@IanDSmith
Actually, the Founders got the idea from a quite perverse understanding of the way that the U.K. government worked in the early 18th century. The "separation of powers" came from Montesquieu's understanding of the way that the U.K. worked.
But it didn't work that way at the time, and we didn't in fact realize what (others thought that) Montesquieu thought that we had until 2005 when the Lord Chancellor was *finally* split into its legislative, judicial, and executive parts. Even now, we are still a monarchy and the "separate" powers still emanate from one person.
Moreover, the Founders did anticipate such things. They wrote about "tyrants" quite a lot, in fact. Remember the "refreshing the tree of liberty with the blood of patriots and tyrants" thing? That was old Tommy son of Jeff. There's lots of stuff from that time about mad tyrants (including George 3, who actually was mentally ill in some way) and preventing them.