noob to RELAY questions

[HatchetMan]

Let's just assume I know practically nothing about them but keep reading about them :redface:
Like this one- http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=330-070

At what point do you need them? How many? Where to connect :confused:

With a Pioneer headunit, if I have 2 amps - (1) 4-ch Alpine for the front and rears (200W RMS total) and (1) 2-ch Fosgate for the sub (bridged at 4ohm mono, so 200W RMS total). Do I need a relay(s)???

I know I would need a relay if I threw in some fans, but why is this one so expensive- http://www.crutchfield.com/S-gmcBILaPaya/cgi-bin/ProdView.asp?i=120E5000&search=120E5000

Now what if I throw in a 120 GB 5400 RPM hard disk drive based MP3 player that has very sensitive circuits and so want a relay to give it power say 5 to 8 sec after I turn the car on and power off after roughly the same amount of time after I turn the car off? Which one/how?

Anything to be cautious of? Thanks :)

[southpawboston]

ha! that $15 relay at crutchfield is a RIP OFF. it is no better, or functionally different in any way, than that $2.19 bosch relay at partsxpress!!! welcome to the world of marketing, my friend! but to be somewhat fair, the crutchfield one does come with the pigtail harness, which is worth a buck or two...

you don't need a relay if you are only hooking two amp remote turn-on leads to your head unit.

the relay shown can not do what you want for the mp3 player. all that this or any standard relay can do is "relay" a signal to connect a separate power source. what you need is a pulse-timer relay. what that is is a microprocessor-controlled relay that can be programmed for the delay that you want, before it "relays" the signal to connect the power source. that will cost more like $25. the PAC-TR7 pulse timer relay can do exactly what you want for the mp3 player...

[HatchetMan]

I was hoping you would chime in spb, thanks!
Looks like partsexpress also has the pigtail for another buck fifty-
http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=339-075
Maybe I'll go that route if I do the fans (which I probably should since I am going to put the 2 amps under a false floor in the hatch with no other ventilation). I'll have to look into that pulse timer relay, couldn't find it on partsexpress, looks like ebay has the best deal.