Cutout Switch
James - 20/3/05 at 08:55 PM
People often seem to fit the plunger type cutout switches:
Cut off switch
(Thanks for picture SVC! 
)
I've got one (one of a large batch that ChrisG sold a couple of years ago) but I'm not sure what to actually cut out with it! 
I've tried looking up the hard to read Hella code for it: 2843 -0.5 but can't find the rating for it on the web so no idea if it's up
to high currents.
Presumably it's unwise to cutout the main battery/starter feed so is it just the ignition wire I want to do?
I have a mechanical fuel pump so no chance of using it to cut fuel.
Thanks!
James
[Edited on 20/3/05 by James]
Ben_Copeland - 20/3/05 at 09:03 PM
They are battery cut off switches, the cut the main battery postive.
The racing ones, stop the engine too by earthing all residual power too.
Also cuts the ignition off too.
I've got the racing one, 6 contacts on the back. 2 big ones cut the battery positive, 2 small ones cut the ignition and other 2 same ones earth
the residual power.
Mine definately looks like the one in the pic
ReMan - 20/3/05 at 09:04 PM
The purpose of this super heavy duty 200 amp? switch is to cut out EVERYTHING, so put it in the battery live lead!
If you just want to cut the ignition, use one like this...
I bought a big one myself and cant find anywhere to fit it
[Edited on 20/3/05 by ReMan]
Rescued attachment jk25.jpg
stevebubs - 20/3/05 at 09:12 PM
Unless you're going racing, it doesn't matter too much what you use it to cut out so long as it immobilises the car.
I've got mine in the battery feed to the main fusebox so it cuts out everything except the direct feed to the starter, i.e.
Main Loom
|
Cutout
|
Battery+
|
Starter Motor Feed
HTH
Stephen
omega 24 v6 - 20/3/05 at 10:03 PM
If the engine is running and you switch off withe the master switch does the feed from the alternator keep the engine running or not. Also as the
alternator then cannot measure a battery voltage (Its disconnected now) will this bugger up the alternator or not. Would it not make more sense to put
this master switch in the main earth wire from the battery.
wilkingj - 20/3/05 at 10:33 PM
quote:
Originally posted by omega 24 v6
If the engine is running and you switch off withe the master switch does the feed from the alternator keep the engine running or not. Also as the
alternator then cannot measure a battery voltage (Its disconnected now) will this bugger up the alternator or not. Would it not make more sense to put
this master switch in the main earth wire from the battery.
Thats just as bad. Disconnect either side of the battery, and there is NO reference voltage for the alternator.
This is why they put a 0.5 Ohm (25 or 50 Watt Rating) resistor across the Main terminals of the cut off switch, this allows a small current to flow,
and not bugger the Alternator. But not enough to start the engine.
The Race versions have additional contacts that Cuts the Ignition or Fuel Pump feed. They DO NOT short the electrics to ground, unless the regs
require you to destroy all evidence by fire, before the marshalls arrive on the scene.
[Edited on 20/3/2005 by wilkingj]
tks - 21/3/05 at 01:23 PM
WY NOT JUST CUT OF THE GROUND TERMINAL FROM THE CHASIS TO BATTERY
I'M SURE THAT IF you try to start with a 50watt res on the switch it melds down loke piece of butter unlike you have a solenoid starter type
first a solenoid
if that one doesn't pull on the gear then the starter cant rotate else on bike for example it will messup your res and i gess their are not
cheap..
TKS
Marcus - 21/3/05 at 08:26 PM
The race switches cut the ignition, and earth the alternator output via a high power, low value resistor (3 ohm IIRC). I have used one of these, but
not used the ign or alt. connections. It's not for stopping the engine in an emergency, it's an immobiliser (couldn't reach it in a
hurry, it's mounted at the front of transmission tunnel)
Marcus
bob - 21/3/05 at 09:15 PM
Mines there just as an imobilser as well,i've got it switching the earth on battery.
NS Dev - 22/3/05 at 09:30 AM
Crikey, there's some billy floating about on this thread!!
Wilkinj and Marcus have it right though!
JamJah - 22/3/05 at 11:31 AM
Having not got anywhere close enought I could be talking a load of naff. But ReMans switch surely wouldnt pass the SVA? Would it?
Rorty - 9/4/05 at 03:23 AM
There's a page on my site explains this topic in some detail if you're interested:
LINK
wilkingj - 9/4/05 at 08:27 AM
quote:
Originally posted by NS Dev
Crikey, there's some billy floating about on this thread!!
