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Discussion Starter #1
I have my table and track laid out and I want to do some track lighting so I can run some night races. It would be 10-20 street lights (maybe LED's for this???) and 1 larger bulb on each end of the track. Im also working on my lap timer stuff. Thats the background of the project so far.

I've read Greg Braun's website about the LED lap timers and I have all my lights set up (under and above track), Im just not too sure about power. He says to power these infrared LED's with a 12v source. The only thing close to a 12v power supply I could find was my ancient Aurora power pack. It says that its 14vDC max power. I noticed Greg says to use a 470 Ohm resistor on the overhead LED's but Im wondering if that is based on if the LED's are operating under 12v of power?? My power supply would be 14v max, but would the other lights on the track create enough draw to lower the voltage to around 12v average for everything on the power circuit? I really dont know much about electronics and what kind of power LED's can handle. I was affraid the 14v power supply could fry the LED's, and I dont like wasting money so the fewer of those I ruin the better. I will likely do a lot of racing w/out the track lights until they get built so I need to make sure the max voltage of the power supply wont blow out the LED's. :) Any advice on this would be great. Thanks gents!

~Dan
 

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Dan,

I could easily be wrong, but I think Greg's site shows a +12vdc and a -12vdc, which (I think) is 24vdc.

If I went LED's above, I was going to wire them to my track power, since my Tomy wall warts are 22VDC max, and I thought that would work.

Good questions, and I'm sure someone will steer us in right direction.
 

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The goal is to reduce whatever input voltage to @ 3+ VDC for LED's. Each LED will require a resistor if your input voltage is higher than 3 VDC. Radio Shack's "Electronics for Beginners" is very helpful in learning basic electronics. Then get yourself an experimenter board kit and start playing with different resistor values. It can be fun and safe as long as you're only playing with low voltage DC applications. You need a resistor charts like these to learn how to reduce voltage properly:



You need to add up the total draw of the LED's/lamps and use that factor to purchase the right power supply. Most LED's have a rating on the back. Ya, ya I know it's math=yuk but that's the right way to do it. Driving bulbs with a single power source is actually much easier to do than light LEDs, but you can use more LED's per power pack because of the low amperage draw. LED's must be run in parallel due the + and - leads when bulbs can go in series as well. Light bulbs draw more milliamps so the more you add the dimmer the rest get.
 

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Good stuff Scott - thanks!

In looking at the back of my Radio Shack infrared LED, it says Forward Voltage is 1.2V, and Forward current is 100ma.

Using the forumula R = (VS - VL) / I, and if I wanted to run the LED's from my track power wall-wart at 22v.

R=(22-1.2)/.1
R=208

So I would use a 208 ohm resistor...I think.
 

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Discussion Starter #7
Thanks for all that Scott, thats very informative and should help others with this as well. Now I just need to start reading and researching.
 

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Scafer-

I didn't try to do the math yet but I think that's wrong. I know a 3 vdc LED requires @ 220 ohm resistor to run from a 9 vdc source, so 22v > 1.2v will need much more resistance.

I started reading the RS books because I had lots of trouble understanding this stuff. Once I bought an experimenter board and some circuits and went through some exercises I started getting it. I even did some logic circuits with IC timers and did fun stuff like theater marquee chase lights and some programmable traffic control light circuits with various capacitors. They show you how to do it all. :cool:
 

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Discussion Starter #9
I thoroughly read the link to the LED page. That was just great! Now I really have a decent understanding of what it takes to power LED's. And to the opposite extreme, now Im going to need more power to do night races heh. I may just go with the highest power pack voltage I have because I wanted to have a handfull of street lights.

I do still have questions for anyone that can answer. I now know I must wire the LED's in series and not parallel. And I understand how much power to use to power X amount of LED's. But what about Infrared? The package listed forward voltage as 1.2V but I wasnt sure if that was the limiting capacity of the LED to operate or not. For 20mA the forward voltage was 1.6V. Not sure how that all shakes dwon for my lap timing use.

Lastly, its hard to say the LED's will have a definite voltage when they are lit up because a 20V power source could drop to 12V when you have 10 LED's lit up all from the same + source, right? Im not using a battery so maybe the wall circuit will keep the power at peak 20V all the time?? Im not real sure on this but I need to know for sure what voltage the LED's will be operating at because thats what determines how much I need to reduce the power with resitors. This is kinda confusing in that regard.
 

