Methods for detecting the current in order to find the one that best suits the measurement.
the simplest method is through a Shunt that we can buy or do it yourself with wire with a very low value or with a power resistance.
As we can see in the image above, there are several shunts, there are also many encapsulated for THT or SMD printed circuit boards, clearly the latter of lower power, but basically these Shunts respond to Ohm's law , where we have a wire that has a specific resistance value, for example 0.1ohm, this means that if a 10A current is circulated through it, we will have to measure a voltage at the shunt terminals that will be
V = I * R = 10A * 0.1R = 1V
as we can see by applying Ohm's law we can know inversely which current flows through the circuit simply by measuring the voltage in the shunt. Unfortunately, shunts are difficult to obtain or expensive, for this we can replace it with a power resistor, for example a 0.1R 10W ceramic resistor, but we will have to take into account the dissipation power of 10W, for Ohm's law of power where
P = I ^ 2 * R ,
solving the equation we can know what is the maximum current that we can circulate through the resistance of 0.1R 10W without exceeding 10W,
I = sqrt (P / R) = sqrt (10W /0.1R)=10A,
means that the maximum current to circulate in that resistance will be 10A, if for example we connect a load that consumes 6,5 At that time by law of ohm we have to measure a voltage of
V = I * R = 6.5A * 0.1R = 0.65V = 650mV
and the dissipation power in that resistance will be
P = I ^ 2 * R = 6.5A ^ 2 * 0.1R = 4.22W.
The advantage of using this current sensing method is that resistance is a linear component, which means that the voltage value proportional to the current flowing through it will be linear.
The cons is that resistance is a contact component that must be in the circuit to be measured, it is not isolated in any way.
Here we see an example of current detection to measure the current of a lamp from 100 W powered a 230 V CA, the voltage measured in the resistance is 0,0435 V, if we apply Ohm's law we can calculate
I = V / R = 0,0435 / 0,1 = 0,435 = 435 mA.
HALL: a hybrid measurement method is called the Hall effect, the Hall effect is the potential difference generated by the separation of charges from the electric field inside a conductor through which an electric current circulates. If we take a certain electric canal and circulate an electric current in one direction, there will be a tendency to generate a separation between electric charges which are orthogonal to the flow of the applied current, by subjecting this conductor to a magnetic field, The charges that are at the orthogonal ends of the current increase and these charges cause a potential difference that can be measured with a voltmeter and that will be proportional to the magnetic field applied to it.
A Hall effect sensor can be found in various formats, but a very common one is in encapsulation similar to a low signal transistor, for example UGN3503 is an analogue Hall effect sensor (there are also digital outputs, with data protocol , with 3 axes, 6 axes, etc …) The only thing we need to polarize the UGN3503 sensor is a source a 5 V is a multimeter to measure its output.
The figure below shows a way to detect current using a sensor coil with a toroidal core.
There are currently many types of Hall effect sensors and current sensors, but there is a very popular one designed exclusively to detect the current (alternating or direct) based on an integrated circuit that has a Hall effect sensor inside, the same inside has a conductor through which the electric current to be measured circulates and on this conductor rests a Hall effect sensor that will measure the magnetic field generated by the conductor. The sensor is the ACS712 available in three versions, 5A, 20A e 30A, this sensor must be powered with 5V and provides us with a voltage output that will be proportional to the measured current value, the cons of this device is that we have to interrupt the electrical circuit as with the Shunt and connect it in series with the load, although the current part is isolated with the internal electronics, we have to break the circuit and we have current limits.
This sensor is already modular with a board, terminals and power supply to be implemented together with an external meter (microcontroller, voltmeter, oscilloscope, etc … ).
This is a very common method when we need to make high current measurements, since it is a non-invasive and contactless method, we do not have to interrupt the circuit to be measured and this provides us with higher measurement currents, it can also be known as streaming.
A current transformer is a simple transformer based on two windings and a core that facilitates the magnetic circuit between both windings by reducing magnetic leakage.
This is a method widely used in current clamps, in which we pass a conductor inside them and then measure the current without contact.
A disadvantage of the current transformer compared to a shunt is that it is not as linear as the shunt, the magnetic field that is generated in the conductor passing through the terminal is too weak, therefore a ferromagnetic core is used which increases Sensitivity thanks to its greater permeability which favors the magnetic circuit, this ferromagnetic core does not have a linear response and this reduces the quality of the measurement.
Although this coil can also be made without a ferromagnetic core, with an air core and this is linear, but the air has a much lower permeability than ferromagnetic, therefore the inductance of the coil decreases and to compensate for this decrease, must be increased. the number of turns in the inductor.
Ampere's law states that an electric current flowing through a conductor produces a magnetic field, this field strength is proportional to the current that the conductor travels.
The magnetic field generated in the conductor is a circular angular field perpendicular to the conductor.
B: magnetic field
dl: length differential
If we solve the closed integral, we will stay with:
Magnetic field reset:
If we apply this law to a toroid rather than to a straight conductor:
How can we see, we added the variable N in which we will insert the number of turns the toroid has, to know clearly the value of the magnetic field we must also know what permeability the magnetic circuit used has.
We will talk a little’ of electronics and the circuit created for our project.
As mentioned earlier, for the ACS712 current sensor or for the shunt, we don't need any other signal conditioning circuit, but in the case of the toroid, we will need to convert the current into voltage and then amplify it to insert it into the measuring instrument.
The operational amplifier as a current-to-voltage converter, also known as a transimpedance configuration.
This configuration produces an output voltage proportional to an input current, with a very low input impedance.
To this transimpedance converter we will add an integrator as a low pass filter with a cutoff frequency close to 50Hz, adding a 330nF capacitor in parallel to the feedback resistor.
Finally we will use another amplifier to increase the gain of the first one, we can also use a series preset for feedback resistance to vary the gain and adapt the meter to our scale.
Here we can see the final circuit, with the first stage of transimpedance and integration, then a second stage of amplification.
The circuit requires a symmetrical source so that the waveform does not deform, therefore a floating mass was created.
The integrated circuit used is a double LM358 general purpose operational amplifier, but it would be ideal to use one of the Rail-to-Rail type such as LMV358 to take advantage of all the supply voltage.
The toroid has a diameter of 25 mm per 10 mm wide, have been made 10 turns and the cable to be detected passes directly through the center of it.