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Beginner's Question 2


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The term "ground" is often not used properly. You'll hear things like... connect it between +5V and ground... this is not correct, it should be connect it between +5V and 5V return. Power supplies are not referenced to ground, they are referenced to their return. Now it maybe that the return is connected to ground, however it is still considered referenced to it's return.

This "floating ground" to which you refer, is called a "return".

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It's one of the most misunderstood things in electronics - voltage. The problem is that people think 9V is 9V and that's that, but really it means 9V higher than somewhere else. On a 9V battery, the '+' terminal is 9V higher than the '-' terminal. In other words, the battery is generating a difference of 9V between its two terminals. It might help if the negative terminal on a battery was labelled "some voltage or other" and the positive labelled "9V more than the other terminal".

Now ask yourself this - what is the difference between the voltage on the - terminal of that battery, and the top of the Eiffel tower? It could be 10 million volts, for all you know. In that case, the positive terminal of the battery is ten million and nine volts higher than the top of the Eiffel tower. All relative.

A floating ground is some point in a circuit that is not connected in any way to anything outside of the circuit, but acts as ground for the circuit itself. It could be thousands of volts different from the ground of your home, because it's completely isolated from it.

If you then connected that floating ground to the earth/ground pin in a wall socket, then it wouldn't be floating any more.

The idea is that you don't always want the 'ground' supply rail of your circuit/appliance to be held at the same voltage as the ground of your building. A nice example is when you have two bench supplies in your electronics workshop, and you want to combine them to form a +15V:0V:-15V dual rail supply. In that case you connect the two in series, and you most definitely do not want the negative rails of the two supplies to share the same ground. When you connect them in this way they need to have floating grounds (completely isolated from the building's ground) to avoid a short circuit.

And anyway, your circuit probably doesn't really care what the rest of the world's voltage is, and since it's not connected in any way to the rest of the world, why should it?

If you do connect equipment together (hifi to TV for example), the ground connections in your home provide a neat way of making sure all the equipment is held at the same voltage level, and you don't end up connecting something floating around 100V to something floating at -50V - sensitive and fragile signal inputs and outputs will be exposed to the difference of 150V! With these crap "double insulated" appliances which claim to not need a ground connection (and use only two core mains cable - live and neutral, but no ground), you'll often feel the buzz of a hundred volts AC or so as you connect them together, because their internal "floating" grounds are different.

This is a great way to explode PCs - connect together two serial ports on different PCs which aren't grounded via the mains socket, and see what happens. No, seriously, don't do that.

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