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PWM with timer on a 12v constant voltage power supply? Answered

I have 5 - 12v LEDs running in parallel. Each lamp has it's own heat sink, fan and resistor,. Each one steadily uses about 850ma. I'm hoping to add a timer of sorts, something that adjusts the light level on a schedule. I've read about people using something like a 555 and/or the Arduino, but I'm kinda lost. Any pointers?

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steveastroukBest Answer (author)2012-02-16

Use a MOSFET driven by the arduino pin, or by a simple 555 timer.

You'll probably have to connect the fans to the supply directly though, because they aren't going to like running on PWM at low duty cycles.

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lford4 (author)steveastrouk2012-02-16

How do I go about picking out the correct one? To save myself some trouble with the arduino sketch I'm hoping to only use one for this project at first. But I would like to know how to pick them out so that I can apply it again on the next project, which will probably be adding the MOSFET to each of the LED's for better control.

Should the source-drain be exactly 12v - 4.250 amps?

How about figuring out what the gate should be rated. Some of these are showing 20v, and don't mention anything about the pulse.

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steveastrouk (author)2012-02-16

Don't worry to much about any other part of the rating but Vgs sat, and Rds on.

The MOSFET is purely a switch here. It needs to be turned on hard, so its Rds is as low as possible. Since you only have a logic pin to drive it, you really need a "logic level MOSFET", Like an IRL 540 - The IRL indicates its a low voltage gate.

So long as the Vds max > 12V, and the maximum Drain current is >5 x 0.85 A, you're good to go.

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lford4 (author)2012-02-15

Perhaps use the Arduino to signal a NPN transistor that's able to handle the amps?

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