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[on:]

I want to convert a pc power supply to power my DFRduino mega 2560, but is the 12 volt rail is too high for the arduino as the upper limit it is rated to is 12V. If it is could I use a circuit like the following to reduce the voltage?
http://powersupply88.com/simple-12-volt-to-9-volt-dc-dc-converter.html

I'm assuming as a power supply is used in a pc all its outputs would be regulated, is that right?
Last question: The 12volt rail is rated as having an 8amp output is that the maximum it can output?

My arduino is arriving tomorrow so the last thing I want to do is blow it on its first use

[Reply #1 on:]

why not use the 5v output?

[Reply #2 on:]

but is the 12 volt rail is too high for the arduino as the upper limit it is rated to is 12V.

Why do you think that. The limit says 12V and your power supply gives 12V, it will be fine.

[Reply #3 on:]

Hi,

Especially if you are not drawing very much 5V current, the limit is not critical.. If it was 12.8V I do not think there's a problem...

At the relatively small current you are drawing you could put a typical rectifier silicon diode (like 1N4002 thru 1N4008 ) in series with the 12V lead which will drop about .7 volts..

[Reply #4 on:]

why not use the 5v output?

Too low

@Grumpy_Mike

I thought if the supply spiked it might be a problem

[Reply #5 on:]

No.
1) regulated power supplies don't spike.
2) The 12V limit is only based on the regulator heat dissipation issue see http://www.thebox.myzen.co.uk/Tutorial/Power_Examples.html the true upper limit is likely to be 16V or 25V depending on the rating of the input capacitors.

[Reply #6 on:]

Thanks that answered my other question about regulation. Thanks for the fast responses 

[Reply #7 on:]

Keep in mind that ATX Power Supplies generally need a minimum load for regulation.  The one I have sitting on my desk says each rail (except standby and -12) needs a minimum load of 1 amp.  Without the min load, the voltage varies quite a bit.

[Reply #8 on:]

Quote from: mmcp42 on November 20, 2011, 07:32:05 PM
why not use the 5v output?

Too low

No it's not, all you need to do is to connect it to the +5V pin not the Vin or power jack.

[Reply #9 on:]

If it is an older PSU you will likely need a load on it to keep the voltages stable.  Plug some old HDD into it, that ought to do it.  Be aware that you may need an oscilloscope to see the voltage variations, they're often too fast to see with a multimeter.

Some modern units don't need a load on them, but they're all >$100 units and slightly excessive for Arduino use.