12vDC: How to connect multiple earths

[HDPete]

Hi,

Any advice gratefully received.

I have a project which requires a 12v scanner to be run from a battery. I have a 12vDC 12AH battery which is all working fine, but I'm not entirely sure that I have the earths connected as efficiently as possible.

The diagram below shows that I need an earth for the battery charger, the illuminated rocker switch and the DC to DC convertor.

At present, I'm using Lucar connectors with three wires coming off of the Negative of the battery with heat shrink to wrap them up and protect the cables.

Is this OK, or should I run a single earth to a termination block and join all earths there? If so, can anyone recommend a suitable termination block? It needs to be small and protected from user interference.

Thanks
Pete[/img]

 

[ban-all-sheds]

Your diagram doesn't show any earths at all.

Is the charger permanently connected?

You might be better off asking in the car maintenance forum for advice on particular wiring accessories you might use.

 

[ricicle]

What makes you think that the switch and convertor need an earth ?

 

[maltaron]

Not an answer but a query. Why do you need such a huge battery and 10 amp fuse for equipment that has a 4 pin din for its power connection?

 

[securespark]

Are we talking earth as in 230V CPC's or are we talking earth as in

12V -ve??

 

[ericmark]

Other than 10A fuse which seems a little on the large size (It does say less than 12W at 12 volt so 1A should be enough) I see no problems.

There is no earth as shown it is often referred to as a earth return system as the non switched line is often considered as most likely to be earthed. However with no motors there is no copper dust likely to produce a connection to the metal chassis in fact no chassis so as a result just positive and negative there is no earth.

However I would agree since the 1950's when it was found the chassis could be negative with respect to the battery but positive with respect to the spark plug most cars and wagons have connected the negative to the chassis as it causes less corrosion than with a positive chassis so negative is considered often as earth.

The butt connector would have been the normal way to connect up to 4 wires together. There are however many ways with spades designed for two cables and with piggyback spades to name just some.

What I assume is the regulated battery charger keeps the battery topped up so will float at 13.2 volts but when mains is lost will drop to 12 volts so the DC to DC converter is to hold the output at 12 volts?

The spec shows the use of a switch mode power supply and does not give the tolerance at 12 volt. However I would be surprised to find it was such a close tolerance that a DC to DC converter was required.

The battery is likely 10 times the size required as most lap tops will only run for an hour and I would have added an extra cell and used 12 NiMH cells and a standard 7812 voltage regulator.

 

[HDPete]

Hi, sorry a couple of mistakes on my part. I meant 12vDC negative,not earth and also the fuse is indeed 1A.

The switch needs the negative leg for the illumination to work.

I'll check out the piggy back connectors, thanks.

Regards
Pete