Choosing a Pixel Voltage: 5V vs 12V

General

There are many Different Styles of Pixels that are available with different design features like shape, integrated circuit and voltage available. Choosing the right pixel depends on your specific use plans and one of the most common questions is which voltage pixel should you buy. Choosing between the two most common voltages, 5vdc and 12vdc depends on how you plan to control and how you layout your pixel setup. There are a few general rules:

• Long strings (>50 pixels) are less likely to need power injection if they are driven by 12vdc.
• Long distances from the controller to the first pixel is easier with 12vdc.
• 12vdc power supplies are more expensive per Amp then 5vdc power supplies.
• Some low cost and homemade controllers only work with 5vdc.
• Some 12vdc pixels use less current then traditional 5vdc pixels.
• Voltage drop and power injection are important issues for both voltages.
• 5VDC pixels are more power efficient then 12VDC pixels.
• 5VDC pixels use less power than 12VDC pixels.

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How RGB LEDs work

Light Emitting Diodes (LEDs) emit a colored light when the proper voltage is applied across their terminals. A typical Common Anode RGB LED used in pixel strings actually has three LEDs (Red, Green and Blue) inside of one housing.

The specific voltage required to light the LED generally depends on the color of the LED. The following table shows the characteristic voltages ( also called Forward Voltage V_F) required for each of the three LEDs inside of the typical RGB LED used in pixels.

Number of Channels (*)

Typical 8mm RGB LED Properties
Color	Forward Voltage VF @ 20ma
	Minimum	Typical	Maximum
Red	1.8V	2.0V	2.2V
Green	3.0V	3.2V	3.4V
Blue	3.0V	3.1V	3.4V

Since we tend to use commonly available 5VDC or 12VDC power supplies to drive the pixels, we have to drop the voltage and limit the current flowing thru the LEDs using current limiting resistors chosen for each color LED to deliver the proper voltage and current.

Current Limiting Resistors

By using a formula called Ohm's Law you can calculate the size and value resistor you need to drop the supply voltage down from the 5VDC or 12VDC to the V_F and current that you need for a specific LED. An example of that calculation is:

1. What resistor would you use with a 5VDC power supply and a Blue 8mm LED with a V_F of 3.1V?
   First you need to calculate the voltage that the resistor needs to drop. That is equal to
   V_Drop=V_PowerSupply - V_F=5-3.1=1.9V
   The table above recommends a current I of 20 ma for a 8mm LED. The formula for resistance gives us:
   R=V/I=1.9/0.02=95Ω Generally you choose a resistor for that value or the next higher standard resistor value , in this case 100Ω.
   To calculate the wattage resistor needed you use the formula for Power:
   P=VI=(1.9)(0.02)= 0.038W. You would use the next standard size up resistor, 1/8 W.
   

Power Efficiency of Pixels

Because the power supply voltage is not exactly equal to the V_F that the LED needs, there is some power wasted as heat in the current limiting resistor. Many common pixels are designed to use 20ma (0.02A) of current per LED color, or a total of 60ma (0.06A) total per pixel. You can then use Ohm's Law to calculate the power used by the LED and the resistor. From the table below, you can see that a typical 5VDC pixel consumes 0.3W of power and the LED uses 55% of the power and the rest is wasted as heat in the resistor. A typical 12VDC pixel that uses 20ma consumes 0.72W of power and the LED only uses 23% of the power and the rest is wasted as heat. To the human eye there is only a small difference in brightness in the LED by lowering the current thru it, so some 12VDC pixels actually run at only 10ma per LED (compared to the usual 20ma) to lower the overall power consumption to 0.36W, basically cutting the power consumption in half by reducing the brightness of the pixel somewhat.

Single RGB LED Pixel Power Efficiency
	5VDC RGB Pixel @20ma per color
	Red	Green	Blue	Total
LED Power (W)	0.04	0.064	0.062	0.166W
Resistor Power(W)	0.06	0.036	0.038	0.134W
Total Power (W)	0.1	0.1	0.1	0.3W
	12VDC RGB Pixel @20ma per color
	Red	Green	Blue	Total
LED Power (W)	0.04	0.064	0.062	0.166W
Resistor Power(W)	0.2	0.176	0.178	0.72W
Total Power (W)	0.24	0.24	0.24	0.72W
	12VDC RGB Pixel @10ma per color
	Red	Green	Blue	Total
LED Power (W)	0.02	0.032	0.031	0.083W
Resistor Power(W)	0.1	0.088	0.089	0.36W
Total Power (W)	0.24	0.24	0.24	0.72W

Instead of wasting all of the power in heating the resistor, some pixel strips actually use 3 LED wired in series for each channel as a more power efficient way to use 12VDC. By doing that the pixel still draws 0.72W per pixel, but the LED power efficiency goes up to 69% compared to 23% when they used only a Single LED! As mntioned above, by cutting the current in half, they can further reduce the power consumed by the pixel in half and therefore some 12VDC pixel strips are actually run at 10ma and only consume 0.36W per pixel of power and are very efficient.

3 LED RGB LED Pixel Power Efficiency
	12VDC RGB Pixel @20ma per color
	Red	Green	Blue	Total
LED Power (W)	0.12	0.192	0.186	0.498W
Resistor Power(W)	0.12	0.048	0.054	0.222W
Total Power (W)	0.24	0.24	0.24	0.72W
	12VDC RGB Pixel @10ma per color
	Red	Green	Blue	Total
LED Power (W)	0.06	0.096	0.093	0.249W
Resistor Power(W)	0.06	0.024	0.027	0.111W
Total Power (W)	0.12	0.12	0.12	0.36W

So as seen by the above analysis, there are many factors involved in deciding not only whether to go with 5VDC or 12VDC, but how they are wired as well and what current per pixel they use.

Different Voltage Configurations

A common pixel type uses the WS 2811 integrated circuit to control the RGB LED. This integrated circuit can run on either 5VDC or 12VDC using the appropriate internal and external dropping resistors. As discussed above, there are three common ways they are wired up.

5V Power and 1 RGB LED

In this configuration the voltage dropping resistors for the LED are integrated into the control chip and the excess power heats the chip.

12V Power and 3 RGB LEDs

In this configuration the voltage dropping resistors for the Green and Blue LED are integrated into the control chip and the excess power heats the chip. The Red LED requires a separate external current limiting resistor.

12V Power and 1 RGB LED

In this configuration, all three LED require separate external current limiting resistors and the excess power heats both the control chip and the IC.

Related Links

Different Styles of Pixels
Controllers
Dumb RGB or Intelligent Pixels??
Things You Will Need To Get Started With Pixels
Pixel Wiring Colors
Power Supplies