Recently, it was necessary to test various very powerful batteries with a voltage of 24 to 55 V. Since it is unrealistic to select resistors for such high currents, I had to build something completely electronic. The design of an artificial load served as the base. Since its power was too low, it increased somewhat.
Electrical circuit diagram EN
As a power element, 8 resistors of 0.68 ohms are used, connected to the power transistor IGBT. Why IGBT? During the tests, several conventional MOSFETs flew out, and IGBTs were noticeably more stable. Resistors are installed on radiators, 4 pcs. Depending on the needs, they are connected in series for higher load voltages or in parallel for weaker ones. The radiators are screwed at a distance of 1 cm from the bottom of the case, holes are drilled under the radiators, the cooling air consumption is significant.
The power transistor is mounted on a heatsink from a PC processor, cooled by two fans.
As a measuring element and a reference for the operational amplifier, a 0.01 Ohm resistor is used, and counters on ICL7107 microcircuits are used as meters - current accuracy is 0.1 A, voltage is 0.1 V.
Electric power for meters and fans - removed from some kind of pulse device with parameters + 5 V to 5 A (indicators), +/- 12 V to 2 A (fans and op-amps). There was a cool metal case from some old device, and it was decided to use it. The front panel is made from a piece of 3mm PVC plate. There are holes for fans in the back.
Load operation test
- Circuit tested at voltages of 28V at 20A - power dissipated in resistors and 560W IGBTs - with cooling and under load for one hour - temperature 40 degrees.
- Another artificial load test was carried out with a 55 V battery at 11 A / h - here the load was 15 - 20 A, which means the power reached 1 kW - the radiators became hot, especially those on which power resistors were installed. The resistors heated up to about 110 degrees, the IGBT transistor to a temperature of 90 degrees, in principle, is acceptable.
- Naturally, you can easily test car batteries with 12 V at 20 A mode - while the temperature was 80 degrees, which is normal.
Ways to improve the device
In the future, further improvement of this homemade electronic load by adding a power meter and mode controller on the Arduino (from Aliexpress).
The construction of the device cost mainly the cost of power resistors - the rest was lying around from disassembling all sorts of things.
It will also add some jacks to have multiple voltage ranges for testing without switching high power resistors.
This device is designed and used to test DC power supplies, voltage up to 150V. The device allows you to load power supplies with a current of up to 20A, with a maximum power dissipation of up to 600 watts.
General description of the scheme
Figure 1 - Schematic diagram of the electronic load.
The diagram shown in Figure 1 allows you to smoothly adjust the load of the power supply under test. Powerful field-effect transistors T1-T6 connected in parallel are used as the equivalent of the load resistance. To accurately set and stabilize the load current, the circuit uses a precision operational amplifier OU1 as a comparator. The reference voltage from the divider R16, R17, R21, R22 is supplied to the non-inverting input of the OU1, the comparison voltage from the current-measuring resistor R1 is supplied to the inverting input. An enhanced error from the output of OU1 affects the gates of field-effect transistors, thereby stabilizing the given current. Variable resistors R17 and R22 are placed on the front panel of the device with a graduated scale. R17 sets the load current in the range from 0 to 20A, R22 in the range from 0 to 570 mA.
The measuring part of the circuit is based on the ICL7107 ADC with LED digital indicators. The reference voltage for the microcircuit is 1V. To match the output voltage of the current-measuring sensor with the input of the ADC, a non-inverting amplifier with an adjustable gain of 10-12, assembled on a precision operational amplifier OU2, is used. Resistor R1 is used as a current sensor, as in the stabilization circuit. The display panel displays either the load current or the voltage of the tested power source. Switching between modes is done with the S1 button.
The proposed scheme implements three types of protection: overcurrent protection, thermal protection and polarity reversal protection.
In the maximum current protection, it is possible to set the cutoff current. The overcurrent circuit consists of a comparator at the OU3 and a switch that switches the load circuit. As a key, a T7 field-effect transistor with a low open channel resistance is used. The reference voltage (equivalent to the cutoff current) is supplied from the divider R24-R26 to the inverting input of OU3. The variable resistor R26 is placed on the front panel of the device with a graduated scale. Trimmer resistor R25 sets the minimum protection operation current. The comparison signal comes from the output of the measuring OU2 to the non-inverting input of the OU3. If the load current exceeds the set value, a voltage close to the supply voltage appears at the output of the OU3, thereby turning on the MOC3023 dynistor relay, which in turn closes the T7 transistor and supplies power to the LED1 LED, which signals the current protection has been triggered. Reset occurs after the device is completely disconnected from the network and reconnected.
