why a Relay?

[phil on a fazer]

now....go easy on me! 
 
im going to (try &) fit a secondry fusebox to both my bikes to run satnavs , heated grips, lights and so on. (ive been putting this off for SO long)
 
I understand switched lives but im a bit confused on relays. (i know what they do)
 
On the Fazers fusebox there is a spare switched live. My orginal plan was to get a cheap halfords blade style fusebox and connect it via this spare switched live. i would then connect all my accessories to this new fusebox and they would all be switched live.
 
So in my head.....my "secondry" fusebox is only powered when the ignition is on....which is what i wont.
 
but...every one (the people who know what there doing!) keep banging on about Relays. So looked on the net....got more confused....adapted a drawing off another site and now im here 
 
So.....is this drawing correct?
 
If so.....why the Relays?    in the drawing it shows another relay on the driving lights...why?

[Fazerider]

Relays can be used for various purposes: in this case it's providing a shorter, lower resistance path for the current to get to your accessories. It also has the benefit that it reduces the current that the ignition switch (and anything else in the switched live supply) has to handle.
I think the additional "relay" in your drawing is just a switch so you can turn off the driving lights independently, though it could be a relay if you want to keep the high-current path as short as possible or could be omitted altogether if you don't mind the driving lights being on all the time your switched live is on.
 
Relays can also be used to provide logic functions. For example, the original Fazer headlights had a left side dipped beam that was on all the time (if the lights were switched on) and a right side H4 bulb that only made use of the main filament. Since the circuit diagram in my manual omitted the fact that there was a switching dipped supply available in the connector box under the tank, I derived my own using a relay and the existing headlight feeds so the dipped filament in the right hand lamp came on when dip was selected and went off when main beam was needed.

[phil on a fazer]

Relays can be used for various purposes: in this case it's providing a shorter, lower resistance path for the current to get to your accessories. It also has the benefit that it reduces the current that the ignition switch (and anything else in the switched live supply) has to handle.

I think the additional "relay" in your drawing is just a switch so you can turn off the driving lights independently,

Yeah i think your right about the driving lights, think thats you so you can have them switchable.
 
i always thought Relays boosted Current??    thats why you have starter relays and fuel relays. These things need a larger current, so smaller current is sent from the battery to the relay that then boosts it to turn / work the starter motor or fuel pump???
 
I guess thats why i need a relay between the switched live from the main Fusebox to the 2nd fusebox - it helps to boost the current???
 
cheers

[Lawrence]

A relay is just a switch, doesn't boost current at all.  It lets you use a low current/voltage circuit to switch on a high current/voltage circuit.
 
For example, on a car the starter motor will have very thick cables running from it.  You don't want to run those fat cables into the cabin up to the ignition switch, so you use a very low current circuit with small wires to energise a relay, which completes the circuit for the starter motor.  Or you could use a 12v DC circuit to energise a relay and switch in a 240v AC circuit.  It's just an electrically triggered switch.
 
It might be better in this situation as you're running lots of stuff (including heaters) off one connector in the fuse box.  You could use this existing connection to energise a relay which will switch on a secondary circuit running the GPS, grips etc.

[garysimmo]

http://foc-u.co.uk/index.php/topic,4819.0.html


gives you the choice of switched and unswitched

[moffmeister]

The reason I used a relay was to avoid inadvertently leaving something switched on - which I did do when I first "lashed in" my heated grips to see if they worked because it was cold and then left them that was for next 6 months - luckily I got he flat battery at home.


So there you are.  I eventually did it all the right way with a relay (energised by a ignition swithed live). 


I used on of these as the "auxiliary fuse box" http://www.daqstuff.com/400122_pat_10-4_w_transzorbs.htm

[moffmeister]

see here too: http://foc-u.co.uk/index.php/topic,4819.0.html