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Schematic:

enter image description here

I am making some light fixtures controlled by an Arduino UNO. Each fixture will host two approx. 1 meter long analog RGB LED strips with a current draw at full power of ~1.2 A per meter. I have a 12 V power supply that I will wire directly to the strips, with the RGB channels wired to the Ardiino PWM pins via NPN MOSFET (note in the schematic I've only shown one color channel for simplicity, but there would of course be three transistors hooked up to three PWM pins). The Arduino itself will be powered by USB.

My main difficulty in wiring the system is that for so much current I need fairly thick wire. The pads on the LED strips are delicate and I have several times damaged them by mechanical stress from the wire while soldering. The thick wire also crowds the narrow space between the pads creating a risk of a short. Therefore my first question is (bear with me, I'm pretty new to this):

1) How should I wire the LED strips?

My thought is to wire up the strips in parallel with thinner, more flexible wires. This brings up two sub-questions:

1a) How should I join the thinner wires to the thick wire coming from the power supply?

1b) At the junction between the wires leaving the lights and running to the transistor, does the wire need to become thick again? (I think the answer is yes – the current should be the same as coming out of the power supply.)

I also am unsure about the ground.

2) Does the wire coming from transistor to ground need to be joined to the ground pin on the Arduino or can it run directly to the ground terminal of the power supply?

Sub-question: 2a) Does the wire coming from transistor to ground need to be thick?

I don't really understand how transistors work (basically my understanding is that the Arduino uses a small amount of current over the PWM pins to somehow control how much current is allowed to flow through the rest of the circuit.

I've drawn a schematic showing how I think maybe this system needs to be wired up. Do I need to make changes?

  • You need to connect the two grounds. Otherwise there isn't a path for the electrons to flow from to Arduino to the mosfe, and back! – Gerben Aug 23 '16 at 8:44
  • Thank you! No one else has addressed that question yet. – Dylan B Aug 23 '16 at 14:43
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It's all a question of losses.

Wire is basically a resistor. The thinner and longer the wire the higher the resistance. Current flowing through a resistor causes a voltage drop across the resistor.

Long thin wires with large amounts of current through them will basically mean you get less than your 12V at the LED strips. The question has to be: how much less a voltage is tolerable?

Each strip takes 1.2A. Take for example 24AWG wire. That has a resistance of 0.0842Ω per meter. With 1.2A flowing through a meter of that you get 0.10104V dropped across the wire. That turns your 12V into 11.9V.

Conversely, a meter of 14AWG wire has a resistance of 0.00829Ω - so you would lose 0.009948V over that length.

Now you want two strips - which is double the current, and thus double the voltage drop. On 14AWG you would lose about 0.02V over a meter. Not really enough to worry about. But you can't connect 14AWG to the LED strips. You can connect thinner wire (say 24AWG as above) to them. But that looses too much voltage. Well - over a meter it loses too much voltage. But what about over, say, 10cm? That's a 10th the length of before, and so a 10th the voltage loss: 0.01V.

So what is to stop you having a short length of thin wire connected to the LED strips, which then connects to a longer length of thick wire? Answer: absolutely nothing.

And how do you join them together? Well, I find the best way is with solder. Insert the thin wire (stripped) into the end of the fat wire (stripped) and twist them together (axially) then flood the joint with solder.

enter image description here

  • Is there any heat dissipation risk? – Dylan B Aug 23 '16 at 14:42
  • From what ... ? – Majenko Aug 23 '16 at 14:43
  • Resistive/Joule heating in the thin gauge wire. – Dylan B Aug 23 '16 at 14:45
  • Not at those lengths / currents. P=I²R = (10cm wire) 1.2²x0.0842 = 0.121W – Majenko Aug 23 '16 at 14:46
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I think you are overestimating the size of wire required to attach to the LED strips, I have connected a lot of single color LED strip (in excess of 1mtr) with the wire that was already attach and it was reasonably light gauge. Look at this Example on Ebay and you can see what i mean.

I am note an Electrical Engineer so i don't know the method to calculate the minimum wire gauge, but if in doubt contact the manufacturers and ask them or search on the web.

As for the Transistor mounting/wiring you would mount that on a small circuit board with terminal block fitted for ease. Any power/connecting wire should not be less that used to the LED strip.

Also you will need a current limiting resistor attached to the Base pin on your transistors. You will have to work out the value of that based on the spec sheet.

There is a bit of learning required first, once you have researched the above you will have you exact answers.

  • For mosfets the base resistor isn't strictly necessary. To calculate the wire gauge you have to measure the resistance. After that combined with the max current you can calculate the voltage drop across the wire, and the power disappated in the wire. – Gerben Aug 23 '16 at 8:43
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    @Gerben It depends on the gate capacitance. Too much gate capacitance can cause too much inrush current which can cause voltage droop on the pin. With big MOSFETs often come big gate thresholds. If the droop drops below the threshold the MOSFET will fail to switch properly and you will get large switching losses. By including a small resistor you limit that inrush current and thus avoid the droop. I like to include a 100Ω resistor in all my gates to be safe. Yes, it slows down switching slightly, but helps protect the pin from stress. – Majenko Aug 23 '16 at 9:21

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