Current measurement

[Uncle Krunkus]

I'm just wondering if anyone knows whether an ammeter in series with the 0V line will give a realistic reading of the current in the +ve & -ve lines added together. Something tells me it won't, but I'm unsure.

[blue hell]

Indeed, in general it will not tell you much knowing the current in the 0 line only.

An opamp for example usually gets it's supply from the + and - lines, and unless a lot of current is flowing to/from ground from its output you'll measure about nothing in the ground reference, yet it does draw some current that directly flows from + to - through the device (which can be a very small current for some opamps)

You'll have to measure two lines to be able to calculate the current in the 3rd line - then you know all there is to know about current consumption.

It might be usefull to learn a bit about mesh analysis, wikipedia has a page about it but it is very confusing I think for an introduction : http://en.wikipedia.org/wiki/Mesh_analysis

Basically it is all derived from two theorems (Kirchhoff's laws), one that states that the sum off all currents flowing into (and out of) a point of connection must be zero and another that states that the sum of all voltages over all circuit elements in any closed loop in a circuit must be zero (going in one direction, some voltages will be positive others will be negative). When this would not be the case energy conservation laws would be violated.

Maybe googling for Kirchhoff's law(s) will help you here.

[Uncle Krunkus]

Thanks Jan,
I've decided to do a separate meter on each rail.
A lot simpler and looks even more retro!