Soffit LED Flashlight project question...(so close)  - Page 1
Forum section: electronics / beginners

author: Mrkirby
author_rank: Newbie
timestamp: August 31, 2013, 12:30:57 pm
content: http://youtu.be/C89mBDfiEB4I created a video (brutal) to explain my problem. I have finished the wiring process and the installation of my recessed lighting. I just can power it safely or sustainably. It looks good for the 2 min that I can leave it on. Specs. 22leds per light20mA3AAA to power one light 4.5vI wired three lights in series GrOUPThen connect four GROuPS together via parallel.There are resistors (in the video) on each light.What are safe transformers I can use to power my system.Thanks for reading!

author: IanB
author_rank: Super Contributor
timestamp: August 31, 2013, 03:49:44 pm
content: You will need to do some measurements and calculations.Those 3AA flashlights look like they are from a dollar store? That kind of light is not designed to last very long or work very reliably. So don't expect long term results from your lighting setup. Some LEDs may fail after a while.But let's see. 3AA per light. On average that is going to be around 4 V (not 4.5 V unless the batteries are brand new and switched off).22 LEDs per light. The current is unknown until you measure it, but let's assume 20 mA per LED. Then you require 22 x 20 = 440 mA to run a light.Put three lights in series and you therefore need 12 V and 440 mA (12 V = 3 x 4 V).If you put four of these groups in parallel, then your current demand becomes 4 x 440 = 1.76 A.So the approximate power supply you need is something like 12 V, 2 A.

author: G7PSK
author_rank: Super Contributor
timestamp: August 31, 2013, 04:27:56 pm
content: Those LED's look over run to me, I would try the 9 volt transformer. I seem to remember that this exact question came up before, last year or early this year, someone was using led lamps around the outside of their house.

author: Mrkirby
author_rank: Newbie
timestamp: August 31, 2013, 11:23:57 pm
content: Thank you for the two responses!As for the first response...12v 2amp... It looked like the lights were over run.... I think the resistors are dampening the circuit. (Lessening, restricting). Essentially I think the resistors are making it so I don't need as many volts. I tried a 12volt 1amp and the lights were still "screaming and flashing"If you tell me what to do I will find the measurement of the draw or amps or volts... The metric that you needed to know.As for the second response.... Yes it is the same guy. I am the guy who was working out the details earlier this year and just finished the work.I will try the 9volt and as high amps as I can...but when I tried anything lower than 12v. I couldn't power the system. I think I had less than .5amps. I will boost the amps this time.Thanks guys for the fast reply. When the system is on the house looks great! Also, not dollar store 3.99 home hardware specials.I bought many over the year and can see where they have off shored the company to spike profits with cheaper wire and less materials within the product!

author: IanB
author_rank: Super Contributor
timestamp: September 01, 2013, 01:31:47 am
content: The way LEDs work is that if you connect them directly to a battery or any other power supply they will fail. To stop them failing you need to put some limit or restriction on the available current, and a resistor is the easiest way. Those cheap flashlights have a resistor for this purpose, but they also (deliberately) use tiny, feeble AAA batteries. It is hard for small batteries to produce enough current to fry the LEDs. The light would run longer on AA batteries, but the LEDs would have a shorter life.This presents you with something of a problem, since you want to power the LEDs from a wall wart which has much more available power than AAA batteries. If you don't do something about it, your LEDs will get fried in short order.If you don't have electronic tools like a multimeter available and the knowledge of what to do with it, then your best bet is to use a 12 V 2 A power brick or wall wart and then add your own resistor to limit the current. My suggestion would be to use a 12 V 2 A DC regulated wall wart as I mentioned above, and then add an additional resistor in series in the circuit. Get a selection of resistors, for example 10 ohms, 5 ohms, 3.3 ohms and try each of them until you are happy. Your resistors should have a power rating of 5 or 10 W (watts). Instead of a resistor you could try an automotive bulb, for example a typical 25 W brake light bulb.

