Hi luiz_aug
I want to use these analog ammeters as a weather display, connected to some uP over the wifi.
But to do so, as a energy efficiency concern, I need to be able to deflect the pointer over all the scale by using the minimum current possible.
I have already opened one and removed the "shunt" resistance, and by applying a 3.3V signal through a potentiometer, I discovered that the new value of the full deflection current is about 5.18 mA.
(1) - Is there a way to reduce even more this current ?
I presume that you want to limit current consumption to extend battery life in the weather display unit.
Yes, there is a way to effectively reduce the current for the full scale deflection (FSD) of the meter. The power used by the meter is 5.18mA * (whatever voltage was across the meter terminals at FSD). This is a constant power (I*V) and cannot be changed practically. But what you can do is to change the I/V ratio to suit the voltage of your power line (battery).
Take an example:
Meter FSD current: 5mA (round figures)
Meter FSD voltage: 100mV (assumed)
Thus meter power requirement = 5mA * 100mV = 500uW
Power line = 3.6V (single LiIon battery cell assumed)
If you drove the meter directly from the 3.6V power line the current would be 5mA and the power dissipation would be 5mA * 3.6V= 18 mW. So 17.5 mW would be wasted in heat.
But, on the other hand, if you used a micro-power switch mode power supply (SMPS) to generate 100mV from the 3.7V supply line, the wasted power would be zero (assuming for the time being that micro-power SMPS were 100% efficient). So now the current taken from the 3.7v power line for meter FSD would only be 500uW/3.6V = 139uA. In practice, I would imagine that you would be able to reduce the current drawn from the 3.6V supply line, for meter FSD, from 5mA to 200uA.
And the micro-power SMPS could be implemented by a microcontroller PWM output pin and a voltage monitoring input pin. A small inductor, small Schottky diode, and a small capacitor would complete the micro-power SMPS (roughly speaking). But there are many ways to realise the micro-power SMPS.
By the way, depending on the total power consumption of your weather station unit, you could think about adding a small solar panel to effectively make battery life infinite. Small solar panels are dirt cheap these days.