Wilkinj and Marcus have it right though!
Thank you....
However, I am wrong about the 0.5 Ohm.
I think it needs to be a little more, someone said 3 ohms, that would be better, and 50w rating as well.
Right.........The Maths proof of it all.
This is for a 3 Ohm resistor wired across the MAIN terminals of the switch.
Disconnect switch in the OPEN position, and try to start the motor through the resistor, goes like this. The starter motor is almost zero ohms!. about
0.05 Ohms (ish)
12V fully across the resistor I=V/R
12/3 = 4 Amps, flow, and 4 amps @12v = 48 Watts, (P=IxV) so a 3ohm 50 Watt resistor is needed as a minimum (50W min).
You need about 200 Amps (ish) to start the car, so it WILL NOT start with the switch in the OPEN position, even with the resistor across the
terminals, as the Resistor will limit the current flow to a max of 4 amps. Also the resistor will keep your clock running.
A 3 Ohm Resistor with a 50 Watt rating will NOT MELT, MELD, or anything else. The mathematical proof is above. However, using a 50w resistor running
at 48w will get very hot, and needs a good air space round it to stop it melting any plastic bits near it.
Or Fit a heat sink, Or better still get a 100w rated job.
It DOESNT matter whether its in the Earth or the the Feed side of the battery, its an ON/OFF switch and will break the circuit regardless of the side
its in. THINK about this, pull off either terminal off the battery it STILL disconnects it, doesnt it 
I do the earth side by choice, as you usually have only have ONE wire on the earth side, Usually on the live side, you have a few extra wires tacked
on there, Earth side just makes a slightly neater job, thats all.
There is a small safety factor as well. Put it in the Live side, and there is Full battery power available at the Switch terminals. PLEASE INSULATE
them, as a short circuit to chassis, ie the odd spanner shorting them out, will be dangerous. Put it in the Earth side, this will NOT happen, ie the
spanner would short out earth to earth, which is slightly safer.
The resistor, prevents you loosing the voltage reference for the alternator and blowing the regulator stage in it.
Also the resistor helps do a nice impression of a flat battery, if you try to start the car with the switch OPEN. ie enough current flows to light the
ign lamp etc, and then DIMS seriously when you put the startermotor load through it, when you try to start it
It might be enough to deter the joyriders.
I hope I am not trying to sound too big headed, but I have worked for BT as Technical Officer for 36 years, and am a Radio Ham (24 years), and have
re-wired several cars etc etc. ie I do have some experience and knowlege, although I am NOT formally qualified in Auto Electrics.
Final Word... If you do not know, or are unsure or concerned, Please consult a Qualified Auto Electrician. The LAST thing you want is to set fire to
your pride and joy, by that I mean your CAR. The above is a guide to WHAT I DO, Do not follow MY advice and them blame me if YOUR car goes up in
smoke. If you dont know what you are doing then DONT DO IT and seek advice as above ie a Q. Auto Electrican.
PS.. Sorry for the long thread, as I think that ALL Electrical workings are important, as many people find this one of the more difficult areas of car
building, its also a safety issue. ie Make sure you get it right first time.
[Edited on 9/4/2005 by wilkingj]
Marcus - 9/4/05 at 10:00 AM
I think it needs to be a little more, someone said 3 ohms, that would be better, and 50w rating as well.
Right.........The Maths proof of it all.
This is for a 3 Ohm resistor wired across the MAIN terminals of the switch.
Disconnect switch in the OPEN position, and try to start the motor through the resistor, goes like this. The starter motor is almost zero ohms!. about
0.05 Ohms (ish)
12V fully across the resistor I=V/R
12/3 = 4 Amps, flow, and 4 amps @12v = 48 Watts, (P=IxV) so a 3ohm 50 Watt resistor is needed as a minimum (50W min).
You need about 200 Amps (ish) to start the car, so it WILL NOT start with the switch in the OPEN position, even with the resistor across the
terminals, as the Resistor will limit the current flow to a max of 4 amps. Also the resistor will keep your clock running.
A 3 Ohm Resistor with a 50 Watt rating will NOT MELT, MELD, or anything else. The mathematical proof is above. However, using a 50w resistor running
at 48w will get very hot, and needs a good air space round it to stop it melting any plastic bits near it.
Or Fit a heat sink, Or better still get a 100w rated job.
