need help powering 5mm LEDs

[bpool21]

Hello, I have searched this forum top and bottom for quite a while but I still cannot find a good source explaining current and resistance since I know almost nothing about the subject.

My local radioshack has a manager who did electronic SMT stuff for some company for a while and has some sort of degree in engineering and she told me that a 5mm LED will TAKE what current it needs but you must regulate the voltage too with a resistor on EACH LED. I have learned here that you must regulate the voltage and typically under-power the current a tad for longer life around 20mA or so. Once you have the voltage, the current will follow. There was a nice explanation I found somewhere involving plumbing but I have reinstalled my OS and lost all of my Favorites.

So I broke down and bought a ton of White LEDs a while back to play with. Once I found time, I soldered up some resistors and made some stuff, one being kitchen cabinet/counter test lighing, the other being camper shell type lighting for my work rig.

The cabinet application required multiple strings of LEDs including 4~8 in each. Each 3.2v LED received a 420ohm 1/4 watt and each was wired Parallel in their string. I have 40 LEDs total and 40 resistors total. Upon applying what I thought should be 20mA x 40 = 800mA @ 12vDC, the wall wort overheated and burnt up in about 10 minutes.
After googling what I did wrong, I found out that the way I went about it was all wrong and it actually drew more energy from the source and wasted it in heat. I also learned what wall warts typically contain inside to help regulate the DC output.

Second attempt was where 3 LEDs and one 150 1/2 watt resistor in series with another 2 sets totalling 9 LEDs in each application, 4 total applications equalling 12 resistors and 36 LEDs. I assumed this would consume approx 240ma using the online led.linear site. These work just fine and I am completely satisfied.

What do I do about the kitchen? I did set it up originally just to play with the 5mm and to see if I liked the color temp on the surfaces but now I dont want to go back and change it. I've read on here that I can assume each resistor would be a 2volt drop also these were the blue multi band resistors from china and I dont know how well the +/-10% really is, they dont get hot. I've driven the setup at 13vdc @ 1.2 amp and this made the adapter warm after a few minutes. This adapter is used for charging battery supply so I dont think its a cheap knockoff.

I understand from reading here that the automotive application should be protected against AC spikes and diodes should be used to prevent reverse voltage but I only have a company vehicle for 6 months or so until they give me another and once I remove these self adhesive strips, they will get totally destroyed anyway. The only question I have is that if we need to regulate current to an LED, how much would one get from a car with only a resistor in-line? Wouldnt this instantly burn it up with an 80amp alternator? I've been experimenting with cell phone chargers since they are regulated and have specific output from 100~500mA.

Can I keep all 40 LEDs and 40 resistors or is this a major heat issue? I wanted to do a motion censor type setup so if someone came into the kitchen they would turn on for 5 ~ 15 minutes just havent really messed with them for a few months. I have a tendency to think I know whats going on and spend all night building something just to find I got the first step backwards before production. I dont have the money at the moment to burn up Crees so I am first learning on 5mm. Larger output stars are the same just more current, more heat, more expensive but have puck drivers available.

Also, later upon switching to star based lighting, does the puck/buck circuits require a specific current? I would like to do a house-wide low voltage circuit to add lighting in the future but if I run 10amps at first to only one driver, this will fry it correct? Once I have 20+ drivers parallel it will be fine correct?

I dont expect anyone to set down and figure out my problem, I want to learn just dont feel like dealing with the forums speed issues for the next 6 or 8 hours again. If you have links or can help specify a search keyword, that would be great. I am mainly concerned with how to provide current when using random sources like wall transformers. Thanks if you read all of this!

I cannot upload pics so I will have to link to the thread I made on my forum. It shows how my kitchen install looks and also some various strips I was putting together to test with as well as showing my members how to use flux since many are interested in doing LED work to their cars.
http://www.nwaimports.com/forum/showthread.php?t=13533

 

[znomit]

I cannot upload pics so I will have to link to the thread I made on my forum. It shows how my kitchen install looks and also some various strips I was putting together to test with as well as showing my members how to use flux since many are interested in doing LED work to their cars.
http://www.nwaimports.com/forum/showthread.php?t=13533

Those pics cannot be viewed without registering. Nice avatar though.

