How to wire tri-colour LEDs with common cathode

Thread Starter

sjwomersley

Joined Oct 4, 2023
8
Please excuse the crude diagram...

I'm looking for some help with wiring up some tri-colour common cathode LEDs please.

I want to use a DPDT switch to change between two colours by reversing the polarity, 4x tri-colour LEDs will be connected in parallel to give a variety of colour swaps.

I have tested this on a breadboard and got it working, but when I have installed it into my project I can no longer get the LEDs to light up at all, even when just connecting a positive with series resistor directly to the anode and negative directly to the common cathode.

On my diagram the vertical lines represent the stripboard, the yellow bars are my resistors and the monstrosity on the left is my DPDT switch.

Am I missing something? Thanks in advance!

1711117867675.png
 

MisterBill2

Joined Jan 23, 2018
19,398
The wiring diagram tells me NOTHING about what is inside the LED package LEDs have cathodes and anodes and the wiring diagram does not show those elements.
The switch I can understand because I am familiar with DPDT switches used for polarity reversal. With such a switch you can easily select between two different colors, but not by reversing polarity.
 

geekoftheweek

Joined Oct 6, 2013
1,250
Since your title mentions common cathode I am going to assume it is a RGB LED with a common cathode.

That means there are three anodes. One anode red, one anode green, and one anode blue.

The anodes are ALWAYS more positive than the cathode... in other words reversing the polarity will do absolutely nothing.

Also your resistors need to be in series with the LED leads instead of parallel across the leads. Power from your supply to one end of the resistor, and from the other end of the resistor to your LED anode.
 

Tonyr1084

Joined Sep 24, 2015
7,991
The wiring diagram tells me NOTHING about what is inside the LED package LEDs have cathodes and anodes and the wiring diagram does not show those elements.
The switch I can understand because I am familiar with DPDT switches used for polarity reversal. With such a switch you can easily select between two different colors, but not by reversing polarity.
Actually you can get three colors by lighting one LED, the other LED or Both LED's. However, with just a single SPDT or DPDT switch you can only select one color or the other.
Since your title mentions common cathode I am going to assume it is a RGB LED with a common cathode.

That means there are three anodes. One anode red, one anode green, and one anode blue.

The anodes are ALWAYS more positive than the cathode... in other words reversing the polarity will do absolutely nothing.

Also your resistors need to be in series with the LED leads instead of parallel across the leads. Power from your supply to one end of the resistor, and from the other end of the resistor to your LED anode.
I have these same LED's; one Green and one Red. Together they make yellow.
Screenshot 2024-03-22 at 4.17.54 PM.png
Using an SPDT switch you can wire it so either one will be lit but you will not be able to light both at the same time.
Using two SPST switches you can select from three colors, Red, Green and Amber (Yellow).
 

Tonyr1084

Joined Sep 24, 2015
7,991
Another noteworthy detail is each different color LED has a different forward voltage (Vf). You will need to select the proper resistors for each color depending on the voltage you're going to use and how bright you want each color to be. The resistors will be placed on the Anode because each will need its own resistor.

Details are the devil in the works. If you don't share all the details then we can't be thorough in answering your question and helping you solve your issue.
 

MisterBill2

Joined Jan 23, 2018
19,398
Since your title mentions common cathode I am going to assume it is a RGB LED with a common cathode.

That means there are three anodes. One anode red, one anode green, and one anode blue.

The anodes are ALWAYS more positive than the cathode... in other words reversing the polarity will do absolutely nothing.

Also your resistors need to be in series with the LED leads instead of parallel across the leads. Power from your supply to one end of the resistor, and from the other end of the resistor to your LED anode.
Very interesting, I have a string of tricolor LEDs that has the opposite setup, and in addition, it has internal resistors. One terminal is marked =12 and the others are marked R, G, and B. So I can connect my 12 volt supply and then tie the negative to which ever color I want to illuminate. I suspect that if I applied a voltage to two different colors that I could light one or the other by reversing the polarity. BUT I would not choose to do that because it means reverse biasing some of the LEDs and they have a low tolerance for reverse bias voltages. And switchhing on more colors can provide more light.
Probably the TS should examine the string closely to see if it contains internal resistors, which my string does. Each blok has 3 LEDs i series with a resistor for all three.
 

geekoftheweek

Joined Oct 6, 2013
1,250
Actually you can get three colors by lighting one LED, the other LED or Both LED's. However, with just a single SPDT or DPDT switch you can only select one color or the other.

