hall-effectpower-measurementpower-metersensorwatts
I read about the Hall effect and understand its usage in sensors like speedometers, but I also read it can be used as a wattmeter. How can it happen?
In order to calculate power (P) we need to multiply voltage and current.
How does a Hall sensor do that?
As shown in the "Functional Description" section of the datasheet, those Hall effect switches use an external sense resistor.
EDIT:
Like so:
simulate this circuit – Schematic created using CircuitLab
And then you measure the voltage across Rsense.
Your diagram shows a bare Hall element.
The current that flows along the element (excitation current) is dictated almost entirely by the resistance along the element, and its excitation voltage Vx.
The output or Hall voltage VH is the product of the B field, the excitation current, and the Hall sensitivity of the element. Any output current flowing is dictated by the magnitude of VH, the resistance of the element across the output terminals, and the load on them. It's good practice to have a load here that draws little or no current.
The Hall sensitivity tends to be so low that the excitation current varies little with output voltage.
However, you rarely see a bare element. What you buy as a 'Hall Sensor' has an amplifier following VH, to boost the mV output to a swing of volts that you can read sensibly on your Arduino. This further isolates any sensitivity of input current to changes in the output voltage.
If you load the sensor output amplifier output with a resistor, and the amplifier is capable of driving the current that resistor demands, then the amplifier will draw the extra supply current to drive the resistor load.
Best Answer
You are correct that for power measurement both voltage and current must be measured.
Figure 1. Hall effect measurement setup for electrons. Initially, the electrons follow the curved arrow, due to the magnetic force. At some distance from the current-introducing contacts, electrons pile up on the left side and deplete from the right side, which creates an electric field ξy in the direction of the assigned VH. VH is negative for some semiconductors where "holes" appear to flow. In steady-state, ξy will be strong enough to exactly cancel out the magnetic force, thus the electrons follow the straight arrow (dashed). Source: Wikipedia Hall effect.
The Hall sensor provides a means of non-contact current measurement.
Figure 2. A typical current sensing arrangement of a Hall sensor in a torroidal core around the conductor. Source: Digikey.
The energy meter will require a voltage measurement as well.
For DC the instantaneous power can be calculated as the product of the voltage and current. For AC it is necessary to continuously sample the voltage and current, get the product and then calculate the average of the power over time. Mathematically, \$ \int_0^T VIdt \$.