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#Post#: 38022--------------------------------------------------
What does negatively earthed actually mean?
DIR By: ChrisJ
Date: September 23, 2012, 4:26 pm
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I'd like to better understand the world of car electrics since I
tend to do electrical jobs by trial and error. Not the best
approach.
I'm fitting some auxiliary lights at the moment and they are
connected to the positive post of the battery to supposedly
switch on when the ignition is turned. In the case of my car
whenever the positive terminal is attached to the battery they
come on - even if the ignition isn't switched on.
Is there a fundamental fault with the ignition sequence or is it
just an attribute of an old car?
I'd welcome any help.
Cheers,
Chris
#Post#: 38026--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: guest13
Date: September 23, 2012, 4:40 pm
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Negative earth
- Negative connection on the battery is the earth (usually
chassis)
Positive earth
-Positive connection of the batter is connected to earth
If you have wired the lights directly to the battery positive
(its sounds like you have) then they will be permanently on
whenever the battery is connected.
To switch them of you need a switch in the system :
You have some options - there are allot more, I just picked 3
that popped into my head
Battery live > switch > light > earth - this will allow the
lights to be switched on with the ignition off, but you need a
switch rated for the current the lights will draw (Bulb power X
no of lights then divide by voltage to get the current - then
allow a bit more to be safe)
Switched ignition feed > Aux light switch> light > earth - this
will only let the lights be on with the ignition on. Not
srecomended as will put to much load on the ignition switch =
fire
I would do it this way (I think ... it is late at night :-?)
Battery live > Fuse > Relay > lights > earth
The relay trigger would be
Ignition switched live > Fuse > Aux light switch > Relay - This
only allows the lamps on when the ignition is on and avoids
putting load on the ignition switch as it only switches the
relay on and off.
Having the system fused is a good idea, as a short will equal
fire if not
#Post#: 38034--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: Lucy
Date: September 24, 2012, 2:21 am
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HTML http://www.classictruckshop.com/clubs/earlyburbs/projects/bosch/foglite.jpg
might make what he^ said less clear :)
#Post#: 38052--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: ChrisJ
Date: September 24, 2012, 7:08 am
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Thanks for the explanation and options. The option which seems
most within my capability would be to add a switch to the wire
which goes to the positive terminal on the battery.
The lights came with a fuse already on this wire. Does this mean
all I need to do is add a switch between the terminal and the
fuse?
Can you recommend a simple switch setup?
I looked at the relay option and it was too much for my head to
handle... :haha:
Would the simpler option be robust enough?
Cheers,
Chris
#Post#: 38054--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: simdeb
Date: September 24, 2012, 7:17 am
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Add the relay or you might burn out your wiring and set fire to
your car seen it happen better still get some one that knows
what they are doing to help you 12 v might not hurt when you
touch it but will cause a fire just the same
#Post#: 38056--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: Snailer
Date: September 24, 2012, 7:41 am
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--- Quote from: simdeb link ---
>
> Add the relay or you might burn out your wiring and set fire
to your car seen it happen better still get some one that knows
what they are doing to help you 12 v might not hurt when you
touch it but will cause a fire just the same
>
--- End Quote ---
^^^^ this, what you describe you would get away with if it was
something that was very low in power but you are talking lights,
they are one of the most current heavy things after your built
in kettle.
The relay is the way to go. What this means you are using a
relay as a switch to operate another switch. Your relay is a low
power switch and is a set of contacts that are in the off or on
position. When you operate the relay the big heavy duty contacts
connect and your lights come on. If you wanted, this could be
fed direct from the battery as long as there is a fuse in the
system somewhere. I am sure there are drawings on how a relay
works to help you. They are a simple but effective way of not
burning your vehicle to the ground.
#Post#: 38060--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: NoelC
Date: September 24, 2012, 8:12 am
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Chris, relays essentially allow switches to be made of shite.
Rather than make a nice beefy switch that can handle the current
the modern trend is to make a weedy little switch that in turn
switches a relay (effectively a switch that rather than a manual
toggle is switched by electric current) to complete the circuit.
For the sake of simplicity if you purchase a beefy enough switch
you won't need a relay.
A rough calculator is as follows:
110 watts (2 x 55 watt bulbs) at 12 volts = 9.2 amps
200 watts (2 x 100 watt bulbs) at 12 volts = 16.7 amps
300 watts (2 x 150 watt bulbs) at 12 volts = 25 amps
AES have a good selection,
HTML http://www.autoelectricsupplies.co.uk/product/820/category/49
HTML http://www.autoelectricsupplies.co.uk/product/820/category/49,<br
/>this for under a fiver seems decent value for a 25amp switch.
A word of warning though - when you add wiring of any type make
sure it is properly secured and fused. My mate lost a s3 hicap
diesel when a PO bodge cable to the starter motor chafed
through, the engine stop cable became an earth, glowed red hot
and set fire to the dash and interior.
#Post#: 38065--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: ChrisJ
Date: September 24, 2012, 9:02 am
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Thanks all. I've been on you tube and watched a few videos on
relays so now I understand how they work it should be pretty
simple to wire up. I've also purchased a Lucas toggle switch for
the mechanical switching part.
One thing I'm still not sure of though is where I connect the
other end of the mechanical switch to. I looked at Lucy's
diagram and it shows attaching to the ignition.
Having taken the ignition apart in the past I know there are
several spare points on the back but how do I know which one to
connect to, to bring the power through from the battery?
I also know that the fuse panel is not in the standard location
under the steering column. I presume It must have one.... It
wouldn't work without one right or would have already burned to
the ground?????
Should I seek out a proper Bosch 40a relay or are Bosch-like
relays OK?
Help, I'm worried I'm going to discover the charred remains of
my car one morning.
#Post#: 38104--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: divie
Date: September 24, 2012, 3:55 pm
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Get yourself a 4 pin relay from the local factors. All relays
have the same numbers on them 30-85-86-87 Connect with suitable
cable for the amps you are going to use number 30 via a fuse to
possitive battery terminal. number 85 to one side of your switch
feed the other side of the switch from the ignition side of fuse
box (green) a lighter rated cable is ok here ( 14 strand x .30
mm minimum) Connect 86 to earth and 87 to your lights. This will
only operate lights when ignition is on. Ensure all cables are
neat and tidy , clipped and sheathed as required to prevent
chaffing.
#Post#: 38142--------------------------------------------------
Re: What does negatively earthed actually mean?
DIR By: ChrisJ
Date: September 25, 2012, 9:08 am
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Thanks all. I've got everything I need.
I really appreciate all your help.
Chris
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