The same ground for different voltage circuits?

Fly-n-hi · Post by **Fly-n-hi** » Tue Nov 11, 2008 12:28 pm

HI guys. Question:

If I have 3 different circuits in my model, and the model is supported by a brass tube, and all three circuits use the same 12 volt power source, can all 3 curcuits be grounded to the brass pole? (I'm pretty sure they can).

Ok. Lets say I have the same model but one of the circuits is a 9 volt circuit. Can I still ground that circuit using the brass tube as long as a negative lead is connected from the tube to the 9 volt supply?

In other words can I use the brass tube to ground a 12 volt circuit and a 9 volt circuit at the same time?

USS Atlantis · Post by **USS Atlantis** » Tue Nov 11, 2008 5:34 pm

Sorry, no

Each voltage source must have it's own, individual ground - else the power sources get confused and blown-circuits or leaky batteries result

Why would you want to ground to the brass rod anyway - it's only one more wire per source to ground back to the battery/wall-wart

Fly-n-hi · Post by **Fly-n-hi** » Tue Nov 11, 2008 8:51 pm

It's not that simple. I have to run the wires down the brass tube. There are about 12 wires running down the tube (because of a flashing board) and space is very tight. Using the brass tube as a ground saves alot of space.

It turns out I can actually use the 12 volt supply on the 9 volt circuit so it won't really matter now but I was still curious to know if it was possible.

Thanks.

USS Atlantis · Post by **USS Atlantis** » Tue Nov 11, 2008 9:00 pm

Tight space, hmmm

For wiring projects with multiple circuits, I use 15-conductor cable - and it'll fit down a 3/8" dia brass tube

Something to think about

kitty · Post by **kitty** » Tue Dec 02, 2008 7:03 pm

If you have a 12V powersupply and for instance a 9V and a 5V regulator that are connected to the +12V and ground of that powersupply, you already have a common ground and there would be no need for seperate leads for the 12V, 9V and 5V gound.

If however you use 2 different powersupplies (for instance 8 x 1.5 volt batteries for 12V + a 9V battery for 9V) you cannot have a common ground, then you need 2 seperated circuits thus seperate grounds.

But why would you need 3 different Voltages?
Nowadays the lighting of a model can all be done with 5V, there is actualy no need for more, since you can use leds (bright whites are usualy 3.8V) for everything, you can even replace a CFL tube with a piece of sidelight fibreoptics with a bright white led on both ends or el sheet with a piece of acrylic that you light at the edge with leds, which eliminates the need for 12V completely.
Look at a macbook, those screens are lit with leds.
And if you realy need that EL sheet you can use a switched voltage regulator to make 12V from 5V.
The advantage of 5V is less heat dissipation by all components compared to 12V.

Sparky · Post by **Sparky** » Thu Dec 04, 2008 4:08 pm

I had to think about this and think about it. And check with some of the other engineers around here.

There is only one issue that we could come up with for why you can't use a common negative return (aka ground) for multiple different battery voltages:

When you run a line from a battery to a circuit load, the negative return, or negative buss only needs to conduct the current from that battery. The positive line and negative line always must be matched for size, and we usually do this without thinking, you buy one spool of red & one of black or green, but they are the same gauge wire.

When more then one circuit drains down one common return, you can overload that line very quickly, this wire will get hot as more circuits, each with their own positive feed, attempt to return down that line, this is very bad and dangerous.

You can use the water system analogy here:
If you have a faucet and a drain, and you add another water feed and faucet but attempt to drain both through the same pipe, you will see that sink start to over flow. The single drain pipe needs to be sized to handle the max flow of both faucets, or you will end up with a mess in certain usage scenarios (that's bad, because if you spot check the setup it might pass but then one day boom a mess).

For our usage, if you use a nice thick brass rod as a negative buss bar, and at the level of gauge difference (positive lead vs. negative lead) you can call the rod a buss bar. You wont' have any current capability issues.

Note this is even an issue with house wiring where you have one power source. All hot lines must have their own neutral return line. The circuit break protects a single line pair, if one neutral is used to return 2 circuit breaker protected hots, then that one return wire will be attempting to conduct double its rated load, if a short circuit occurs on both hot lines rare but possible.

Kylwell · Post by **Kylwell** » Thu Dec 04, 2008 4:25 pm

Mouser has some damned skinny wires, like 44/32. For most our current usage we use way thick wires. Somewhere around here I had a calculator for wire gauge... Sparky, you got anything like that? For example a 4.5v source with a 1 amp draw?

Sparky · Post by **Sparky** » Thu Dec 04, 2008 5:18 pm

I found a website that had a chart I'll see if I can find the bookmark.

But it also depends on the wire. Some of the stuff from mouse is all copper, some is tinned copper, those alloys will have different resistance per foot specs and thus a different loss chart.

Locke_333 · Post by **Locke_333** » Mon Dec 08, 2008 1:33 pm

SCC-7107 USS Atlantis wrote:Sorry, no

Each voltage source must have it's own, individual ground - else the power sources get confused and blown-circuits or leaky batteries result

Why would you want to ground to the brass rod anyway - it's only one more wire per source to ground back to the battery/wall-wart

well there are a few possible reason. for example, i've got a scale solution SSD build up in the bench section and the brass rod won't allow for more then 1 wire throught because of the bends in it so i use the brass rod for the negative return.

For the rest, i tottally agre with the others - no can't do that. if you would absolutely need a 12 and a 9v circuit and that your 12v power supply has enough amps, run the 12v through the rod( using the rod or not, tour choise) plug in the 12v circuit and use a resistor to drop the 12v down to 9v with a 150ohm resistor.

Madman Lighting · Post by **Madman Lighting** » Mon Dec 08, 2008 8:25 pm

Well I R an Engineeer and yes you can run all your grounds through a common conductor, as long as its able to carry the current of all the circuits, just like Sparky sez. (Engineeers cant spel)

Just connect all your source (battery, wall wort, etc) grounds together but keep all the positive connections with different voltages separate. If you're worried about how thick a wire to use, how about a nice chunk of stranded 22 gauge wire? Should handle at least an amp which is TEN Madman lighting cards (300 LEDs), or 50 LEDs and resistors in parallel.

Have fun!

-John C.