I'm pretty new to electronics, capacitors especially. I'm building circuit with a 12v battery and a small RFID instrument. The RFID instrument uses 7-18v and is internally protected from voltage spikes. I have a 1 amp fuse off the battery to the line power of the instrument, a ground, and a communication lead to the rest of my project. What I'm most confused about is on this diagram I've been using they have a "backing capacitor" between the line power fuse and the line leading to ground. What is this for and where can I find a capacitor for this circuit? Again the circuit is 1A 12V and thus 1W.