Smart Garden – Service Lights

Wiring in LED Strips for Bright Work Lighting

I never throw out any electronic device without at least harvesting its transformer. No matter how old it is; lead solder blah blah blah. Transformer, power supply, driver, wall adapter. All names for the same thing. I recommend you always collect them as they are all useful gems.

Good work lighting is critical. Not the grow lights for the plants, which put off lots of red and blues. Which are great for plants, but poor for the human eye. But pure bright whites that will let me service my garden with a crisp well lit view of what I’m doing. Lucky for my garden, I have several dozen feet of LED strips leftover from lighting installations for customers. I decided that running LED strips on the underside of the shelves’ supporting braces would give me the most distributed and unobstructed lighting for my laboratory. This means 16.6 feet of led strip.

Some Electronic Background

Before wiring any circuit together you should understand this very basic principle:
—Volts x Amps (Current) = Watts
Watts are a cumulative measure of the actual power being used by a circuit. When you pay your electric bill you pay by the Kilowatt Hour Kwh. Typically 20 to 40 cents per Kwh.

To build and power a circuit you must use a power supply that matches correctly the voltage required by the device, and one that provides at least the amperage consumed by the circuit. Remember, when working with electronics that it’s the amps that kill you! A transformer putting out just 1 amp can hurt you very badly.

Building the Service Lights

Invariably, all LED strips operate at 12v (rare exceptions exist), so we have this important number as a given. The strips I’m using draw 2.6Watts per foot according to their label. They also incidentally put out 4000k light, meaning a nice ‘cool’ bright white, but not so white as to make you dizzy like a 5000k or above would. At 2.6Watts per foot my lights will draw 43.16 Watts over their 16.6 foot length (16.6ft*2.6). To determine what transformer to use I now have to figure out my amperage. I reverse the equation above:
–43.16 Watts) / 12 Volts = 3.6 Amps
I require a 12 V power supply putting out at least 3.6 Amps. Take note that every wall adapter will always have its input and output written somewhere, even if it’s in tiny fine print. Problem is, I don’t have any 12V power supplies at the current rating and I’m not about to buy one. 12V LED Strips on Parallel Circuit.. it works! What I did have was a 36Volt 1.6Amp transformer from something or other.  That’s 57.6W, just above what I need.  If it were possible to wire three strips in parallel I could use the 36V transformer I have on hand.

12V LED Strips on 36V Power Supply… it works!

LED Strips come to you wired in parallel which makes this a little tricky.  If you do a google search for wiring LED strips in parallel you will read a lot of naysayers, a couple people who say it would fail because of uneven heat distribution, and as I recall no photos of anyone actually succeeding.  But I’ve wired dozens of series-parallel circuits successfully in gizmos and gadgets, I know it’s possible!
So I built a demo on the back of a piece of molding.  I cut three strips at very uneven lengths, wired their ends in parallel and connected the ends of the circuit to the 36V power supply.   Success!

Series-Parallel LED Circuit

The LEDs came right on, nice and bright. My shortest length of LED strip did get very hot almost immediately. Therefore, it’s very important that if wiring led strips in parallel each strip MUST be the same length.

Bill of Materials

-20 feet of four conductor wire (telephone wire)
-3 LED Strip Connectors
-36V Power Supply
-16.6feet of LED Strip
-Soldering Tools

To get three equal LED strips across only two shelves was a bit tricky. I measured and cut where needed, using the four conductor wire to emulate three strips in parallel (really 6 pieces) as well as form the 36V series connection.

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