PWM led dmming and power supply questions
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PWM led dmming and power supply questions

by MattPete on Thu Aug 09, 2018 8:19 pm

I plan on building a color-shift led light, using an Adafruit Metro to dim and control the color temperature.

For example, if I wanted to dim down from 3200 to 2200 smoothly, I might have 3 sets of leds (3200k, 2700k, and 2200k) with 4, 2, and 1 of each, respectively. When full on, the brightness would be 4 units of 3200k. At some point in the dimming range, it would be 2 units of 2700k. The key is to figure out how to mix the 3200k and 2700k leds (and 2700k and 2200k at lower levels) for smooth dimming and color temperature shifting between those two levels.

I have all the formulas figured out for mixing the leds (including logarithmic dimming) so I get the proper color and lumens, but I have two questions about the electronics:

1) For PWM at frequencies > 1000Hz, is an Adafruit Metro sufficient, or do I need a PCA9685 breakout board?

2) Power Supply: I’m basing my plans on this mosfet circuit (https://learn.adafruit.com/rgb-led-strips/usage). I assume that I only need a single power supply, and it needs to provide sufficient amps to power the 4 units of 3200k leds (the maximum used at once). Is using a larger power supply dangerous, or is it recommended? Is there a special type of power supply I should be looking for (other than correct amps and volts)?

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Re: PWM led dmming and power supply questions

by adafruit_support_bill on Fri Aug 10, 2018 6:37 am

1) You didn't specify which Metro. But all of them are capable of frequencies well above 1KHz. The upper bound on the PCA9685 is about 1.6KHz.

2) Your supply should be rated to supply at least the required amps and volts. If it will be in continuous use and/or enclosed with limited ventilation, it is a common precaution to over-size the supply by 20% or more.

With any supply capable of delivering more than a few amps, you should size and fuse the wiring appropriately. For permanent installations, we recommend consulting with an engineer or licensed electrician familiar with your local electrical codes for low-voltage DC installations.

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