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I have no vocabulary for this, so forgive my clumsiness in explaining.
I need to hook up a power supply to use with solenoids and an Arduino, but I'm not sure where to cut it. Just before the power plug there is a thing (not sure of the name, bottom right of picture in the link below), do I need to cut before or after it?
Here is a link to the picture, I'm a new user so I can't embed it.
If you know the name of the mystery thing, I'd also love to know.
That thing is a Ferrite Filter. It's there to dampen Electromagnetic Interference (EMI) or high frequency noise from your power lines from interfering with your circuit. If you're not doing audio (or radio) work or you're not picky you can cut it off. If you are and it gets in the way, you can cut it off and add a ferrite bead elsewhere in your circuit.
Better than cutting it would be to add a proper plug to your circuit, but you've probably already considered that. If you're using a breadboard, this type of terminal block works well: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail&name=ED2580-ND
A bit of warning: That looks like a laptop power supply, so it's probably capable of sourcing a fair amount of current. It's a little more powerful than a typical "wall wart". Be careful not to short anything, and don't work on your project while it's powered up.
The thing just before the power plug is a common mode choke. It is used to reduce EMI that could be conducted on the power cable.
If you can leave it in that would be better, but if it gets in the way, or the cable isn't long enough just chop it off.
