LED strip power supply capacity with dimmer
Question: I have two reels of your 2700K 12V 95 CRI LED strip lights. I will be using the entirety of both reels, so a total of 33 feet, which based on my calculations, will require 178 Watts at 12V (5.5 watts / ft x 33 ft). I plan on connecting the system to a wall-dimmer to reduce the brightness. If I don't ever plan on exceeding 50% on the dimmer setting, can I get away with using just one power supply, even though it can only supply a max of 120 watts @ 12V?
While we completely understand the the logic behind this, the LED strips and power supplies do not work in this manner. The LED strips operate at a fixed power, voltage and current, so there is no way to adjust this, even if the LED strips are used with a dimmer. As such, you will need two separate power supplies for your two LED strip reels.
We know this is counter-intuitive, so below is a technical explanation of why this is the case.
If you operate the LED strips at a 50% dimmer setting, you will indeed get 50% of the brightness and use only 50% of the power. This is accomplished via a mechanism called PWM (pulse-width modulation). In PWM, the light output is actually "chopped up" into short bursts of on/off periods.
During the "on" period, the LED strips operate at 100% power, and during the "off" period, the LED strips operate at 0% power. The power supply adjusts the perceived brightness level by adjusting (i.e. "modulating") the proportion of "on" vs "off" time. Even at a 50% dimmer setting, the LED strip will draw its maximum power during 50% of the time period, and will cause the power supply to exceed its power capacity.
Below is on the product page, you will find the wiring diagram for the multiple power supplies connected to a single dimmer switch: