Connecting the pressure sensor to the controller. Connecting sensors with current output to secondary devices

Sensors with a unified current output of 4-20, 0-50 or 0-20 mA, which are most widely used in the field of industrial automation, can have various connection schemes to secondary devices. Modern sensors with low power consumption and a current output of 4-20 mA are most often connected in a two-wire circuit. That is, only one cable with two wires is connected to such a sensor, through which this sensor is powered, and transmission is carried out through the same two wires.

As a rule, sensors with 4-20 mA output and two-wire connection have a passive output and require an external power supply to operate. This power supply can be built directly into the secondary device (into its input) and when the sensor is connected to such a device, a current immediately appears in the signal circuit. Devices that have a power supply for the sensor built into the input are said to be devices with an active input.

Most modern secondary devices and controllers have built-in power supplies to work with sensors with passive outputs.

If the secondary device has a passive input - in fact, just a resistor from which the measuring circuit of the device "reads" the voltage drop proportional to the current flowing in the circuit, then an additional one is needed for the sensor to work. The external power supply in this case is connected in series with the sensor and the secondary device to break the current loop.

Secondary instruments are usually designed and manufactured in such a way that they can be connected to both two-wire 4-20 mA sensors and sensors 0-5, 0-20 or 4-20 mA connected in a three-wire circuit. To connect a two-wire sensor to the input of a secondary device with three input terminals (+U, input and common), the "+U" and "input" terminals are used, the "common" terminal remains free.

Since the sensors, as mentioned above, can have not only an output of 4-20 mA, but, for example, 0-5 or 0-20 mA, or they cannot be connected in a two-wire circuit due to their large own power consumption (more than 3 mA) , then a three-wire connection scheme is used. In this case, the sensor supply circuits and the output signal circuits are separated. Sensors with a three-wire connection usually have an active output. That is, if you apply a supply voltage to a sensor with an active output and connect a load resistance between its output terminals "output" and "common", then a current proportional to the value of the measured parameter will run in the output circuit.

Secondary devices usually have a fairly low-power built-in power supply to power the sensors. The maximum output current of built-in power supplies is usually in the range of 22-50 mA, which is not always enough to power sensors with high power consumption: electromagnetic flow meters, infrared gas analyzers, etc. In this case, to power a three-wire sensor, you have to use an external, more powerful power supply that provides the necessary power. The power supply built into the secondary device is not used.

A similar circuit for connecting three-wire sensors is usually used when the voltage of the power source built into the device does not correspond to the supply voltage that can be supplied to this sensor. For example, the built-in power supply has an output voltage of 24V, and the sensor can be powered from 10 to 16V.

Some secondary devices may have multiple input channels and a powerful enough power supply to power external sensors. It must be remembered that the total power consumption of all sensors connected to such a multi-channel device must be less than the power of the built-in power source designed to power them. In addition, studying the technical characteristics of the device, it is necessary to clearly distinguish the purpose of the power supplies (sources) built into it. One built-in source is used to power the secondary device itself - for the operation of the display and indicators, output relays, the electronic circuit of the device, etc. This power supply can have quite a lot of power. The second built-in source is used to power only the input circuits - connected to the sensor inputs.

Before connecting the sensor to the secondary device, you should carefully study the operating manuals for this equipment, determine the types of inputs and outputs (active / passive), check the correspondence between the power consumed by the sensor and the power of the power source (built-in or external) and only after that make the connection. The actual designations of the input and output terminals of sensors and devices may differ from those given above. So the terminals "In (+)" and "In (-)" can be designated +J and -J, +4-20 and -4-20, +In and -In, etc. The "+U supply" terminal can be designated as +V, Supply, +24V, etc., the "Output" terminal - Out, Sign, Jout, 4-20 mA, etc., the "common" terminal - GND , -24V, 0V, etc., but this does not change the meaning.

Sensors with a current output having a four-wire connection scheme have a similar connection scheme as two-wire sensors, with the only difference that the four-wire sensors are powered by a separate pair of wires. In addition, four-wire sensors may have both, which must be taken into account when choosing a connection scheme.

In the process of automation of technological processes for the control of mechanisms and units, one has to deal with measurements of various physical quantities. This can be temperature, pressure and flow of liquid or gas, rotational speed, light intensity, information about the position of parts of mechanisms, and much more. This information is obtained using sensors. Here, first, about the position of the parts of the mechanisms.

Discrete sensors

The simplest sensor is a conventional mechanical contact: the door is opened - the contact opens, closed - it closes. Such a simple sensor, as well as the above algorithm of work, often. For a mechanism with translational movement, which has two positions, for example, a water valve, you will need two contacts already: one contact is closed - the valve is closed, the other is closed - it is closed.

A more complex translational motion algorithm has a mechanism for closing the mold of an injection molding machine. Initially, the mold is open, this is the starting position. In this position, finished products are removed from the mold. Next, the worker closes the protective fence and the mold begins to close, a new work cycle begins.

The distance between the halves of the mold is quite large. Therefore, at first the mold moves quickly, and at some distance before the halves close, the limit switch is triggered, the movement speed decreases significantly and the mold closes smoothly.

Such an algorithm avoids impact when the mold is closed, otherwise it can simply be split into small pieces. The same change in speed occurs when the mold is opened. Here, two contact sensors are indispensable.

Thus, contact-based sensors are discrete or binary, have two positions, closed - open or 1 and 0. In other words, you can say that an event has occurred or not. In the example above, several points are "caught" by the contacts: the beginning of the movement, the point of deceleration, the end of the movement.

In geometry, a point has no dimensions, just a point and that's it. It can either be (on a sheet of paper, in the trajectory, as in our case) or it simply does not exist. Therefore, discrete sensors are used to detect points. It may be that a comparison with a point is not very appropriate here, because for practical purposes they use the value of the accuracy of a discrete sensor, and this accuracy is much greater than a geometric point.

But in itself, mechanical contact is an unreliable thing. Therefore, wherever possible, mechanical contacts are replaced by non-contact sensors. The simplest option is reed switches: the magnet approaches, the contact closes. The accuracy of the reed switch operation leaves much to be desired; such sensors are used just to determine the position of the doors.

A more complex and accurate option should be considered various non-contact sensors. If the metal flag entered the slot, then the sensor worked. BVK sensors (Proximity Limit Switch) of various series can be cited as an example of such sensors. The response accuracy (stroke differential) of such sensors is 3 millimeters.

Figure 1. BVK series sensor

The supply voltage of the BVK sensors is 24V, the load current is 200mA, which is quite enough to connect intermediate relays for further coordination with the control circuit. This is how BVK sensors are used in various equipment.

In addition to BVK sensors, sensors of the BTP, KVP, PIP, KVD, PISCH types are also used. Each series has several types of sensors, indicated by numbers, for example, BTP-101, BTP-102, BTP-103, BTP-211.

All mentioned sensors are non-contact discrete, their main purpose is to determine the position of parts of mechanisms and assemblies. Naturally, there are many more of these sensors; it is impossible to write about all of them in one article. Even more common and still widely used are various contact sensors.

Application of analog sensors

In addition to discrete sensors, analog sensors are widely used in automation systems. Their purpose is to obtain information about various physical quantities, and not just like that in general, but in real time. More precisely, the conversion of a physical quantity (pressure, temperature, illumination, flow, voltage, current) into an electrical signal suitable for transmission via communication lines to the controller and its further processing.

Analog sensors are usually located quite far from the controller, which is why they are often called field devices. This term is often used in the technical literature.

An analog sensor usually consists of several parts. The most important part is the sensitive element - sensor. Its purpose is to convert the measured value into an electrical signal. But the signal received from the sensor is usually small. To obtain a signal suitable for amplification, the sensor is most often included in a bridge circuit - Wheatstone bridge.

Figure 2. Wheatstone bridge

The original purpose of the bridge circuit is to accurately measure resistance. A DC source is connected to the diagonal of the AD bridge. A sensitive galvanometer with a midpoint, with zero in the middle of the scale, is connected to the other diagonal. To measure the resistance of the resistor Rx by rotating the tuning resistor R2, the bridge should be balanced, the galvanometer needle should be set to zero.

The deviation of the arrow of the device in one direction or another allows you to determine the direction of rotation of the resistor R2. The value of the measured resistance is determined by the scale, combined with the handle of the resistor R2. The equilibrium condition for the bridge is the equality of the ratios R1/R2 and Rx/R3. In this case, zero potential difference is obtained between the points BC, and no current flows through the galvanometer V.

The resistance of resistors R1 and R3 is selected very accurately, their spread should be minimal. Only in this case, even a small imbalance of the bridge causes a fairly noticeable change in the voltage of the BC diagonal. It is this property of the bridge that is used to connect sensitive elements (sensors) of various analog sensors. Well, then everything is simple, a matter of technology.

To use the signal received from the sensor, its further processing is required, - amplification and conversion into an output signal suitable for transmission and processing by the control circuit - controller. Most often, the output signal of analog sensors is current (analog current loop), less often voltage.

Why current? The fact is that the output stages of analog sensors are based on current sources. This allows you to get rid of the influence of the resistance of the connecting lines on the output signal, to use connecting lines of great length.

Further transformation is quite simple. The current signal is converted into voltage, for which it is enough to pass the current through a resistor of known resistance. The voltage drop across the measuring resistor is obtained according to Ohm's law U=I*R.

For example, for a current of 10 mA across a 100 Ohm resistor, the voltage will be 10 * 100 = 1000 mV, as much as a whole 1 volt! In this case, the output current of the sensor does not depend on the resistance of the connecting wires. Within reasonable limits, of course.

Connecting analog sensors

The voltage obtained on the measuring resistor is easily converted into a digital form suitable for input into the controller. The conversion is done with analog-to-digital converters ADC.

Digital data is transmitted to the controller in serial or parallel code. It all depends on the specific switching scheme. A simplified analog sensor connection diagram is shown in Figure 3.

Figure 3. Connecting an analog sensor (click on the picture to enlarge)

Actuators are connected to the controller, or the controller itself is connected to a computer included in the automation system.

Naturally, analog sensors have a complete design, one of the elements of which is a housing with connecting elements. As an example, Figure 4 shows the appearance of the overpressure sensor of the Zond-10 type.

Figure 4. Overpressure sensor Zond-10

At the bottom of the sensor, you can see the connecting thread for connecting to the pipeline, and on the right, under the black cover, there is a connector for connecting the communication line with the controller.

The threaded connection is sealed with an annealed copper washer (supplied with the sensor), and by no means with fum-tape or linen. This is done so that when installing the sensor, the sensor element located inside is not deformed.

Analog Sensor Outputs

According to the standards, there are three ranges of current signals: 0…5mA, 0…20mA and 4…20mA. What is their difference, and what features?

Most often, the dependence of the output current is directly proportional to the measured value, for example, the higher the pressure in the pipe, the greater the current at the output of the sensor. Although sometimes an inverse connection is used: a larger value of the output current corresponds to the minimum value of the measured value at the output of the sensor. It all depends on the type of controller used. Some sensors even have switching from direct to inverse signal.

The output signal in the 0...5mA range is very small and therefore susceptible to interference. If the signal of such a sensor fluctuates with a constant value of the measured parameter, then there is a recommendation to install a capacitor with a capacity of 0.1 ... 1 μF in parallel with the sensor output. More stable is the current signal in the range of 0…20mA.

But both of these ranges are not good because zero at the beginning of the scale does not allow you to unambiguously determine what happened. Or did the measured signal actually take on a zero level, which is possible in principle, or did the communication line simply break? Therefore, they try to refuse the use of these ranges, if possible.

The signal of analog sensors with an output current in the range of 4 ... 20 mA is considered more reliable. Its noise immunity is quite high, and the lower limit, even if the measured signal has a zero level, will be 4mA, which allows us to say that the communication line is not broken.

Another good feature of the 4 ... 20mA range is that the sensors can be connected with just two wires, since the sensor itself is powered by this current. This is its consumption current and at the same time a measuring signal.

The power supply for sensors in the range 4 ... 20mA is turned on, as shown in Figure 5. At the same time, Zond-10 sensors, like many others, according to the passport, have a wide supply voltage range of 10 ... 38V, although they are most often used with a voltage of 24V.

Figure 5. Connecting an analog sensor with an external power supply

This diagram contains the following elements and symbols. Rsh - measuring shunt resistor, Rl1 and Rl2 - communication line resistances. To improve measurement accuracy, a precision measuring resistor should be used as Rsh. The passage of current from the power supply is shown by arrows.

It is easy to see that the output current of the power supply passes from the +24V terminal, through the line Rl1 reaches the sensor terminal +AO2, passes through the sensor and through the sensor output contact - AO2, the connecting line Rl2, the resistor Rsh returns to the -24V power supply terminal. Everything, the circuit is closed, the current flows.

If the controller contains a 24V power supply, then the connection of a sensor or measuring transducer is possible according to the scheme shown in Figure 6.

Figure 6. Connecting an analog sensor to a controller with an internal power supply

This diagram shows another element - a ballast resistor Rb. Its purpose is to protect the measuring resistor in case of a short circuit in the communication line or a malfunction of the analog sensor. Installing a resistor Rb is optional, although desirable.

In addition to various sensors, the current output also has measuring transducers, which are used quite often in automation systems.

Measuring transducer- a device for converting voltage levels, for example, 220V or current of several tens or hundreds of amperes into a current signal of 4 ... 20mA. Here, the level of the electrical signal is simply converted, and not the representation of some physical quantity (speed, flow, pressure) in electrical form.

But the matter, as a rule, is not enough with a single sensor. Some of the most popular measurements are temperature and pressure measurements. The number of such points in modern production can reach several tens of thousands. Accordingly, the number of sensors is also large. Therefore, several analog sensors are most often connected to one controller at once. Of course, not several thousand at once, it’s good if a dozen is different. Such a connection is shown in Figure 7.

Figure 7. Connecting multiple analog sensors to the controller

This figure shows how a voltage is obtained from a current signal, suitable for conversion into a digital code. If there are several such signals, then they are not processed all at once, but are separated in time, multiplexed, otherwise a separate ADC would have to be installed on each channel.

For this purpose, the controller has a circuit switching circuit. The functional diagram of the switch is shown in Figure 8.

Figure 8. Analog sensor channel switch (clickable image)

The current loop signals converted into voltage across the measuring resistor (UR1…URn) are fed to the input of the analog switch. The control signals alternately pass to the output one of the signals UR1…URn, which are amplified by the amplifier, and are alternately fed to the input of the ADC. The voltage converted into a digital code is supplied to the controller.

The scheme, of course, is very simplified, but it is quite possible to consider the principle of multiplexing in it. Approximately this is how the module for input of analog signals of MCTS controllers (microprocessor system of technical means) produced by the Smolensk PC "Prolog" is built. The appearance of the MCTS controller is shown in Figure 9.

Figure 9. MSTS controller

The release of such controllers has long been discontinued, although in some places, far from the best, these controllers are still in use. These museum exhibits are being replaced by controllers of new models, mainly imported (Chinese) production.

If the controller is mounted in a metal cabinet, it is recommended to connect the braided shields to the cabinet earth point. The length of connecting lines can reach more than two kilometers, which is calculated using the appropriate formulas. We will not count anything here, but believe that this is so.

New sensors, new controllers

With the advent of new controllers, new analog transmitters with HART protocol(Highway Addressable Remote Transducer)

The output signal of the sensor (field device) is an analog current signal in the range of 4 ... 20 mA, on which a frequency-modulated (FSK - Frequency Shift Keying) digital communication signal is superimposed.

