# Total LED Calculations

Hello everyone.

I have a quick question about using the RD to control LED strips. How do I calculate the maximum number of LEDs that each channel on the RD can support?

Usually, it is just the sum of all the forward voltages. However, if that were true, the example given in the blog with three LEDs per strip, would be over the recommended supply voltage of 9v.

How does one determine maximum number of LEDs, in series, that each individual channel can support? Also, how does the supply voltage affect this?

Secondary question, anyone have any experience regularly using a 12v supply on the RD?

The constant current drivers are designed to have a single LED on each output channel. I don’t think they will work if you put the LEDs in series. They might work with LEDs in parallel but the brightness won’t be uniform. In the LED strip example they have each LED connected to a separate channel.

With single LEDs you can have up to 192 LEDs with multiplexing, or 24 LEDs without it. The multiplexing is done in the firmware, by switching on one of the VCC pins at a time and putting the data for that row on the 24 RGB pins.

I really appreciate your response.

However, the example that I am referring to using this led strip. Which is three separate RGB LEDs, I assume wired in parallel. My other assumption is that, because the blog example is using 7 of these for the digit display, that each one of them is connected to its own RGB channel.

I guess that the only limits would be the 500mA limit on each source channel, and brightness due to the sum of the forward voltages. Which, if I’m understanding the implementation correctly, would be 500mA/20mA = 25 individual LEDs, or about 8 RGBs. Not accounting for the what kind of brightness you would get, which wouldn’t be much, if any, at 9v.

For anyone else that may stumble across this post asking the same question, I read in another post, from one of the Seeed guys that for high power LED, he would recommend not using the onboard source channels for those high current devices, you will let the smoke out, post haste.

So basically, if you want to use several LEDs in parallel or a high power LED, on a single channel of an RD, get yourself a separate power supply with suitable current rating, and high enough voltage rating for the number of LEDs (in the case of a strip), tie the ground to the common ground on the RD and use the positive from the separate power supply for the anode of the current hog(s) instead of the onboard source driver.