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MANCHESTER
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I am using an uno for a modification in work. I have everything sorted apart from powering my arduino. On the machine I am making the mod on, there is a power rail where I can get access to either 5V or 15V dc. I also have a centre positive adaptor which fits the barrell on the uno unit. Can I wire the adaptor to the 15V supply or will I have to make a voltage divider for the 15V? I have read the 5V supply will not be enough as it could lead to the unit becoming unstable.
Thanks for looking
Dave
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Just wire the UNO directly to the 15V.  The on-board regulator should be able to handle that as long as you do not have too much other stuff being powered by the the UNO.
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The on-board regulator is actually rated for 20V.  However, its only heat sink is the PCB, so there is a lower practical limit you can use. 

The concern is how much current your 5V node (pin) is drawing.  What else is connected?
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"I have read the 5V supply will not be enough as it could lead to the unit becoming unstable."

That seems wrong. If your 5 Volt supply is well-regulated and can supply the current, it should work fine. But you don't use the barrel connector with 5 Volt power. It goes directly to the board.
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"I have read the 5V supply will not be enough as it could lead to the unit becoming unstable."

That seems wrong. If your 5 Volt supply is well-regulated and can supply the current, it should work fine. But you don't use the barrel connector with 5 Volt power. It goes directly to the board.

Yes, putting 5V on the barrel connection will cause the on board regulator to be unstable,

It isn't clear I'd that is what the OP intended.
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I'm not sure what Dave ("substance") had in mind either. But it sounded like he has 5 Volts regulated power available, yet planned on using 15 Volt power instead. My suggestion was, if that is indeed the case, to use the 5 Volt power.
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Dave, you probably know this, but just in case you do not.

If you plug the 5 Volt power into the barrel connector or the Vin pin, the power will go through the voltage regulator and will not be high enough voltage. You need at least 7 Volts to get a stable 5 Volts out of the voltage regulator.

If you plug the 5 Volt power into the 5V pin, the power will bypass the voltage regulator and will be high enough voltage to work well. Be sure to connect the grounds as well.
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MANCHESTER
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Thanks for your advice.

It is the first time I have used an arduino in a real world environment so I want to get everything right.

The ardunio is going to monitor how long gasses are flowing on a varian implanter. There are no other electronics involved. Just 5 inputs into the chip and when any of them go low, I want it to give me a high on one of the 3 outputs. The outputs go to timers and these are battery powered.

The supplies are regulated so there are no worries there.

Sorry for not being clear with what I wanted. If I remember correctly fron the ardunio spec, it said that the input power ranged from 5-20V dc but it recommended 7-12. I just wanted to be on the safe side and go with the 15 or come up with a voltage divider to bring it down to the recomended levels.

Thanks for your help

Dave
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I just wanted to be on the safe side and go with the 15 or come up with a voltage divider to bring it down to the recomended levels.

Voltage dividers for "power", high current, or varying current situations are never a good idea.  Voltage dividers are only good in situations where the divided voltage is a reference, not a source.
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Even at fairly low current using power supplies greater than 9V can result in the voltage regulator failing in time...

I did some testing with different power supplies rated between 7 and 20 volts and a fixed load of 0.25 mA (a 4 by 20 LCD)...

20 volt supply: 3.75 Watts @ 250mA
15 volt supply: 2.5 Watts @ 250mA
12 volts supply: 1.75 Watts @ 250mA
9.0 volts supply:  1 Watt @ 250mA
7.1 volts supply:  0.525 Watt @ 250mA

To be on the safe side connect up your power supply and measure the current drawn by the board...
Multiply the current time the voltage drop across the regulator (the supply voltage - 5 volts) to see how much heat is being dissipated...

It appears given a proper heatsink, a maximum rating of 7 watts can be achieved (but for how long?)
IMHO, it will fry long before you reach these levels... I suggest you keep it below 1 Watt and use a 7 or 9 volt power supply...

If you draw less current you can probably use a 12 volt supply but if using 20 volts, I hope the current is less than 60mA...

The device is probably capable of dissipating a lot more but why take the chance...
I used 12 volts with a load of 0.25mA for 5 minutes and could smell the heat... I almost burned my finger too... smiley-wink
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"If you plug the 5 Volt power into the barrel connector or the Vin pin, the power will go through the voltage regulator [ and the reverse polarity protection diode ] and will not be high enough voltage"

Boards run fine when powered from the 5V/Gnd pins on the power header, especially just  hi/lo levels as descibed.
Be sure to connect the grounds.
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