Figure 10. HART Analog Transmitter Output

The figure shows an analog signal with a sinusoid coiling around it like a snake. This is the frequency - modulated signal. But this is not a digital signal at all, it has yet to be recognized. It is noticeable in the figure that the frequency of the sinusoid when transmitting a logical zero is higher (2.2 kHz) than when transmitting a unit (1.2 kHz). The transmission of these signals is carried out by a current with an amplitude of ± 0.5 mA of a sinusoidal shape.

It is known that the average value of the sinusoidal signal is equal to zero, therefore, the transmission of digital information does not affect the output current of the sensor 4 ... 20mA. This mode is used when configuring sensors.

HART communication takes place in two ways. In the first case, the standard one, only two devices can exchange information over a two-wire line, while the output analog signal 4 ... 20mA depends on the measured value. This mode is used when configuring field devices (sensors).

In the second case, up to 15 sensors can be connected to a two-wire line, the number of which is determined by the parameters of the communication line and the power of the power supply. This is the multipoint mode. In this mode, each sensor has its own address in the range 1…15, by which the control device accesses it.

The sensor with address 0 is disconnected from the communication line. Data exchange between the sensor and the control device in multipoint mode is carried out only by a frequency signal. The current signal of the sensor is fixed at the required level and does not change.

Data in the case of multipoint communication means not only the results of measurements of the controlled parameter, but also a whole set of all kinds of service information.

First of all, these are the addresses of sensors, control commands, settings. And all this information is transmitted over two-wire communication lines. Is it possible to get rid of them too? True, this must be done carefully, only in cases where the wireless connection cannot affect the security of the controlled process.

It turns out that you can get rid of the wires. Already in 2007, the WirelessHART Standard was published, the transmission medium is the unlicensed frequency of 2.4 GHz, on which many computer wireless devices operate, including wireless local area networks. Therefore, WirelessHART devices can also be used without any restrictions. Figure 11 shows a WirelessHART network.

Figure 11. WirelessHART network

These are the technologies that have replaced the old analog current loop. But it does not give up its positions either, it is widely used wherever possible.

Here I separately took out such an important practical issue as the connection of inductive sensors with a transistor output, which are ubiquitous in modern industrial equipment. In addition, there are real instructions for the sensors and links to examples.

The principle of activation (operation) of sensors in this case can be any - inductive (approximation), optical (photoelectric), etc.

In the first part, possible options for sensor outputs were described. There should be no problems with connecting sensors with contacts (relay output). And with transistors and with connecting to the controller, not everything is so simple.

Connection diagrams for PNP and NPN sensors

The difference between PNP and NPN sensors is that they switch different poles of the power source. PNP (from the word “Positive”) switches the positive output of the power supply, NPN - negative.

Below, for example, are the connection diagrams for sensors with a transistor output. Load - as a rule, this is the input of the controller.

sensor. The load (Load) is constantly connected to the “minus” (0V), the supply of discrete “1” (+V) is switched by a transistor. NO or NC sensor - depends on the control circuit (Main circuit)

sensor. Load (Load) is constantly connected to the "plus" (+V). Here, the active level (discrete “1”) at the output of the sensor is low (0V), while the load is powered through the opened transistor.

I urge everyone not to get confused, the work of these schemes will be described in detail later.

The diagrams below show basically the same thing. The emphasis is on the differences in the circuits of PNP and NPN outputs.

Connection diagrams for NPN and PNP sensor outputs

On the left figure - a sensor with an output transistor NPN. The common wire is switched, which in this case is the negative wire of the power source.

On the right - the case with a transistor PNP at the exit. This case is the most frequent, since in modern electronics it is customary to make the negative wire of the power source common, and activate the inputs of controllers and other recording devices with a positive potential.

How to test an inductive sensor?

To do this, you need to apply power to it, that is, connect it to the circuit. Then - activate (initiate) it. When activated, the indicator will light up. However, the indication does not guarantee the correct operation of the inductive sensor. You need to connect the load, and measure the voltage on it to be 100% sure.

