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Hey everybody,

Building my first Arduino project and I would like to supply power to my relays and devices from something other than the arduino. I have an old lenovo AC adapter that outputs 20v at 4.5 amps. The regulator (round 1.5 inch barrel in the cord) on the AC adapter was frayed and sparking. I cut the regulator(?) off the cord and would like to replace it. Can anyone suggest a what part number I should replace it with?

Once i get the AC adapter back up and running I was planning on using a 7812 to provide 12v to some devices and then splitting that off with a 7805 for all the 5v devices. Will that work? Does that sound like a realistic plan? Thanks!
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Ontario
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If you're taking about one of those black bricks with a figure-8 mains plug on one end and a cable that you plug into your laptop on the other end, then the "lump" near the end of the laptop cable is not a regulator.  It is a ferrite bead designed to reduce RF noise being transferred into the cable from the laptop.  For your application you likely do not need it and you can safely do without.
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Hey gardner, thanks for the reply. It is a black brick (it was cheap and handy), but I'm open to a better suggestion.I put a MM to the wire (red and black probes to the same wire) before removing the ferrite bead and was able to read 20.1V, but after removing it I get 0v? Any idea why that might be? That's what made me think it was a regulator.
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Poole, Dorset, UK
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Cheaper and quicker to get a new on than to fix that one.

Mark
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Not sure what "regulator" you are talking about (it could be the ferrite toroidal).

In general, what you are trying to do is doable. Most of thsoe adapters produce an output that can be further reduced / step-down to your desired voltage.

You can get a plug (at Radioshack for example) to add for some convenience but that's not absolutely necessary.
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before removing the ferrite bead and was able to read 20.1V, but after removing it I get 0v? Any idea why that might be?
It probably "sparked" (shorted) one to many times, and it could be dead. smiley-sad   (An internal fues may have blown, but those those things often have to be cut-apart and epoxied or Bondoed back together... Not really recommended...)

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Once i get the AC adapter back up and running I was planning on using a 7812 to provide 12v to some devices and then splitting that off with a 7805 for all the 5v devices. Will that work? Does that sound like a realistic plan? Thanks!
If you can get voltage out of it, Yes... Maybe...   The 78xx series are linear regulators rated for about 1 Amp maximum.   And since they are linear regulators, the more voltage you drop across them, the hotter they get (at the same current).    That means you have to watch the power dissipation (Voltage x current = power) as well as the current (you can overheat it at less than it's maximum current rating), and you may need a heatsink.
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Thanks for all of the great input from everybody. Im fairly confident at this point the black box has shorted out. My ultimate plan was to to use the black box, regulators (78xx), terminal blocks (12v and 5v), and a ground terminal block (spring loaded) to provide power where ever it was needed. I'm looking for something a little more permanent than a breadboard type design that can live in a nema box(20x20x9). I thought the black box would be great glued/double taped to the door being so low profile. Can someone suggest a better setup?

I know i'm really new at this and Im doing my best to read everything I can get my hands on to learn as much as quickly as i can so I don't ask too many dumb questions. thanks for bearing with me. 
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