Power supply ripple rejection

So far my Arduino has only been powered via a USB port, but something more permanent will soon be needed.

If I were to full-wave rectify the output from a "12V" mains transformer and provide basic smoothing with a 63V 100uF capacitor, would this be OK for powering an Arduino, or do I need to provide a voltage regulator?  And if the latter, should it be 9V or 5V?

Sorry for the basic question -  better to ask first rather than regretting later!

 

 

Robert Wall's picture

Re: Power supply ripple rejection

Yes, you need a regulated supply, or you risk exceeding the absolute maximum ratings of the Arduino. A "12 V" mains transformer is probably too high, bearing in mind it will be running at very light load and the voltage will be in excess of 12 V, and the rectified peak voltage might exceed 20 V in the worst case.

You ideally want to end up with 5 V to the Arduino. The absolute maximum rating is 6.0 V.

(You might find it cheaper to buy an adapter - http://shop.openenergymonitor.com/power-supplies/   Beware, some users have reported problems with 'cheap' USB adapters).

calypso_rae's picture

Re: Power supply ripple rejection

Thanks for the warning Robert, but I'm still puzzled about these basics.

I wish to power my Arduino via its DC adapter socket, not the USB. http://www.arduino.cc/playground/Learning/WhatAdapter  says that I will need a 250mA dc supply of 9 to 12V.  This voltage is presumably stepped down by the Arduino as required. 

The AC power-controller that I'm using requires 24V AC, and I already have this available using two '12V' ac transformers in series.  I'm intending to divert some rectified power from one of these to supply the Arduino.  My question is whether I need to include a voltage regulator for this purpose.  Seems that I should.  I'll call by Maplin this afternoon and buy a selection of useful looking bits.

Is there a complete description anywhere which covers every pin on the Uno?

Robert Wall's picture

Re: Power supply ripple rejection

What you are asking is a bit like "How long is my piece of string?". What you have not said is whose kit/circuit diagram etc you are using. Without knowing what other components are around the processor IC itself, it is very hard to give you any reliable advice.

calypso_rae's picture

Re: Power supply ripple rejection

While rummaging around, I found a centre-positive' 200mA 9V dc power adapter which appears to do the trick.  Using this item, the example sketch, "blink", works fine.  My Arduino can now be powered via USB for upload and serial comms purposes, and via the 9V adapter for unsupervised duties in the garage, hopefully while heating our water.

Thanks for your help.

Robert Wall's picture

Re: Power supply ripple rejection

In that case, there is probably an on-board regulator!

"Is there a complete description anywhere which covers every pin on the Uno?"

 "Arduino Uno pinout" ->Google turned up a lot, but I don't know which is relevant to you. If none of that is, I think you'll need to relate the circuit diagram of your pcb to the pinout of the processor itself to find that information from the Atmel Data Sheet.

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