Replacement of sensors

As I already wrote, there are basically 4 types of sensors with a transistor output, which are divided according to their internal structure and switching circuit:

• PNP NO
• PNP NC
• NPN NO
• NPN NC

All these types of sensors can be replaced with each other, i.e. they are interchangeable.

This is implemented in the following ways:

• Alteration of the initiation device - the design changes mechanically.
• Changing the existing scheme for switching on the sensor.
• Switching the type of sensor output (if there are such switches on the sensor body).
• Program reprogramming - changing the active level of this input, changing the program algorithm.

Below is an example of how you can replace a PNP sensor with an NPN one by changing the wiring diagram:

PNP-NPN interchangeability schemes. On the left is the original diagram, on the right is the modified one.

Understanding the operation of these circuits will help the realization of the fact that the transistor is a key element that can be represented by ordinary relay contacts (examples are below, in the notation).

So the diagram is on the left. Let's assume that the sensor type is NO. Then (regardless of the type of transistor at the output), when the sensor is not active, its output “contacts” are open, and no current flows through them. When the sensor is active, the contacts are closed, with all the ensuing consequences. More precisely, with current flowing through these contacts)). The flowing current creates a voltage drop across the load.

The internal load is shown by the dotted line for a reason. This resistor exists, but its presence does not guarantee stable operation of the sensor, the sensor must be connected to the controller input or other load. The resistance of this input is the main load.

If there is no internal load in the sensor, and the collector is “hanging in the air”, then this is called an “open collector circuit”. This circuit ONLY works with a connected load.

So, in a circuit with a PNP output, when activated, the voltage (+V) through the open transistor enters the controller input, and it is activated. How to achieve the same with the release of NPN?

There are situations when the required sensor is not at hand, and the machine should work “right now”.

We look at the changes in the scheme on the right. First of all, the mode of operation of the output transistor of the sensor is provided. For this, an additional resistor is added to the circuit, its resistance is usually of the order of 5.1 - 10 kOhm. Now, when the sensor is not active, voltage (+V) is supplied to the controller input through an additional resistor, and the controller input is activated. When the sensor is active, there is a discrete “0” at the controller input, since the controller input is shunted by an open NPN transistor, and almost all the current of the additional resistor passes through this transistor.

In this case, there is a rephasing of the sensor operation. But the sensor works in the mode, and the controller receives information. In most cases, this is sufficient. For example, in the pulse counting mode - a tachometer, or the number of blanks.

Yes, not exactly what we wanted, and interchangeability schemes for npn and pnp sensors are not always acceptable.

How to achieve full functionality? Method 1 - mechanically move or remake a metal plate (activator). Or the light gap, if we are talking about an optical sensor. Method 2 - reprogram the controller input so that discrete "0" is the active state of the controller, and "1" is passive. If you have a laptop at hand, then the second method is both faster and easier.

Proximity sensor symbol

On circuit diagrams, inductive sensors (proximity sensors) are designated differently. But the main thing is that there is a square rotated by 45 ° and two vertical lines in it. As in the diagrams below.

NO NC sensors. Principal schemes.

On the top diagram there is a normally open (NO) contact (conditionally marked as a PNP transistor). The second circuit is normally closed, and the third circuit is both contacts in one housing.

Color coding of sensor outputs

There is a standard sensor marking system. All manufacturers currently adhere to it.

However, it is useful to make sure that the connection is correct before installation by referring to the connection manual (instructions). In addition, as a rule, the colors of the wires are indicated on the sensor itself, if its size allows.

Here is the marking.

• Blue (Blue) - Minus power
• Brown (Brown) - Plus
• Black (Black) - Exit
• White (White) - the second output, or control input, you have to look at the instructions.

Designation system for inductive sensors

The sensor type is indicated by an alphanumeric code that encodes the main parameters of the sensor. Below is the labeling system for popular Autonics gauges.

Download instructions and manuals for some types of inductive sensors: I meet in my work.

Thank you all for your attention, I'm waiting for questions on connecting sensors in the comments!

