I just got a PZEM-051 Power meter and I can see that in order to measure the power, one needs a shunt. At least that's what I can see from different YouTube videos and I was wondering why would I need one? By looking at the scheme on the back of the meter, I can see that it has a shunt integrated (please see picture).
Do I understand the scheme wrong? What would be the role of the shunt and do I need one to read the power and the voltage from a power supply and a small pump?
The shunt is used to measure the current to the load.
Power is the product of the voltage applied to a load and the current consumed by it.
The shunt is low value resistor that may only be a small fraction of an ohm (depending upon the load current). This example is designed for currents up to 100A and so the value of the shunt may be just 500 micro-ohms. At 100 Amps that would cause a voltage drop of 50 milli-volts.
It is desirable to avoid dropping too much voltage across the shunt because any voltage dropped doesn't get to the load. Also the power dissipated by the shunt maybe excessive. In this example with 50mV drop and 100A flowing the shunt resistor will dissipate 5 Watts.
It looks like this power meter expects the user to supply an external shunt, wired according to the diagram. It should say somewhere on the meter or in the manual what the maximum voltage is expected from the shunt. Something like 50mV would be common. The display is scaled to 100A when the maximum shunt voltage is presented. It is up to the user to calculate the required value of the shunt.
Note the '4-wire' connection to the shunt (also called 'Kelvin' connection) that is used to avoid the resistance of the connecting wires causing an error. The shunt resistor should have 4 terminals.
I haven't found a manual online but there is a YouTube vide explaining the connections Connecting the PZEM-051
The role of the shunt is to enable measurement of the current drawn by the load. The shunt is a low value resistor that provides a small voltage proportional to the current (Ohm's Law). Power is calculated by multiplying the voltage and the current.
Sensing power consumed by a load will require measuring the current that passes through it. If the current-measurement device were located within the power meter, then all current that flows through the load would need to flow through the power meter. A power meter capable of handling a large load would thus need to be connected using very large connectors and wires.
If one instead uses a power meter that is designed to be wired as shown using an external shunt, the wires that connect the supply to the shunt and the shunt to the load will need to be large enough to handle the load current, but the wires that connect to the meter will only need to be large enough to supply current for the mater. Even if a load would consume thousands of amps, the meter itself might still be connected with 24ga wire (though there should probably be fuses to protect the wires going to the meter in that case).
Note that there is no limit to the amount of power that may be sensed by a power meter using an external shunt. If a meter is designed to measure up to 200mA using a 0.1ohm shunt, the same meter would work just as well to measure up to 200A using a 0.0001ohm shunt. If the meter has a numeric readout along with annunciators to select power, current, etc. one might label the "current" LED as "mA" when using a 0.1ohm shunt, and "A" when using a 0.0001ohm shunt, but the meter wouldn't otherwise need to know or care what kind of shunt was being used.
