The current measurement requires the use of an
ammeter placed in series with the load. An ideal ammeter has no voltage drop, i.e.
it is a short circuit. But most of the current sensors are based precisely on
the measurement of the voltage drop on a resistor which, according to Ohm's
law, is proportional to the current that passes through it.
For what has been said, a good real ammeter
must have a very small voltage drop, this in order not to alter the circuit
under measurement with its insertion. The sensor I present has a voltage drop
of only 50 mV and uses easily available components.
The shunt
The most accurate system is to use a shunt,
i.e. a resistor with very low resistance compared to that of the load and the
generator. Of course today we also find on the market sensors based on the Hall
effect, that is a device that measures the magnetic field generated by the
current passing through a wire for the well-known Ampere’s law, but these
semiconductor sensors do not have comparable stability and precision. Figure 1 shows the typical appearance of a 10A
and 75mV shunt.
For low currents it is possible to realize the
shunt with 0.1 Ω wire resistors, possibly put in parallel, or with a piece of
constantan wire of adequate section and length.
Measurement techniques
There are two ways to insert the shunt:
1)
low-side: the shunt is
connected between the load and the mass.
2)
high-side: the shunt is
connected between the power supply and the load.
Figure
2 shows the two connection modes.
The first mode has as advantages a low common mode voltage, inputs and
outputs related to ground and circuit simplicity but is affected by
disturbances of ground returns
The second way is not affected by mass disturbances but the amplifier
must work with high common mode voltage values and the circuit is more complex.
A low-side solution
There are several breakout boards available on
the market ready to measure the current with Arduino, but for those who love
self-construction and also to understand the functioning of the circuits, I
suggest to carry out my project. It is a fairly accurate current sensor while
using common components.
I use the shunt in low-side mode connected to a
differential amplifier with Arduino-compatible output with internal reference,
therefore with ADC overall voltage measurement range equal to 1.1 volts. With
this solution a more stable and less noisy reference is obtained with a
resolution of 1100/1024 = 1.074 mV.
Using operational power amplifiers with a
single power supply there are problems if the signals approach zero. Even using
special operational amplifiers called "rail to rail", whose output swing
is closer to the rails, rarely they have outputs below 20 mV at zero input. A
similar output can also be obtained using a most common LM358 suitable for
working even with single power supply.
An expedient to overcome this drawback is to
operate on a value higher than zero and therefore to subtract this constant at
software level. To have a zero in output and also to eliminate the off-set I
added a Vsh constant of about 100 mV. For this circuit I used a differential
amplifier U1a, whose output is equal to:
Vo = (Vin+
- Vin-)R2/R1+Vsh
The second operational U1b, connected as voltage follower, generates the
voltage Vsh by means of a divider on the Arduino Vref.
For correct operation the following relations must be respected: R1=R3
and R2=R4 with very low tolerances. I have selected them among metal film
resistors with a 1% tolerance, now easily available, using a good ohmmeter.
Since the double LM358 does not have a very low offset I preferred not to
overdo the gain, so R2/R1 = 20.
The ADC converter has a full scale of 1100 mV and I take 100 out of it
to make the zero, so I can have 1000 mV for the measurement, this means that
the maximum input voltage must be 1000/20 = 50 mV.
The following table shows some shunt values and related flow rates.
I max [A]
|
Shunt [Ω]
|
0.5
|
0.1
|
1
|
0.05
|
5
|
0.01
|
10
|
0.005
|
It is also possible to use commercial shunts. Many of them have a
voltage drop of 75 mV so we can vary the gain that becomes 1000/75 = 200/15 =
13.33 or calculate the new full scale which is equal to 2/3. For example, a 10
A and 75 mV shunt has a resistance Rs = 7.5 mΩ and a full scale I = 50 / 7.5 =
6.66 A.
To have a sensitivity of 75 mV, suitable for many commercial shunts, it
is sufficient to set R1 = R3 = 15kΩ and R2 = R4 = 200kΩ, standard and easily
available values.
Figure 3 shows the circuit and its connections with Arduino.
To test this circuit I used a 0.1 Ω shunt
resistor and a 0.1 A current source.
Significant improvements are achieved by
replacing the LM358 with a low off-set input like the OP290, OPA2196, OPA2277 and
similar chips.