author: Jeff1946
author_rank: Regular Contributor
timestamp: September 01, 2013, 02:51:39 am
content: Let's consider one lamp, looking at your video there are 22 leds in parallel with a 2.5 ohm resistor. Assuming a current of 20 ma which the max current for long term operation of these types of leds you are drawing about 0.44 ma. Thus the resistor is dropping about 1V. Additionally AAA batteries have about .25 ohms of internal resistance, so .75 for three of them dropping about another .3 volts. This seems about right as white leds need about 3 to 3.3 volts.With time some of the leds will fail, which will cause more current on the remaining ones stressing them more leading to a death spiral.So here is what I would recommend to power them. Get a used laptop power supply. They put out about 19 V and 3-5 A depending on the model. You could put 3 or 4 lamps sets in series with an appropriate resistor. To be safe assume 4 volts is sufficient for each lamp set. So 3 sets = 12v 19V-12V = 7V 7V/0.4A =18 ohms, call it 20 ohms, note it will dissipate 3W so use a 5W resistor. For four sets in series try a 10 ohm 2 watt resistor.

author: Mrkirby
author_rank: Newbie
timestamp: September 01, 2013, 04:26:52 am
content: Thanks for all your knowledge.Made this video to keep you in the loop. If you are interested. When I answer questions in a field that I know, visuals always help. I am not doing this because of the youtube hits or because I enjoy hearing my voice.This was with a 9v 1amp transformer.I tried a 9v 850mA transformer after this video and more lights started to flash which made me question if perhaps the flashy flash could be because of not enough Amps.Question 1...Could this be true?I explained my diagram that I got from this forum earlier this year to better explain my wiring as some comments suggested a whole system from series.Question 2. I am thinking of connecting that parallel wiring back to the source to equalize the voltage across the whole system? Good idea or bad?Any suggestions for transformer? 9v ?? Amps? What's my top end for amps with my 9v transformer?I have to replace some LEDs, no problem, but boy I would like a way I could shut off the system if there was a light failure. Something that could measure the correct functioning of the system and cut out if a light fails to save the system.Anything reasonable because this sounds like a device the power grid uses.Thanks AgainJames

author: IanB
author_rank: Super Contributor
timestamp: September 01, 2013, 04:36:16 am
content: Quote from: Mrkirby on September 01, 2013, 04:26:52 amYes! That's why I have been mentioning 2 amps in my posts.It has been explained that 22 LEDs in parallel at 20 mA each will be 440 mA. If you put four of these in parallel you will need 1760 mA, which is way more than 850 mA. If you overload the power supply it will go into overload protection and try to shut down to protect itself. Hence the flickering.QuoteI don't know what you mean by this, but whatever you mean it doesn't sound right. Not a good idea.QuoteYes, 12 V (or more), 2 A (2000 mA) or so. Then include enough resistance in the circuit to limit the current and protect the LEDs.

author: Corporate666
author_rank: Unknown
timestamp: September 01, 2013, 04:43:21 am
content: Mrkirby,I think you are probably making this a lot more complicated than it needs to be.It appears that the LED boards have no active control on them - just batteries and a resistor. So the power draw of each LED is kind of irrelevant. They are run from 3xAAA batteries, which is 3V-4.5V depending on charge. So if you have three in series, that is 9V-13.5V. I would choose 12V as others have previously suggested.You say the lights flicker. If there is no "active" circuitry on the back of the LED board, then the flickering cannot be caused by the board, but only by your power supply. You can't have too many amps... but you can have too little.I would take one of your strings of 3 of these units in series... connect it to your 12V power supply with your meter in-line and see what the current draw is. Once you know, and you know how many of these strings-of-three you want to run, you can figure out how big a power supply you need (in terms of amps).But I do not think the flickering is caused by lack of voltage or by too much voltage - certainly not if there is no driver on the board of that LED board. I think you are overloading your power supply.