wiring lots of LEDs together? - Page 1
Forum section: electronics / beginners

author: algorithm
author_rank: Regular Contributor
timestamp: August 19, 2014, 05:07:10 am
content: Ok, so I am trying to use 98 5mm LED's to create 1 big light.The LEDs are 3v 20mA, reverse current (uA) vr=5v max=10My first question is what would be the best way to wire this up, pairs of 5 in series, then wiring those bundles in parallel? or...?Another question, what would be the best way to do this? both inside and out?Should I get a bunch of little blank boards for all those LEDs? or...?Also, what would be the best way to make a case for this light? Would a pan work? I think I will need something to soak some heat up.Thanks in advance for any advice.

author: Monittosan
author_rank: Regular Contributor
timestamp: August 19, 2014, 06:10:50 am
content: leaving the full lead lengths of the leds and having them sit standing off the pcb should allow for some cooling.Else you could drive them at a lower current to increase life and reduce heat.

author: mariush
author_rank: Super Contributor
timestamp: August 19, 2014, 11:39:48 am
content: My advice would be to wire them in such a way that you'll be able to use a common power supply.For example, I would aim for a 12v or 24v power supply or a 18.5v laptop adapter.As the forward voltage won't be exactly 3v for all leds, it's a good idea to make individual chains of leds that have the total voltage a bit lower than the power supply's voltage so you can properly limit the current going into the leds instead of blowing them up.For a 12v power supply, 3 leds per chain would work fine... 3 x 3v = 9v. For 24v, I'd make chains of 7 leds .. 7x3 = 21v ... this would also make it exactly 14 chains of leds. With 18.5v , you'd make chains of 6 leds 6x3 = 18v.You need to limit the current going into the leds, and the simplest way is to add a resistor in series with the leds. The value of the resistor you calculate with the simple formula :[ Input Voltage - Led Forward Voltage (total) ] / Current.For 12v , 3x3 leds you would have (12v - 9v ) / 0.02 = 150 ohm ... and the power dissipated in resistor is P = IxIxR = 0.02x0.02x150 = 0.06w so a resistor rated for 0.125w will do.For 24v , 7x3 leds you would have (24-21v) / 0.02 = 150 ohm .. same value.For 18.5v , 6x3 leds you would have (18.5 - 18) / 0.02 = 25 ohmOnce you figure out how many you want in series, with the resistor limiting the current each chain will use exactly 20mA at that input voltage, so you can multiply with the number of chains to get the current amount. For example, for 24v power supply, you'd have 98/7 = 14 chains so that means you need a power supply capable of 20mA x 14 = 280mA.

author: algorithm
author_rank: Regular Contributor
timestamp: August 22, 2014, 02:55:50 am
content: Quote from: mariush on August 19, 2014, 11:39:48 amBeautiful, thank you very much.I suppose that just about solves it. The only thing left now is to decide what to mount the strip board onto.Thanks again

author: christopher iles
author_rank: Regular Contributor
timestamp: August 24, 2014, 07:31:03 am
content: Hello,I made some Christmas lights with 100 leds, All I did was to solder a 1k 1/4 watt resistor to one leg of each led and then connected them all up in parallel just like you would bulbs.I powered the lot from an old 12v dc power supply and they worked (and still do) fine.They don't even get warm at all.Hope this is of use.Regards,Chris.

author: mariush
author_rank: Super Contributor
timestamp: August 24, 2014, 04:04:28 pm
content: QuoteAssuming a forward voltage of 3v, that means your LED limited the current to 9mAV = I X R => 12v (psu) - 3v (forward voltage) = I x 1000 = > I = 9 / 1000 = 0.009 A = 9mAThe power dissipated in resistor is IxIXR = 0.009x0.009x1000 = 0.081 W which is well below the 0.125w rating of your resistor, so that explains why it works fine for you: the resistors aren't heating up much, the leds are underdriven.But it's a waste of energy (and ugly design, considering the amount of resistors).. 0.081w x 100 = 8.1 watts of wasted energy.

author: algorithm
author_rank: Regular Contributor
timestamp: August 14, 2019, 02:53:10 am
content: Bumping this ancient topic. Sorry if that bothers anybody. But this seemed better than starting a new topic as my question is simple and stems vicariously from the OP.Quote from: mariush on August 19, 2014, 11:39:48 amSo mariush, I never finished the project of my OP. Just have had the LED's laying around since then collecting dust.Now I want to make use of some of them in a laptop. I want to put the LED's inside around the keyboard edge.Im struggling to grasp how I could power it easily though.Ideally, I want to use a cr2032 or something of a similar size.But since each LED is 3v, Im not sure that is possible?Also since I cant actually have the LED's below the keyboard, have to rely on light coming in from the edge,I guess having 2 chains of leds for bottom/top of keyboard would be ideal.Am I wrong for picturing being able to run leds in series with a resistor between each led off the 3v cell?Or, would that make it dimmer with each additional led?Also am I wrong for thinking there is no limitation on how many chains you can run off the VCC? Like in your last answer?If so I guess I could get creative with slack in wiring and manage to make smaller chains of say 3, and wire the chains in parallel and maneuver the slack in wiring to line them up like a strip?The other choice I have is, tap the 7 LED chains using 24v off the DC input on the laptop. Although I worry, could that interfere with the power enough to disrupt the computers functionality?If I made 2 chains of 7 that would merely be 40mA right?When I try to find the answer to "tapping led string off DC psu voltage drop" the results arent helpful.Any advice is appreciated : )