In figure 4 we can see the layout and the
connections of the components of my prototype, made on a small pre-drilled pcb
board.
The op-amp offset
Not considering here the problem of saturation
already seen for the single power supply, a real operational amplifier has no
zero-volt output with zero input, the input offset is defined as the voltage I
have to give to the input to have zero in output.
Figure 5 shows the model for evaluating the
offset of a differential amplifier.
Considering the Vos offset voltage,
the output voltage with zero input is:
Vo = (1+R2/R1)Vos
For example: if an LM358 has an offset of 2 mV,
with R2/R1= 20 there would be a difference of 44 mV (2x21 + 2, including that
of U1b) between the voltage Vo and Vsh. The offset is temperature
dependent and causes a thermal drift, this operational has an input offset
voltage drift of about 7 µV/°C.
For this application the input offset is very
important as this constant is amplified by the amplifier.
Ignoring the thermal drift, we can remove the
offset using a trimmer on the divider that produces Vsh. The rest
will do the software by subtracting a constant. For Vsh= 100 mV this
number is equal to:
Nsh =100*1023/1100 = 93
This is the constant to be removed and we
should set the Rp1 trimmer until zero is read for zero current. In the test
phase I increased this value to 98 to work better with the trimmer.
Components
list
component
|
description
|
component
|
description
|
R1, R3
|
11 kW ± 1% metal film
|
Rp1
|
500 Ω multi-turn trimmer
|
R2, R4
|
220 kW ± 1% metal film
|
Rs
|
0.1 Ω shunt
|
R5
|
10 kW ± 2% metal film
|
C1
|
10 µF,25V Aluminum
electrolytic
|
R6
|
1 kW ± 2% metal film
|
Arduino
|
Arduino UNO or Nano board
|
R7
|
10 kW ±5%
|
U1
|
Dual op amp LM358
|
The program
The example program is very simple, the values
used are relative to my system. To calibrate the Rp1 trimmer, just run the
program and turn the trimmer to read zero on the serial monitor with zero input
current. If it is not possible to zero the trimmer, it is necessary to slightly
modify the Nsh constant to be subtracted from the measurement.
The constant mVtomA is derived from experimental
measurements using a good precision ammeter, a power supply and some resistive
loads such as car bulbs or wire resistors of adequate power.
/* program ArduAmmeter.ino Arduino
current meter
Giovanni Carrera, rev. 11/07/2019 */
float NtomV;
const float VREF = 1095;// in
mV, this value can be read on VREF pin
const int Nsh = 98;// shift
value corresponding to about 100 mV
const float mVtomA = 2.17;// value
obtained by a calibration
void setup() {
Serial.begin(9600);
analogReference(INTERNAL); // internal ADC
reference input = 1100 mV
NtomV = VREF/1023;// constant of conversion
into millivolts
}
void loop() {
int val = analogRead(A0)-Nsh;// read the
current sensor and remove the shift
float mvolt = NtomV*val;// convert to
millivolt
float mamp = mvolt/mVtomA;// convert to
milliampere
Serial.print("Vo = ");
Serial.print(mvolt,1);
Serial.print(" mV - Current = ");
Serial.print(mamp,0);
Serial.println(" mA");
delay(1000);
}
References
1. “AN1332, Current Sensing Circuit Concepts
and Fundamentals”, Microchip DS01332B, 2011.
2. “AN39, Current measurement applications
handbook”, Peter Abiodun Bode, Zetex Semiconductors, 2008.
3. “AN105, Current Sense Circuit Collection”,
Tim Regan, Linear Technology, dec 2005.
4. “Fully
Integrated, Hall Effect-Based Linear Current Sensor IC with 2.1 kVRMS Isolation
and a Low-Resistance Current Conductor”, ACS712-DS,
Rev. 15, Allegro MicroSystems – 2013.
5.
“VARDULOG
- Data logger dei consumi di un apparato elettrico”, Giovanni Carrera, rivista
FARE ELETTRONICA n. 361-362, Novembre-Dicembre 2015;
6.
“Progetto
ArduWattmeter”, Giovanni Carrera, rivista FARE ELETTRONICA
n. 367/368 Giugno/Luglio 2016;
7. “Op Amp
Input Offset Voltage”, Analog Devices, MT-037 Rev.0, 10/08, WK.
