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We have about 170 feet of LEDs (12VDC, 2.88W/foot) wired up in one chain. Currently they are driven by two switching power supplies (12V/27A).

We'd like to control the whole chunk of them together, dim them via an Arduino's PWM output.

Most dimming / extensions boards out there are very limiting in their load. See Sparkfun Mosfet board 10256 - it has a Mosfet ready for up to 30A, but PCB tracing and components limit it to a usage of 4A. Or this 4-channel Mosfet board - each channel can sustain 10A, but the power input terminal can only sustain 16A.

Is there even a way to wire the whole current for all LEDs through one channel (=one Mosfet?), or is this impossible due to component limitations? Could i use for example several of these smaller boards and then simply add their outputs together and feed the whole LED chain with one input? Or are there any hardware devices out there that are build to sustain higher currents (like wall dimmer switches, those seems quite sturdy...)?

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That 4-channel board looks so cheap it would be worth customizing it for higher current. Add a beefier input connector, and maybe solder some solid copper wire to the traces that route power or ground to the output connectors. The web page you linked even suggested replacing the power connector for higher-current use.

That 170 foot antenna you're using should be good for broadcasting radio station "PWM Arduino" to the neighborhood :-)

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So the total current is 27A? That's easily switched by a MOSFET or MOSFETs in parallel.

I don't know of anything off-the-shelf that will do this, but a 1mOhm FET would dissipate about 3/4 watt with 27A of current. With a reasonable heatsink that's certainly a possibility.

If the LED string is grounded and you have to switch the high side you would need a FET driver with a bias supply that's far enough above the 12V to get good enhancement. (5-10V above the 12V rail)

Or, you could parallel a couple of P-Channel FETs, and pull the gates to ground to turn the LEDs on. (P-channel FETs have higher RDSon for the same die size, so they are more expensive and typically the best are still higher RDSon than the best N-FETs.)

If you can put the FET on the low side (grounded source, LED string connected to drain) then you can drive the gate directly with 5-10V. Gate capacitance will be high, so you will want a gate driver between the Arduino and the gate.

If you find a TO-247 FET, you could probably build the circuit with point-to-point wiring and a copper clad board for the driver.

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