Common Cathode, or Common Anode RGB LEDs question - Page 1
Forum section: electronics / beginners

author: Shadetreeprops
author_rank: Regular Contributor
timestamp: March 13, 2016, 11:30:13 pm
content: Okay, so i think my batch is mixed. So now when hooking them up how do i tell the diffrence.Becasue on one set, the POS is the RGB legs, and on the other set the POS is the LONG LEG...but on inspection they look identical, even the flat side is on the same side of them all...This has made one of my projects a Do over...becasue of thisSo my question is, how do i test this, or spot this, without having to hook them all up and individually test each one for Common Anode or Cathode..so i know which way to wire the polarites...casue this gonna be a facking pain...thanks CHINA...lol

author: retrolefty
author_rank: Super Contributor
timestamp: March 13, 2016, 11:33:22 pm
content: I recall a maker type in South Africa, on another site, that had the same thing happen to him on his Asian bulk led purchase. When he complained to the seller they said they were of the reverse polarity variety.

author: Shadetreeprops
author_rank: Regular Contributor
timestamp: March 13, 2016, 11:41:01 pm
content: odd thing is, i still have to put the resistors on the RGB legs..no matter the polarity or my 9v batter burns them up in a split second.it is definate aggrivation. but nothing i cannot correct by flipping the suckers around..but it would be nice just to know without hooking each one up. especially for a project that calls for 60 of them. Then have 25 of them not light up..but i do see the funny behind it..

author: Buriedcode
author_rank: Super Contributor
timestamp: March 14, 2016, 08:48:00 pm
content: So you have TH RGB led's, with 4 legs.That implies you're hand soldering these, so its not an automated pick-n-place job.That means you'll be handling them anyway.I would knock up a small jig with a 4-pin socket, resistors on all pins, and a switch or logic that will reverse the power to the common and the LED's. Shouldn't take more than a couple of seconds per LED, maybe employ a younger person to help, and give them $10 for their trouble

author: PatrickG_
author_rank: Newbie
timestamp: March 15, 2016, 01:28:47 am
content: I would agree with BuriedCode. I test every through-hole LED prior to soldering.For a quick and dirty check of Common Cathode vs Common Anode I use a coin cell battery (CR2032) and place it between the long leg (usually lead 2) and the red leg (typically lead 1) with the + side of the battery on the long lead. If it lights it is common anode, it not it is probably a common cathode. To confirm reverse the battery and test again.I also rigged up a bread board with multiple positions to test not only common anode/cathode, but also each red, green, and blue which I could compare against a known working RGB LED. That way I had no color surprises in a project using multiple LEDs (real helpful if you obtained the LEDs from multiple sources).Since my last few projects contained numerous RGB leds and no circuit board (currently working on a 4x4x4 cube) I rigged up test leads with a 220 ohm resistor attached to the end of positive lead (I had already confirmed they were all common anode). After each string of 4 leds (as wells as each layer) I attached the ground to one of the RGB leads, and raked the resister across each anode to confirm no bad solder joints. I am also testing after the addition of each new layer.PatrickG_

author: Shadetreeprops
author_rank: Regular Contributor
timestamp: March 15, 2016, 02:01:44 am
content: yeah i will have to set that tester up for sure. may take time to do, but will save me so much time in the long run due to this annoyance.another question to this is..Why do i place the resistors on the RGB legs no matter the polarity of the LEDs becasue i was taught to alway connect to the anode or positive of the LEDs?so with my reverese polarity ones, if i do that ZAP they burn..tested 2 to see...i have a few hundred they were cheap..one google answer was, it does not matter where you place it, becasue it still limits the current. Unless using things like transistors where placement is key in the use. especailly in the case of using a MCU on the B lead of the transistor...but seeing i have not got that far in my knowledge yet...i have not encountered this as of yet..but i am sure i will.

author: Ian.M
author_rank: Super Contributor
timestamp: March 15, 2016, 03:16:44 am
content: Some LEDs have a maximum reverse voltage rating as low as 4V, though 5V is more typical. Their actual breakdown voltage may be significantly higher, but if you don't stay within the ratings, here be dragons! There is the possibility of localised avalanche breakdown of the junction resulting in hotspot damage resulting in reduced emission or total failure when its forward biassed again. Build your tester, but add a LDO 5V regulator to keep the maximum voltage applied safe. Assume the minimum Vf you'll ever see will be 1.2V, if you want 25mA, and the supply voltage is 5V, 150R would be a good resistor to use. Even if you connect that to a 3.5V Vf white or blue LED, you'll still get 10mA. You probably don't need anything like that current for a LED tester, so may want to increase the resistor value.Once you've identified a few, if they have clear lenses, you may be able to see the lead-frame which is commonly asymmetric due to the die attach area. If so, you will find that for most types of LED, the polarity tends to match up consistently with the lead-frame orientation.