Help with led light strip set up  - Page 1
Forum section: electronics / projects

author: GoatZero
author_rank: Regular Contributor
timestamp: October 12, 2013, 01:41:28 am
content: Sorry if I did it wrong this is the first circuit of this kind I make so please feel free to correct or give feedbackI wanted more light in my desk so I went and got a led light strip which its supposed to use 12v-24v 220mASo, I used a knife to cut it in several parts and ended up placing them as you can see in the picture 3 at the roof, 1 below desk, 1 small in the middle of the desk), all in parallel and with at least 3-4 metters of cableMeasuring with the multimetter right after the AC adapter it seems that the leds are pulling 4.38A DCHowever im using an AC adapter eliminator rated at 19V 3.42A when I plug it everything seems to be ok however the eliminator makes a sound which leads me to believe it wont last long since Im demanding more current than it can provideAlso, with this current setup i want to at a potentiometer to regulate the amount of light along with a switch to turn it on/off in all the light strips however according to the numbers P=VI = 12(4.38) = 81 Watts, if im right I need an 81 Watts potentiometer?.... Because i only have a 2 watts and several 1/4 watt onesSo in shortcan I dim the leds using an potentiometer or any other way?? If yes howWhy is the AC adapter making that noise ?Is there a better setup for what I just did?How the heck did i go from 220mA in series to 4.38A in parallel?

author: SeanB
author_rank: Super Contributor
timestamp: October 12, 2013, 06:47:35 am
content: The strips should be connected in series, the cut on the one being connected to the matching cut on the other with the appropriate number of cable cores ( typically 2 wires). Power is fed in only at the one original point. Both the power supply and the LED strips will not last long with the 4A current through them.

author: tszaboo
author_rank: Super Contributor
timestamp: October 12, 2013, 05:07:22 pm
content: What theis a "voltage eliminator"? You mean AC-DC power supply? Notebook AC adapter?If it is pulling that much, probably you made a short circuit somewhere. Did you put your multimeter in series with the load, not in paralel?The other possibility, is that the LED strip is rated 220mA per segment, about 3-4 LED per segment (small strip). And you put 15-20 there.Using a potentiometer it is pussible to regulate the current, but it is not economical. You need PWM to switch on and off the LED fast, anything above 500Hz.

author: GoatZero
author_rank: Regular Contributor
timestamp: October 12, 2013, 07:44:33 pm
content: Quote from: NANDBlog on October 12, 2013, 05:07:22 pmyes its an AC adapter from a printer i dont have anymore, sorry in my country we call them voltage eliminators so i just translated it and it came out like that, i just noticed that, i might edit my postthe multimetter is in series with the load at the positive lead after getting rectifiedI also meassured before the rectification, the ac adapter its pulling only 1.19 A from the main line (outlet) and after getting rectified its giving 4.40 A kind of expected since all of this its in the label (3.42)however im pulling 30% more current than the ac adapter says it can give, also its getting hotter (65c-90c) so it worries me a bit,QuoteI do believe this since for example aht the roof i have 3 strips each with 25 segments of 3 leds thethe leds get to 30c-35c, but the ac adapter its what worries me the most,I already tried using 2A adapters and leds just keep blinking, im assuming because they need more current

author: IanB
author_rank: Super Contributor
timestamp: October 12, 2013, 07:51:24 pm
content: If the light strip was supposed to use 220 mA before you cut it up, it should still use 220 mA after you cut it up.Did you test it when it was still in one piece?When you cut it into pieces you need to maintain exactly the same electrical connections after the cutting as before. It sounds like maybe you have not done that right.

author: GoatZero
author_rank: Regular Contributor
timestamp: October 13, 2013, 02:47:35 am
content: i actually did test it using a 12v 300mA nad it barely turned, it wasnt even able to light up the room in the dark, it was until i went up to the 3.42 A that i was able to see the leds "shine"i guess i will buy another one just for the sake of testing and understand whats going on, i will buy the ac adapter if they sell it too

author: bdivi
author_rank: Regular Contributor
timestamp: October 16, 2013, 09:24:40 am
content: Usually the LED strips are 12V, although I have seen few that take 24V.The schematic is such that you have power lines along the strip and groups of diodes (3 in the 12V scenario) together with a resistor for current limiting that are individually connected to the power lines all in parallel. You have to check your specifications and see the supply voltage and current per meter (you can cut the stripes at certain locations as indicated). Then you supply the correct voltage and current to the stripe segments all connected in parallel.cheers

author: mianchen
author_rank: Regular Contributor
timestamp: October 17, 2013, 08:53:46 am
content: Quote from: NANDBlog on October 12, 2013, 05:07:22 pmBattery eliminator

author: sleemanj
author_rank: Super Contributor
timestamp: October 17, 2013, 09:09:34 am
content: The cuttable sections of strip are paralleled together anyway your uncut strip obviously doesn't have a hundred led's in series running off 12v, judging by the equipment on your workbench I don't have to tell you why that is :-)So by cutting the strips and putting in parallel there should be no change in the current requirements, since all those parallel sections on the uncut strip are still parallel in the cut strip, no matter how you connect those cut strips. You haven't added any more parallel pathways.So, back to basics young man. What is the voltage drop across one of the LEDs, look at the strip and find out how many leds are in series on each parallel chain, there's probably a current limiting resistor for each series chain, now take all that, apply ohm's law, and work out what current you expect to see.The current rating you have given "220ma" is probably per meter is my guess.