The purpose of this project is to provide a
4-20 mA output from a PWM signal generated by a microcontroller ATmega328 and numerous
other chips, such as the PIC. One of the more interesting applications of this
circuit would be to replace or to realize a smart sensor with Arduino.
Last year I had designed a circuit suitable
only for Arduino Due, this new work makes use of a common Arduino Uno or
similar to create a standard 4-20 mA analog output.
Arduino Uno, or systems based on the ATmega328
chip has no a true analog output. The easiest way is to use one of the PWM
outputs and filter the signal with a passive RC filter to obtain an analog
signal proportional to the duration of the pulses. This expedient creates a
considerable noise due to the frequency of the PWM itself. To eliminate the
noise I used a second order active low-pass filter, Sallen-key type. The
frequency of the Arduino PWM (with 16 MHz clock) on pin 9 is about 490 Hz, so I
used a very low cutoff frequency (11 Hz) but with a bandwidth sufficient for
the majority of industrial controls.
By connecting the filter directly to the PWM output
is obtained a signal which varies from 0 to 5 V which would give an output
current of 0 to 20 mA. The pulses duration is programmed with a word of 8 bits,
losing 1/5 of the full scale. To improve the current resolution from 20/255 to
16/255, I modified the minimum amplitude of pulses from 0 to 1 volts, giving at
the output a 4 to 20mA current. The block diagram is shown in figure 1.
Figure 2 shows the complete diagram of the
circuit. To obtain pulses from 1 to 5 volts I had to use a 1 V source realized
with U1A and the transistor Q1 that works as a switch. The operational U1B
operates as a separator; the filter uses U1C and the voltage / current
converter uses U1D and Q2.
The transistor Q1 inverts the PWM signal, so
the software must complement the number of PWM duty cycle.
The trimmer pot Rp1 is used to adjust the minimum
output current (4 mA) and the Rp2 to adjust the maximum (20 mA). The
theoretical value of the emitter resistor is Re = 5/0.02 = 250 W, but that does not take into
account the tolerances of the voltage supply of Arduino and of the resistors.
The resistor R8 is used as U1D output current
limiter in the situation of absence of load.
A step down converter is a good solution for
powering the system because of the 24 V, this value can be varied from 12 to 30
V, depending of the load circuit.
Arduino Uno has a +5 V output pin, It does not
recommend using it as a power input inasmuch
this would be in parallel with the internal regulator but it can be powered at
+5V using the USB connector, other boards as Arduino Pro Mini, have a +5 V
input.
Hardware components
1x Arduino board,
1x Step-down switching converter,
Components list
R1= 27 kW ±5%
|
R2= 47 kW ±5%
|
R3= 10 kW ±5%
|
R4= 27 kW ±1% metal film
|
R5= 6.2 kW ±1% metal film
|
R6= 100 kW ±1% metal film
|
R7= 100 kW ±1% metal film
|
R8= 1 kW ±5%
|
R9= 270 W ±1% metal film
|
R10= 1.8 kW ±1% metal film
|
Rp1= 1 kW trimmer
|
Rp2= 10 kW trimmer
|
C1= 100nF Mylar
|
C2= 100 nF Mylar
|
C3= 200 nF Mylar
|
C4 = 10 MF,50V Electrolytic
|
C5 = 100 nF Mylar
|
U1= LM324 quad op amp
|
Q1 = 2N3904 or eq.
|
Q2= 2N2219A or eq.
|
The capacitors used for the filter must be
measured with a capacimeter, for my prototype I selected for C3 some 220 nF
capacitors to search for a value that approached 200 nF and C2 have selected a
value half of C3. Q1 is a transistor
that must have a low Vce(sat.) and Q2 must have a current gain of at least 100
and a Vceo of at least 40V with a minimum power of 500mW.
The operational amplifier U1 must be suitable also
for single-rail power supply, such as LM324.
The components layout of my prototype is shown
in Figure 3, the resistor on the top is a precision load used for calibration
of the system. Q2 has a small heat sink because, with at 20 mA and a low
voltage load, as in this case, dissipates: (24-3-5) *0.02 = 320 mW. In these
circumstances is better to reduce the 24 v.
The test program
To test the system I used an Arduino with an
LCD display and a potentiometer connected to analog input A0, as pin PWM I used
D9. The program is very simple: read the potentiometer, converts 10 to 8-bit
Analog reading and produces the PWM.
//
program to test Arduino Uno PWM
// G. Carrera 2 ago 2016
#include <LiquidCrystal.h>
int
PWMpin = 9; // PWM out on digital
pin 9
int
analogPin = 0; // potentiometer
connected toA0
int
val = 0; // variable to store the
read value
char
spacestring[17] ="
";
//
initialize the library with the numbers of the interface pins
LiquidCrystal
lcd(7, 6, 5, 4, 3, 2);
void
setup() {
pinMode(PWMpin,
OUTPUT); // sets the pin as output
lcd.begin(16, 2);// set up number of columns
and rows
lcd.setCursor(0, 0);// set the cursor to
column 0, line 0
lcd.print("Stalker PWM test");//
Print a message to the LCD
}
void
loop() {
val = analogRead(analogPin) >> 2;// 10
to 8 bit conversion
analogWrite(PWMpin, val);
lcd.setCursor(0,
1);
lcd.print(spacestring);
lcd.setCursor(0, 1);
lcd.print(val);
delay(500);
}
I reported on the spreadsheet the PWM values
and the measurements in volts made on a precision resistor (150 W ±0.5%) that worked as a load,. The
PWM / output current diagram is shown in Figure 4.
The linearity is very good as confirmed by the coefficient
of determination R2 = 0.999992.
If you want a positive slope, the value must be
complemented to 255 in this mode:
val
= 255-val;
In my program, you could generate a new value
every 500 milliseconds (2 Hz), but you could reduce this period of up to 100 ms
(10 Hz).