I don't want to carry this thing on for long, but you'll be causing fires if you use this system!!!
The one thing you've forgotten is the cranking current of the starter motor. 100A is not unusual.
Using your maths, P=VI, power would be 100x12=1200W
Putting 1.2kW through a 100W resistor is madness. It'll fry.
Haven't seen many 2kW resistors around!!
Marcus
Peteff - 9/4/05 at 11:41 AM
Electric fire element.
paulf - 9/4/05 at 07:55 PM
I have wired my car in a similiar way and used a 30 ohm resisitor to keep a supply to the alarm and imobiliser.There is no way of starting the car but
a short will cause no problems .I also connected the alternator direct to the starter +ve terminal so the switch only cuts out the main supply to
ignition dash lights etc and has no effect on the charging circuit.However the engine will not run on from the alternator when switched off.
Paul.
quote:
[I don't want to carry this thing on for long, but you'll be causing fires if you use this system!!!
The one thing you've forgotten is the cranking current of the starter motor. 100A is not unusual.
Using your maths, P=VI, power would be 100x12=1200W
Putting 1.2kW through a 100W resistor is madness. It'll fry.
Haven't seen many 2kW resistors around!!
Marcus
wilkingj - 9/4/05 at 10:47 PM
QUOTE:
*******
I don't want to carry this thing on for long, but you'll be causing fires if you use this system!!!
The one thing you've forgotten is the cranking current of the starter motor. 100A is not unusual.
Using your maths, P=VI, power would be 100x12=1200W
Putting 1.2kW through a 100W resistor is madness. It'll fry.
Haven't seen many 2kW resistors around!!
Marcus
QUOTE:
********
Absolute coblers.
I have NOT forgotten. I might be old, but have not got Alzhiemers yet!
You CANNOT draw that amount (100A) of current through a 3 ohm resistor, when its supplied with12V.
I=V/R and you only have 12Volts.
The starter Motor DOES draw 100+ amps... (nearer 2-300Amps) However the motor will be in series with 3 ohms and the TOTAL resistance in circuit is
3.05 Ohms, and will ONLY draw about 4 AMPS.
You obviously do NOT understand electrical systems and Ohms Law.
I politely suggest you get the Text Books out, Draw out the circuit and work it all out. I am standing on ground made from re-inforced concrete as
opposed to the running sand that you are standing on.
A starter motor is nearer 2-300 Amps not 100 Amps. To draw 240 Amps at 12V you need a total resistance in circuit of 0.05 Ohms, The starter motor in
series with the 3 Ohm job will give 3.05 ohms. (approx)
I=12/3.05 which is as near as dammit 4 Amps. Mr Ohms Say its impossible, and I agree with him. I learnt this stuff 35 years ago as an apprentice and
still use it at work and in my hobby.
If you want a 1kw rated resistor I can let you have one, its wire wound on a ceramic former and made from Nickel Chromium Wire, and glows nice and red
when pushed hard
I pick them up at the re-cycling centre
Please think before you put pen to paper.
If you dont understand it, U2U me, and I will phone you and explain it.
However, you are quite within your rights to challenge anything, especially electrics, where getting it wrong could cause a fire and loose you car or
far WORSE.
*****************
NO offence intended.
*****************
Best Regards
Geoff
JoelP - 10/4/05 at 07:55 AM
geoff is of course quite right here - a starter only draws high current due to its low resistance. the current is worked out from the resistance of
the entire circuit, which includes, in this case, the resistor.
Marcus - 10/4/05 at 09:26 AM
Good point.
I stand (or sit actually) corrected.
Marcus
wilkingj - 10/4/05 at 11:42 AM
quote:
Originally posted by Marcus
Good point.
I stand (or sit actually) corrected.
Marcus
Marcus,
No problem, we are all here to help each other, and some have more expertise in one area but not another.
Otherwise whats the point of this Forum?
There is a wealth of knowlege and experience on here, Dive In... and learn... along with th rest of us.
I am a crap fabricator and even worse welder. I cant cut metal straight, and rely on Mr Angry Grinder to even it up
If it's Soldering... I can do that very very well indeed.
Marcus - 10/4/05 at 06:43 PM
Who'd have thought I've got by for 14 years repairing electronic stuff without actually thinking!!
I've given it all up to work with injectuion moulding (perhaps just as well)
Marcus
NS Dev - 11/4/05 at 08:14 AM
all's well that ends well!
