Is a 1 ohm resistor really necessary for this led circuit? - Page 1
Forum section: electronics / beginners

author: Jay112
author_rank: Regular Contributor
timestamp: May 18, 2016, 09:01:13 pm
content: I'm using this led calculator thing to help me arrange my led array:http://ledcalculator.net/This is the image it gives me for 21 LEDs at 12V. The LEDs have a 2V drop and can take 30mA.Will those 1 ohm resistors really prevent the LEDs from dying early? Is it safe to omit them?

author: Fred27
author_rank: Unknown
timestamp: May 18, 2016, 09:05:26 pm
content: I'd really not recommend six in series. That 2V won't be exact. A little above and the LEDs may not light at all. A little below and you'll get a higher current than expected. You'd be better with 7 sets like the bottom row.

author: Jay112
author_rank: Regular Contributor
timestamp: May 18, 2016, 09:10:19 pm
content: Quote from: Fred27 on May 18, 2016, 09:05:26 pmThanks! This is my first time making an LED array, so the advice is much appreciated!

author: Jay112
author_rank: Regular Contributor
timestamp: May 18, 2016, 09:12:14 pm
content: If they are cheap LEDs from ebay rated at 30mA, is it safer to treat them as if they maxed at 20mA? Or can most cheap LEDs handle 30mA?

author: danadak
author_rank: Super Contributor
timestamp: May 19, 2016, 01:43:11 am
content: LED datasheet threshold specs are pretty sloppy, all you get is a max.So your power must be >= than sum of all + drop across R.So Vled1 + Vled2 +......+ Iled x Rled = Vcc,solve for R worst case. Vledprobably varies as much as 40%, take this into account when you calcRpowerrating."Normally" you want to drive LEDs with a current source, this helps inbrightness matching for the individual LEDs. In your case the R is "trying"to effect a crude current source, not close to best practice.https://engineering.purdue.edu/~ece495/Power_Electronics_Lab/LED_Basics.pdfRegards, Dana.

author: Jay112
author_rank: Regular Contributor
timestamp: May 19, 2016, 01:14:52 pm
content: Quote from: danadak on May 19, 2016, 01:43:11 amThanks for the info, Dana! That was a helpful document about LEDs!It's nice to know that the LED specs are usually sloppy. Since there's potentially so much variance, I just treated them as if the max current was 20mA instead of the listed 30mA.I'm just making a decorative light for a friend, so it's nothing serious. I put a PWM dimming controller on there, and I think most of the time the lights will be on a dimmed setting, so I think they won't be under much stress at all. My biggest concern is that maybe some of the LEDs will have a different color or intensity than the others, but I won't be able to test that until I test everything together (probably tomorrow).

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2016, 01:46:16 am
content: The 1 ohm resistor value is stupid. The LED calculator assumes that you tested and sorted all the LEDs into groups with exactly 2.0V for each LED and that the power supply is exactly 12.0V. Do you think that ebay tested and sorted the LEDs? No way! They simply guessed that some will be 2V.But if your LEDs are 1.8V then the six have a voltage of 10.8V and the remaining voltage of 1.2V causes a current of 1200mA which will instantly destroy the LEDs.5 LEDs with a voltage of 1.8V have a total voltage of 9V. If the remaining 3V causes 30mA then the resistor value is 3V/30mA= 100 ohms. If the LEDs are all actually 2V then the current is 20mA which is only a little dimmer.

author: Jay112
author_rank: Regular Contributor
timestamp: May 20, 2016, 01:58:09 am
content: Quote from: Audioguru on May 20, 2016, 01:46:16 amThanks for the clarification!I completed the LED project today, and it was successful. My friend who received it really liked it a lot.I learned from this thread that none of the pieces are exact. I had never tested forward voltages before on LEDs, and was surprised that most of the LEDs were below their rating at 1.75V.I didn't like how some of the light series were brighter than others, but as I think Dana noted above, if I really wanted the lights to all have the same intensity I should use constant current drivers.Thanks everyone for the help!

author: Audioguru
author_rank: Super Contributor
timestamp: May 20, 2016, 03:01:04 am
content: Constant current drivers will still allow some LEDs to be brighter than others.You get what you pay for. Cheap ebay LEDs are cheap so some are brighter than others. I do not know if the bright ones or the dim ones will fail first because I do not buy cheap ebay LEDs.