 

[bpool21]

I do apologize, I sometimes forget that option is turned on, it should be viewable to guests now. I should get a photo hosting account but I've never gotten around to it. Thanks for letting me know!

 

[JohnR66]

You have a lot said here, I'll cover a few things.

The car alternator can be 80 amps or 800. No difference for what your doing. What matters for you is the voltage. When the engine is running. The alternator puts out around 13.8 volts, so base your calculations around that voltage.

An electrical circuit starts at some voltage and ends at 0. Each component in the circuit "drops" the voltage, dissipating energy as it does. At 20 ma, the typical 5mm white LED drops 3.2 volts. If you start with 12 volts, you have 8.8 volts left to drop in the resistor. This is very wasteful. It is much better the put 3 LEDs in a series circuit = 9.6v. Now you have only 2.4 volts to drop in the resistor. 2.4v / 20ma = 120 Ohms, So I'd rewire your 40 LED, 40 resistor circuit.

Set the drive current based on the usage of the LEDs. For continuous or long duty, drive the 5mm LEDs around 10ma. LEDs are more efficient at lower currents and they last much longer. For Automotive use, 15-20ma is fine. For flashlight use, 20-40ma is okay since flashlight are used very intermittently.

 

[bpool21]

Thank you for the response, I do tend to get long winded but I like to cover every detail - its a habit from my job.

So if for example, I were to find a resistor large enough to displace the voltage drop to 5vdc on a engine starter, it would turn slowly (if 5v would turn it) but it wouldnt hardly have any amperage for torque and the resistor would turn into a heating device from sucking up all the amperage?

I guess what the lady at radioshack does make some since where once the voltage is similar to what the device needs, it will only "TAKE" what it needs to work given the wiring is suitable to withstand the amperage. I did wire my LEDs in my rig at 3 per resistor and they are noticeably weaker during engine off but that was based on the 13.9v that registered when it was running making the LEDs something like 22mA. I had always heard it was a terrible decision to wire more than one in series because one could take more energy and leave the others weak and/or one could blow out causing the rest to, but I did learn after my cabinet install that this typically only refers to different color LEDs that require different voltages. If they are the same and from the same manufacturer I should be fine.

I really dont want to rewire the cabinet install since I havent even really got to use the lighting to experience the color temp or luminosity but now I do understand the resistors are eating a ton of juice and its more of a fire hazard than efficient even though they dont feel hot to the touch.
It was after all just a temporary experiment to see what I felt and would be later replaced by something like a star or similar I just didnt want to invest at the moment and havent played with higher current stars yet.

I suppose from what you said, my supply current can be whatever I choose from 20mA to 15AMPs as long as the voltage is within reason and I dont exceed my 15AMPs repository and this can be the same with a driver in a torch? I have seen plenty (non-hotwire) that utilize multiple AW cells but I know the driver doesnt need that much (2200mAh x 4 = 8.8) so it all goes into runtime then right?

I use to have that neighbor I could run to for assurance but I've lost touch of all my electronics friends and for some reason I just cant get LEDs right with current requirements. I appreciate you guys taking time out to answer my questions, I know everyone has the same questions and it gets old but searching this forum takes a long time to find the correct responses for something as well known as current on technology as outdated as 5mm LEDs:candle:

 

[JohnR66]

Motors present a dynamic load, so the voltage drop would change across the resistor depending on the current drawn. Yes, the resistor would be dissipating heat. Any device with a voltage drop across it and current going through it is dissipating energy. Lets say you had a device and you measured the voltage across was 6 volts and the current going through it is 3 amps. Multiply the volts x the amps to get the power dissipated. 6v x 3a = 18 watts.

You might want to grab a electronics book and learn about Ohms law. voltage, current, resistance and power are all related by simple mathematical formulae. Knowing any two values will allow your to find the other two.

You can't parallel different color LEDs due to the difference voltage drops. You can series them as long as you don't exceed the current of the one rated at lowest current.

That sounds correct about the torch runtime.