I'm trying to modify a Arduino Uno R3 to run on 28 V Nominal, 18-34V supply input. I'm changing the linear regulator to a LM340MPX-5.0/NOPB and changing RN1A from 10 k? to 54.9 k?.
I haven't built anything yet, but I was hoping to get some feedback on the modifications before testing.
If that's the power supply you have to use for whatever reason, why not reduce the voltage before the Arduino, and put its usual 7-12 in as it expects.
To be able to use 28V, you need a DC-DC converter. Some can go up to 23V, so you need a newer and better one that can go up to 34V input voltage. For example one with the LM2596.
The LM340 datasheet states a 20W max (with infinite heatsink) dissipation. So if you're taking 28V to 5V that's a 23V drop. Multiply 23V by however much current you'll be pulling through it and that's how many watts of heat you need to dissipate.
An Uno board's heat dissipation is certainly less than 20W. For a passive heat sink (no fan) you'd need something the size of a CPU heatsink to do that. All in all you'd be much better off using a regulator like Caltoa has linked.
I should be able to dissipate the heat. I'm designing the board, and the layer it's going to be connected to is actually 2 oz copper. I will verify the pad below has enough area to heatsink for 1 A.
Assuming it doesn't overheat, are those two component changes enough?
Some background, there is already an Arduino Uno R3 with a Sparkfun CAN-BUS shield working. I'm tasked with replacing the existing setup with single custom board.
I would use a step-down converter in front of the Arduino, like this: http://www.pololu.com/product/2108
It is good to 42 volts input and would be far more efficient than a linear regulator.
The more I think about it, the more I like the idea of using a step down DC-DC converter before the normal on board components. It adds an extra layer of voltage conversion, but it let's me use 'stock' Uno components all around.
Welcome to the DC-DC converter enthusiasts' club.
They also reduce the current that is requested from the higher voltage. If the Arduino uses 9V 200mA, the DC-DC converter might use 20V 100mA. Sometimes I think I can do without a DC-DC converter, but end up using it anyway to reduce the generated heat.
drbuild:
I should be able to dissipate the heat. I'm designing the board, and the layer it's going to be connected to is actually 2 oz copper. I will verify the pad below has enough area to heatsink for 1 A.Assuming it doesn't overheat, are those two component changes enough?
Some background, there is already an Arduino Uno R3 with a Sparkfun CAN-BUS shield working. I'm tasked with replacing the existing setup with single custom board.
1A? At 28V in 5 out? You won't dissipate anything remotely like 23W on 2oz copper.
You'll need aluminium circuit board and forced air cooling to even begin contemplating
that - some soldering irons give out less heat than that.
You should probably be using a 24V -> 5V converter (much less waste heat, much
less current taken from the supply).