I have these same LED's; one Green and one Red. Together they make yellow.
View attachment 318185
Using an SPDT switch you can wire it so either one will be lit but you will not be able to light both at the same time.
Using two SPST switches you can select from three colors, Red, Green and Amber (Yellow).
I have seen them before and that actually makes a lot more sense than my thoughts. RGB was just the first thing that came to mind since tri color was mentioned.

Very interesting, I have a string of tricolor LEDs that has the opposite setup, and in addition, it has internal resistors. One terminal is marked =12 and the others are marked R, G, and B. So I can connect my 12 volt supply and then tie the negative to which ever color I want to illuminate. I suspect that if I applied a voltage to two different colors that I could light one or the other by reversing the polarity. BUT I would not choose to do that because it means reverse biasing some of the LEDs and they have a low tolerance for reverse bias voltages. And switchhing on more colors can provide more light.
Probably the TS should examine the string closely to see if it contains internal resistors, which my string does. Each blok has 3 LEDs i series with a resistor for all three.
Interesting. Those could be quite handy for the right situation. The original post mentioned the yellow bars were resistors so I assumed they were supposed to be the current limiting resistors. Since there was no mention of a strip I also assumed they are single LEDs. I guess if you left the cathode disconnected you could in theory make it work by reversing the polarity, but why would you? I actually bought a lot of 1000 common anode RGB LEDs when I bought my 3D printer for some custom lighting ideas that didn't work out according to plan... kind of the same, but different.
 

Audioguru again

Joined Oct 21, 2019
6,782
Your sketch shows an LED with only 3 wires. One wire is the common cathode and the other 2 wires are for colors.

BUT post #7 talks about an RGB LED with 4 wires. I assume the wire marked "=12" is the common cathode (negative).
I assume that your LED also has a negative common wire.

An LED will explode if connected directly to 12V (they are 1.8V to 3.5V) so it must have more LEDs and some resistors in it.
 

Attachments

Last edited:

AnalogKid

Joined Aug 1, 2013
11,196
Your drawing does not agree with your description. I bounced over to Digi-Key, and all of their3-color LEDs have 4 leads.

Please post a photo of the LED, or a link to a product page, or a (link to a) datasheet, or something.

ak
 

Tonyr1084

Joined Sep 24, 2015
7,991
LED's come in many varieties. I have somewhere in my stock pile a two lead two color LED. In fact, on a garage door opener control board there are indicators that are SMT with only two points of solder. They can glow red, blue or purple depending on polarity and whether they are powered at high frequency AC. A two color LED can have two wires or three. In the case of an LED with three wires, one being common, it can produce up to three colors. A true 3 color LED having four wires can produce the entire rainbow of colors depending on how bright each element is lit.

As has already been noted, each color will have an independent Vf. Each color must be treated as a single LED when powered. It's Vf must be considered when powering it and using a resistor to limit the current. I won't make any examples of how to power an LED, I'll just say it's not difficult to figure out the proper resistor and how to choose the correct current.
 

ThePanMan

Joined Mar 13, 2020
803
A 3 color LED is considered to have three elements, each capable of producing only one color. If you have a tri-color LED then it will have four leads, not two or three. A 2 color - or bi-color LED will have two elements and either two or three wires, one being common cathode or common anode. And a bi-color LED with three leads can produce a range of colors as well. Suppose you have a red/green LED. You can have Red or Green. Or by controlling the brightness of each element you can mix a range of colors that the two base colors are capable of making. I would assume R & G at the same intensity would produce a yellow color. More red should turn it more toward orange. I don't know the exact color mixing but you CAN get a range of colors from a bi-color LED. I'd suppose it's possible to perceive color using just a 2 wire LED but each element would be lit by an AC source meaning only one color will be lit at any time. The human eye would have to perceive a mixed color, which might be how it can be done, but I don't know that for sure.
 
Top