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Discussion Starter #11 (Edited)
I was just trying to rap my head around how many LED's I can power with my power supplies. This makes zero sense to me, but thats not a surprise. I have a 22v power supply and it just seems flat out stupid that I can only power 5-6 white LED's with that much power. I mean these little lights are so tiny and I just cant comprehend them requiring that much voltage to light up. Does anyone know if LED's will burn out if you underpower them slightly? For my setup I would need 9 or 10 white LED's to have the street lighting and it seems crazy to need 2 seperate 20v+ power sources to power tiny twinkle lights. Am I the only person who thinks its silly? Anyway Im open to suggestions before I waste any more money. :p
 

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Think of the LED's as consuming the available voltage when wired in series. If they are rated at 3.3V, you'd need 33V to power 10 of them and could do so with very little or no resistance added to the circuit.

As long as you can supply them with enough current, which is most likely the case, you can wire them in parallel or in a series/parallel configuration, as in two parallel banks of 5 series-wired LED's.

You can get lots of LED info at this site: http://led.linear1.org/led.wiz
 

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Discussion Starter #14
One final question before you all ban me from the forums lol... I tried hooking up just one LED to the powerpack to see if it would at least light up. That didnt work. Here is exactly how I test the LED, whether its 100% right or not I dont know.


I wasnt sure if it mattered which way the resistors were but I didnt think it mattered because I made a circuit board years ago and polarity didnt matter so I was guessing it didnt matter. But the image shows how I put them in line the same direction but I didnt know if it was necessary or not. Im getting 14v on the power pack and everything was soldered so the connections were good. But when I flip the power switch, not even a faint twinkle. I must be doing something wrong. If not Im going to pull my hair out! :p
 

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I'll try to answer some of your LED questions.

Your circuit for testing the LED is correct and should have resulted in the LED lighting up. You were also correct that the polarity of resistors doesn't matter. They can go in either way. There are a few possible explanations as to why the LED didn't light. One is that the LED might be defective. Another is that it may be in backwards. The (-) side or cathode is normally the shorter of the two leads. The (+) or anode is normally the longer of the two leads. If you change the LED to the opposite direction and it still doesn't light then it may be defective.

I don’t know the particular operating parameters of the LED that you are testing, however, it may be that your particular LED has a higher current requirement. You could try removing the 100 ohm resistor and see if that makes any difference. That will increase the circuit current from about 20 ma. to around 25 ma. Just don’t exceed the LED’s maximum current rating.

Reading your previous posts, I’m sure which type of LED you’re trying to test since you were talking about both infrared and white LED’s. If you are testing an infrared LED you won’t see any light at all. You can’t see infrared light with the naked eye. You’ll need some type of infrared detector to tell if an infrared LED is emitting light. If it was a standard LED you should be able to see it light up.

You can wire LED's in series, parallel or a combination of both. It all depends on your preference. I'd rather wire them in parallel with a current limiting resistor than in series for the simple reason that if they are wired in series (using the sum of their internal resistances to obtain the desired operating current) and one burns out then the entire string is out and it makes it more difficult to find out which one is defective. A lot depends on your wiring scheme. If you plan ahead and incorporate a means to locate a burned out LED in a series configuration then, by all means, go ahead and wire them in series.

You won’t burn out an LED by under-powering it. The lower the current, the dimmer the light will get until you reach the point at which the LED is no longer forward biased and the light goes out. If you have an adjustable power supply you can taylor your track lighting to your particular taste by varying the output.
 

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There is so much good detailed information here I hate to add anything simple.

I have two different 3 volt transformers working lights on my Christmas layout. One is a neat Department 56 transformer with a 12 place switchable power bar (#56.53204). It comes with bulbs and leads with jacks to light the buildings and stuff they make, but would be an easy adaptation to LED's. The other came from Hobby Lobby, similar setup except fewer receptacles, powers LED streetlights in the same way. LEMAX is the brand, and it was only about $6.
 

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Discussion Starter #17
Thanks for all that info sir!

Some things I found out a few days ago, the LED was wired backwards and it did light up after I switched it. The only way I've found to see an infrared light is using a digital camera view finder. It showed up bright and purple with the lights off. That was kinda cool to see.

I still cant get my software to work with the LED's because my laptop is goofy. I cant get my ports straight and the wire harness is probably wrong based on what the computer things my 25 pin port is. But anyway, at least I know the LED's are the problem now. Thanks for that tid bit of info!
 
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