Kill switch and Ignition wiring

[MK1_Oz]

OK I am confused now. Looking at my pretty picture above, doesn't the current used to energise teh solenoid (ie engage the starter motor) come direct from the battery? The current going through the switch is NOT the same as the current energising the solenoid?

If it's a starter switch it should be rated for the load.

I still assume the load through the switch is negligible?

[exboyracer]

OK, look at it this way.
The solenoid on the starter motor is, in effect, a bloody big relay. It has a set of contacts which switch the heavy current to the starter motor, and a coil which creates the magnetic field to move the switch contacts.
Now, because the switch contacts are large, the coil needs to be pretty beefy too in order to generate a large enough magnetic field. So, it draws more current than a 'standard' automotive relay would, especially when it is energised and you get a large inrush current (though obviously a lot less than the starter motor does).

So, you need to make provision for the current draw. Don't assume it's the same as a standard 40 amp relay because it isn't!

[exboyracer]

A little bit of internet research shows the starter solenoid can draw around 15 amps, so definitely not negligible!

http://www.aeroelectric.com/articles/strtctr.pdf

[MK1_Oz]

Ah....so the starter switch has sufficient current through it to create that magnetic field that moves the solenoid to switch the heavy current to the starter...got it ........... phew

[Miniliteman]

OK thanks Ex. The "fair bit of current when it energises" is not being drawn through the starter switch though is it?

It is. The resistance of a normal starter solenoid from a Bosch starter for example is approx. 0.6 Ohms.
So peak current through the switch will be 20 Amps.

[MK1_Oz]

Sorry for being slow lads. A 20+Amp switch will be sourced.

[doughman]

Hi guys,
The main electrical feed onto pole 1- should that be the batt + feed that supplys the fuse box?
Thanks

[exboyracer]

Yes, but obviously from the isolating side of the main switch. The purpose of pole 1 is to provide a discharge path for the alternator so it will shut down if the switch is used with the engine running.

[doughman]

So basically could just loop a wire from the main terminal of the switch (switched side not from battery side) onto the pole 1?

[exboyracer]

Yeah, make sure it's got sufficient capacity though. You only really need to hang the alternator off pole 1.