power supply and wiring for 12V project - Page 1
Forum section: electronics / beginners

author: paulskiogorki
author_rank: Newbie
timestamp: February 20, 2019, 12:42:01 pm
content: HelloI'm a woodworker with really minimal technical knowledge about electrical and electronics. I am blue skying an idea about incorporating electronics into some wood lamp designs I have.One idea is to make a lamp that includes LED strip lighting and also a spot to wirelessly charge your smartphone.I'm not sure how I would go about determining power supply requirements, and if any special wiring or circuitry is required.For example, I might include:- a PCBA Circuit Board and Coil (a Qi wireless charger). It has 5V output and Input:12-24V/2.4A. I have used these by themselves before and connect to a USB cable to wall wart or computer.- a length of 12V LED strip light. Stuff I might use is consuming 500mA/metre.So supposing I would have 1m of LED strip in the light plus the Qi charger, how would I choose a power supply? Is it simply a 12V adapter with at least 2.9A (2.4A + 500mA)?And, what is a good way to manage the wiring inside a small lamp assembly, i.e. splitting the power to regular DC and USB cables? Is there some sort of junction box or can I just cut open the cable from the adapter and solder on wires to the two power using items?ThanksPaul

author: mariush
author_rank: Super Contributor
timestamp: February 20, 2019, 01:16:09 pm
content: A QI charger will probably be around 75% efficient at converting the DC energy into "air", to charge phones. For safety and simplicity, I'd say just go with 50%. So if the device says it can charge phones at up to 1A, then it means it produces 5W of energy. If we go with the 50% efficiency of conversion, that means the device would need 10 watts to produce that energy.Since the charger says it can work with 12v ..24v, then it more likely means it has a switching power supply inside, which converts a higher voltage to 5v, with a certain efficiency, let's say around 90% or better.So with 12v input, you're looking at 10w + something for the conversion losses in the switching power supply ... so you could just say 12v 1A (12w)LED light strips ... in a light strip, you'll have groups of 3 LEDs in series, limited to a certain current by a resistor. Depending on the LEDs used and the resistor value, each group of 3 leds will consume an amount of current, anything between 20mA and 100mA, but typical values are between 20mA and 50mA.You can sort of approximate the current set by the led strip, using the classic Ohm's law formula: Voltage = Current x Resistance ... so (12v power supply voltage - 3 leds x 3.3v average forward voltage of a white led) = Current x Resistance ... so Current = 2 / Resistance value ... if you see a 100 ohm resistor, then your current is 2/100 = 0.02 = 20mASo " a meter" doesn't say much, how much the strip would consume in total depends on how close together the leds are, how many groups of 3 LEDs you have on the strip.My suggestion would be to use a barrel jack connector, either one that goes on a circuit board you make, or one that you can mount on the case.This way, you can simply plug an external wallwart / adapter into the lamp and inside the lamp, you can solder two pairs of wires to the connector (or have two pairs of traces on the circuit board)Here's pre-selected connectors with 2.1 or 2.5mm inner diameter and 5mm or 5.5mm outer diameter (the most common barrel plugs used) :https://www.digikey.com/short/p312j3Here's only the connectors that you can mount on case :https://www.digikey.com/short/p312pmtypically a led will consume around 15-20mA per color, so a RGB led strip will consume around 50mA per LED. A plain white led strip may consume 20-50mA per LED, depending on brightness.However, LEDs have a voltage between 3v and 4v, so 12v led strips have series of 3 leds grouped together to run at 12v, so each group of 3 leds in the strip will consume 12v and around 50mA. I don't know how many leds are in a meter, but you can simply count the leds and you can get an idea of how much power you'll need.

author: paulskiogorki
author_rank: Newbie
timestamp: February 20, 2019, 04:55:43 pm
content: Excellent info and explanation. Thanks a bunch