diode question

now i've found that the 1noo4 is 400 volts.  why would my book say anything about that one?

If it's being used to clamp an inductive spike from a big relay turning off, the 400V that the 1N4004 can take without damage gives more margin than the 1N4001, at 50V. Most likely the 1N4001 would still handle it without problems though. 1N4002 can block 100V reverse voltage, and the 1N4003 blocks 200V, if you need a bit more. Doubt the application would really need 400V-

I see its just for the locks.  I was told i'll need one.  the 1n4001 should works thnx

Yeah, the 4001 should be fine. Actually, the diodes for these applications should never see more than the car voltage when reverse biased anyway, so even the 4001's 50V is more than adequate. The inductive spike doesn't reverse bias the diode, so it doesn't really factor into it. So just use whichever you have on hand, or can get the cheapest-

what about a spike when the relay turns on

The voltage spike only occurs when a relay turns off; when it turns on, the voltage ramps up slowly. Think of the current in the coil of a relay like water in a long hose, having a lot of intertia; it's hard to get flowing when it starts, but hard to stop when it gets going. When you turn to turn off a relay, the current wants to keep flowing, and that's what causes the voltage spike. Diodes can be used to protect the device trying to stop the flow, by temporarily allowing a bypass route for the current.

what am i getting. when i cut the car on the two headrest screens come on for a less than a second and shut right back off. like a flash