# Multi bay solar watch charger using LEDs.

#### Matt Bermes

Joined Nov 21, 2016
4
I am new to building electronics and I am working on a watch caddy. I own multiple watches that are charged by light sources. The problem with these watches are when they are in storage the batteries die. I want to create a watch caddy with LEDs to keep the watches charged. I want my watch caddy to have 10 watch bays. Each bay would have 5 LEDs providing the light. I know I need 50 LEDs and the specs on the ones I want to use are White: DC 3.0-3.2V / 20mA. I was thinking using a 16V power supply with these specs Input: 100-240VAC 50-60Hz Output: 16VDC @ 4.5A. I am not sure what the wattage is. I am thinking 10 strings of 5 LEDs in series. That leaves me with 16V per string using 200mA. Again I am a novice and I want this to last for years so I do not want to pop any LEDs.
Here are my main questions.
Will this power supply work?
Will I need any resistors? If so what kind?

Any and all recommendations are appreciated.

#### Matt Bermes

Joined Nov 21, 2016
4
I just found out that the power supply is 72W.

#### Matt Bermes

Joined Nov 21, 2016
4
I just found this power supply. Would it work?
ROBERTSON 3P30010 LD012C050LRAP LED Driver, 12 Watt, 120Vac. Input, 500 mA Constant Current, 5-24Vdc Output, High Power Factor, Forward Phase Dimming 100% - 5%

#### blocco a spirale

Joined Jun 18, 2008
1,546
A 72W PSU is overkill for this project, it would be better to opt for an inexpensive and compact 12V 1A wall-wart and power clusters of 6 LEDs made up of 2x 3 LEDs in series.

12V - (3x 3.2V) = 2.4V

R = 2.4V / 0.02A = 120 Ohm

The total power consumption will be 12V x 0.02A x 20 = 4.8W

So that would be 20 strings of 3 series-connected LEDs, each string shall include a 120Ω 0.25W resistor.

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