Sip speedo has gone wrong

I think my speedo has fried my regulator went  and my speedo will not would when the engine is running ,i can put a 12v dc supply though the main feed of the loom and the speedo lights up and appears ok .is there a reason for this ,what do i need to get the speedo back up and running .plus would it be worth putting a fuse in line to protect the speedo from a regulator failure