Connecting the current sensor to the microcontroller

Having become acquainted with the basics of the theory, we can move on to the issue of reading, transforming and visualizing data. In other words, we will design a simple DC current meter.

The analog output of the sensor is connected to one of the ADC channels of the microcontroller. All necessary transformations and calculations are implemented in the microcontroller program. A 2-line character LCD indicator is used to display data.

Experimental scheme

For experiments with a current sensor, it is necessary to assemble the structure according to the diagram shown in Figure 8. For this, the author used a breadboard and a module based on a microcontroller (Figure 9).

The ACS712-05B current sensor module can be purchased ready-made (it is sold very inexpensively on eBay), or you can make it yourself. The capacitance of the filter capacitor is chosen equal to 1 nF, a blocking capacitor of 0.1 μF is installed on the power supply. To indicate power on, an LED with a quenching resistor is soldered. The power supply and output signal of the sensor are connected to the connector on one side of the module board, the 2-pin connector for measuring the flowing current is located on the opposite side.

For experiments on measuring current, we connect an adjustable constant voltage source to the current-measuring terminals of the sensor through a series resistor 2.7 Ohm / 2 W. The sensor output is connected to the RA0/AN0 port (pin 17) of the microcontroller. A two-line character LCD indicator is connected to port B of the microcontroller and operates in 4-bit mode.

The microcontroller is powered by +5 V, the same voltage is used as a reference for the ADC. The necessary calculations and transformations are implemented in the microcontroller program.

The mathematical expressions used in the conversion process are shown below.

Current sensor sensitivity Sens = 0.185 V/A. With a supply Vcc = 5 V and a reference voltage Vref = 5 V, the calculated ratios will be as follows:

ADC output code

Consequently

As a result, the formula for calculating the current is as follows:

Important note. The above relationships are based on the assumption that the supply voltage and reference voltage for the ADC are 5 V. However, the last expression relating the current I and the ADC output code Count remains valid even with fluctuations in the power supply voltage. This was discussed in the theoretical part of the description.

It can be seen from the last expression that the current resolution of the sensor is 26.4 mA, which corresponds to 513 ADC samples, which exceeds the expected result by one sample. Thus, we can conclude that this implementation does not allow measuring small currents. To increase resolution and increase sensitivity when measuring low currents, you will need to use an operational amplifier. An example of such a circuit is shown in Figure 10.

microcontroller program

The PIC16F1847 microcontroller program is written in C and compiled in the mikroC Pro environment (mikroElektronika). Measurement results are displayed on a two-line LCD display with an accuracy of two decimal places.

Output

With zero input current, the output voltage of the ACS712 should ideally be strictly Vcc/2, i.e. the number 512 should be read from the ADC. A drift of the output voltage of the sensor by 4.9 mV causes a shift in the conversion result by 1 LSB of the ADC (Figure 11). (For Vref = 5.0V, the resolution of a 10-bit ADC would be 5/1024=4.9mV), which corresponds to 26mA of input current. Note that in order to reduce the effect of fluctuations, it is desirable to make several measurements and then average their results.

If the output voltage of the regulated power supply is set to 1 V, through
The resistor must carry a current of about 370 mA. The measured current value in the experiment is 390 mA, which exceeds the correct result by one unit of the LSB of the ADC (Figure 12).

At a voltage of 2 V, the indicator will show 760 mA.

This concludes our discussion of the ACS712 current sensor. However, we have not touched on one more issue. How to use this sensor to measure alternating current? Keep in mind that the sensor provides an instantaneous response corresponding to the current flowing through the test leads. If the current flows in the positive direction (from pins 1 and 2 to pins 3 and 4), the sensitivity of the sensor is positive and the output voltage is greater than Vcc/2. If the current reverses, the sensitivity will be negative and the sensor output voltage will drop below Vcc/2. This means that when measuring an AC signal, the microcontroller's ADC must sample fast enough to be able to calculate the RMS current.

Downloads

The source code of the microcontroller program and the file for the firmware -