When you measure current (AC or DC) you need to place the measuring device (meter) in series with the load. You don't mention a load? The fact that you mention a center tapped transformer leads to confusion. Whart exactly are you trying to do? Do not place a meter set to measure current accross any source! Bad things will happen.
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That's the only method I was thinking about, but what about the secondary? The secondary is connected to a split 7915 &7815 and finally to 2 Op-Amps, And because I have 3 wires out of the secondary coil of the transformer then I have no idea how to exactly measure the WHOLE current draw of the circuit (Yea, I do know how to use an Ameter In a circuit).
2 other questions, an AC CURRENT/VOLTAG meter measures RMS values, So what do you mean by using a true RMS meter??!
The second question which now arrived is I have 2 options, One is measuring the AC current at the primary and the second is to measure DC current values at the Output of the transformer, One option gives me an RMS value and the other a DC value, right, If so how to translate these values whit each others?
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That's the only method I was thinking about, but what about the secondary? The secondary is connected to a split 7915 &7815 and finally to 2 Op-Amps, And because I have 3 wires out of the secondary coil of the transformer then I have no idea how to exactly measure the WHOLE current draw of the circuit (Yea, I do know how to use an Ameter In a circuit).
if you measure the ground you won't get far I think, you will basically be measuring ac if the two halves of the circuit are drawing the same current alternatly (for example your amplifying a symetrical AC signal), if your circuiit is drawing current from both sides wou will be measuring the difference. if this is you idea of measuring two currents with one meter you will come unstuck
The problem of measuring the current into the primary is that the power factor of a lightly loaded transform sucks! The current lags the peak voltage by as much as 60 or 70 degrees, so if you multitply Line voltage by primary current, you will get an artificially high power. It would take a "power meter", or a o'scope reading of voltage/current simultaneously to determine the relative phase of the current to the voltage.
If you do not care about power losses in the transformer or regulators, then measure the DC load current right at the output of each regulator. The load power calculation is trivial there...