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measuring current


Hi :)

I'd like to measure both mA and A for my motherboards consumptions,
so is there anyway wire both analog ammeter without burning them? I need them both as i need to monitor the startup ma and the standby consumption.  :-BROKE

thank you in advance.

Combining the 2 meters is not that easy. Depending on the details of the meters it could be done by adding anti-parallel schottky diode to the mA meter to clamp the voltage / signal and than have them in series. However the voltage loss / burden voltage may still be too high for the computer to work. The probably better choice would be unsing only the shunt from the A meter and than add a custom amplification for the low range. This may than need a different meter for the low range (may use the old one by removing the shunt).

It is more common to use just one meter, and then add some circuitry to switch between different ranges. quite similar to the old analog Multimeters, that use a single pointer with many current, voltage, and resistance ranges.

Another method is to build an analog logarithmic amplifier to measure the voltage across a shunt resistor.
This is not easy to build, but it is straightforward to calibrate the indicated voltage output as a function of input voltage/current.
The ancient (ca. 1970)  -hp- 7562A and 7563A logarithmic units can be found at surplus sites:  the 7562A includes a true-RMS converter, and is capable of AC or DC input.  The 7563A is DC only.
The 7563A has a dynamic range of 110 dB (3.2 x 105 V/V), and a rise time of 2 ms (input < 1 mV) decreasing to 40 us (input > 10 mV) and 2 us (input > 1 V).
It has a single input range, but you need to select positive or negative input voltage.


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