What resistor for SMD LED board? - Page 1
Forum section: electronics / beginners

author: Jackster
author_rank: Frequent Contributor
timestamp: January 18, 2015, 10:18:50 pm
content: Hi all,I have these LED boards with 6 LEDs on them.I was told that they are 12v but looking at other sites that list the same thing as 10v.They are 5630 SMD LEDs.From the looks of it the 6 SMDs are in parallel pairs and then the 3 pairs in series.I am looking to power these with 8 to 13v batteries.What would be the best resistor to add before this unit?Thank you!

author: sleemanj
author_rank: Super Contributor
timestamp: January 18, 2015, 11:10:20 pm
content: A single white led die will be around 3.2-3.3v forward voltage. 3 (parallel pairs) in series is therefore somewhere around 9.6 to 9.9v.Now you can use Ohm's law to calculate based on your supply voltage, the forward voltage, and the current you want to pass through them.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 19, 2015, 02:29:49 am
content: Quote from: sleemanj on January 18, 2015, 11:10:20 pmI don't have the spec sheet for the LEDs so I don't really know what I want to put through them :/

author: sleemanj
author_rank: Super Contributor
timestamp: January 19, 2015, 03:43:41 am
content: Quote from: Jackster on January 19, 2015, 02:29:49 amLooking around, it seems the typical die current for that size white led is 60 to 150mA, so since you have parallel pairs, multiply that range by 2.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 20, 2015, 12:26:29 am
content: So if these are 10v, being 3watt means each disk would pull 300mAh?300 / 6 = 50 so these are 50mAh leds?Going off being 10v again, the LEDs are 3.3v?Calculator says 1.2 ohm resistor.Sounds about right?

author: sleemanj
author_rank: Super Contributor
timestamp: January 20, 2015, 12:29:45 am
content: Quote from: Jackster on January 20, 2015, 12:26:29 ammAh is not the units you are looking for. The unit is mA.QuoteWith what supply voltage?

author: Jackster
author_rank: Frequent Contributor
timestamp: January 20, 2015, 12:34:18 am
content: Quote from: sleemanj on January 20, 2015, 12:29:45 am10v supply, I can go lower and it will work?

author: sleemanj
author_rank: Super Contributor
timestamp: January 20, 2015, 12:41:20 am
content: Quote from: Jackster on January 20, 2015, 12:34:18 amIf you have established that you want to put 300mA into the disk, and you know that the VFwd is going to be (in the vicinity of) 3 x 3.3 = 9.9Known:Current through the resistor = 0.3AVoltage over the resistor = 10 - 9.9 = 0.1So it follows by Ohm's law:Resistor = 0.1v / 0.3A = 0.33' Ohms

author: Jackster
author_rank: Frequent Contributor
timestamp: January 21, 2015, 05:13:02 pm
content: Could I play it safe and have the max of 12v, what would I need then?Thanks

author: Jackster
author_rank: Frequent Contributor
timestamp: January 21, 2015, 07:53:12 pm
content: Okay so I got this variable resistor out of a broken multimeter (need a new one now) and found a good level. 30c for the last 20 mins.Will also have more cooling from the attached object I am putting it on.Going to borrow my granddad's multimeter so I can find out what I need.

author: sleemanj
author_rank: Super Contributor
timestamp: January 21, 2015, 08:58:11 pm
content: I suspect you are probably only running a few tens of ma there, which is fine if you only need that, but far below what it would be capable *if* attached to a heatsink and you were not using a trimmer potentiometer meant only for low currents.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 21, 2015, 09:16:41 pm
content: So a resistor measuring the same (as close as) would work the same or are these variable things not the same?

author: sleemanj
author_rank: Super Contributor
timestamp: January 21, 2015, 10:11:49 pm
content: Quote from: Jackster on January 21, 2015, 09:16:41 pmIt should.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 22, 2015, 02:43:23 am
content: Quote from: sleemanj on January 21, 2015, 10:11:49 pmAwesome! Thanks for the help.

author: Jackster
author_rank: Frequent Contributor
timestamp: January 25, 2015, 08:49:10 pm
content: So I wanted to check the amp pull on the LED today.Turns out I have never used a digital amp meter before :pI set it to Amps and the 2 setting.It reads 0.074That is 74mA right?