures because I still have not found my charger. I could buy another one, but I'm not that motivated yet. I'll just keep looking through the mess that we call home until I find it.

My kids (ages 6, 8, and 10) had an "LED" party where we twisted a bunch of sets together. We twisted the posts together to form series of three lights each, which is the most we can put together for 12V or less, then I tested each set for coninuity. Last night my oldest son and I soldered them together so that they'll be stronger than when they are just twisted. We have a total of 30 sets of three diodes each, or 90 diodes total. Now that we have them soldered the next step will be to add the resistors and then arrange the sets into the base I want to use for a lamp.

The lamp itself is a sturdy plastic apple juice bottle. I cut the bottom off, trimmed the edges of it, and then pushed it up into the body of the bottle. It's not glued yet, so I can still take it apart to put all the pieces in. I'll also paint the inside shiny silver or white to help reflect the light outward, or maybe shiny copper or gold to add some glow. The base that is being attached up inside the body provides a flat plate that is now located close to the top of the bottle; it will be drilled to allow the LED's to be pushed through from the back side, with the wires running out through the cap. The cap will be attached to a flex tube so that I can move it wherever I want it and to conceal the wire inside.

I also need to get a 12V wall adapter, since all my outlets are currently wired for 110.

I'm taking my time, so it may even be months before I finish. I'm mostly spending my free time building a chicken coop, messing with the WVO, taking care of the yards and the garden, and in general getting ready for spring. I'll let you all know when I finish the LED project, and hopefully I'll get pictures.

Dan


99.5 Jetta TDI, Elsbett single-tank (incl. modified injectors & glow plugs, FPHE, heated filters), running on WVO/Diesel blend
 
Location: Oregon | Registered: March 19, 2008Reply With QuoteReport This Post



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