Thermal protection is made on the comparator OU4, temperature sensor RK1 and executive relay RES55A. An NTC thermistor is used as a temperature sensor. The response threshold is set by the trimmer resistor R33. Trimmer resistor R38 sets the hysteresis value. The temperature sensor is mounted on an aluminum plate, which is the base for mounting the radiators (Figure 2). If the temperature of the radiators exceeds the set value, the RES55A relay closes the non-inverting input of the OU1 to the ground with its contacts, as a result, the transistors T1-T6 are locked and the load current tends to zero, while the LED2 LED signals the thermal protection has been triggered. After the device cools down, the load current resumes.
Reverse polarity protection is based on a dual Schottky diode D1.
The circuit is powered by a separate mains transformer TP1. Operational amplifiers OU1, OU2 and the ADC chip are connected from a bipolar power source assembled on the L7810, L7805 stabilizers and the ICL7660 inverter.
For forced cooling of the radiators, a 220V fan is used in continuous mode (not indicated in the diagram), which is connected directly to the 220V network through a common switch and fuse.
Schema setup
The scheme is configured in the following order.
A reference milliammeter is connected to the input of the electronic load in series with the power supply under test, for example, a multimeter in the current measurement mode with a minimum range (mA), a reference voltmeter is connected in parallel. Handles of variable resistors R17, R22 are unscrewed to the leftmost position corresponding to zero load current. The device is receiving power. Next, the trimming resistor R12 sets the bias voltage of the OU1 so that the readings of the reference milliammeter become zero.
The next step is to configure the measuring part of the device (indication). The S1 button is moved to the current measurement position, while the dot on the display panel should move to the hundredths position. With the trimmer resistor R18, it is necessary to ensure that zeros are displayed on all segments of the indicator, except for the leftmost one (it must be inactive). After that, the reference milliammeter switches to the maximum measurement range mode (A). Further, the regulators on the front panel of the device set the load current, with the trimming resistor R15 we achieve the same readings with the reference ammeter. After calibrating the current measurement channel, the S1 button switches to the voltage indication position, the dot on the display should move to the tenth position. Next, with a trimming resistor R28, we achieve the same readings with a reference voltmeter.
MTZ setting is not required if all ratings are met.
The thermal protection setting is carried out experimentally, the temperature regime of power transistors should not go beyond the regulated range. Also, the heating of an individual transistor may not be the same. The response threshold is adjusted by the trimmer R33 as the temperature of the hottest transistor approaches the maximum documented value.
Element base
As power transistors T1-T6 (IRFP450), MOSFET N-channel transistors with a drain-source voltage of at least 150V, a dissipation power of at least 150W and a drain current of at least 5A can be used. The field effect transistor T7 (IRFP90N20D) operates in the key mode and is selected based on the minimum value of the channel resistance in the open state, while the drain-source voltage must be at least 150V, and the continuous current of the transistor must be at least 20A. As precision operational amplifiers op-amp 1.2 (OP177G), any similar operational amplifiers with a bipolar 15V supply and the ability to control the bias voltage can be used. A fairly common LM358 chip is used as operational amplifiers op-amp 3.4.
Capacitors C2, C3, C8, C9 are electrolytic, C2 is selected for a voltage of at least 200V and a capacity of 4.7µF. Capacitors C1, C4-C7 are ceramic or film. Capacitors C10-C17, as well as resistors R30, R34, R35, R39-R41 are surface mounted and are placed on a separate indicator board.
Trimmer resistors R12, R15, R18, R25, R28, R33, R38 are multi-turn firms BOURNS type 3296. Variable resistors R17, R22 and R26 are domestic single-turn types SP2-2, SP4-1. As a current-measuring resistor R1, a shunt was used, soldered from a non-working multimeter, with a resistance of 0.01 Ohm and rated for a current of 20A. Fixed resistors R2-R11, R13, R14, R16, R19-R21, R23, R24, R27, R29, R31, R32, R36, R37 type MLT-0.25, R42 - MLT-0.125.
The imported microcircuit of the analog-to-digital converter ICL7107 can be replaced with a domestic analog KR572PV2. Instead of the BS-A51DRD LED indicators, any single or dual seven-segment indicators with a common anode without dynamic control can be used.
The thermal protection circuit uses a domestic low-current reed relay RES55A (0102) with one changeover contact. The relay is selected taking into account the response voltage of 5V and the coil resistance of 390 ohms.
To power the circuit, a small-sized 220V transformer with a power of 5-10W and a secondary winding voltage of 12V can be used. Almost any diode bridge with a load current of at least 0.1A and a voltage of at least 24V can be used as a rectifier diode bridge D2. The current regulator chip L7805 is mounted on a small heatsink, the approximate power dissipation of the chip is 0.7W.
Design features
The body base (picture 2) is made of 3mm thick aluminum sheet and 25mm angle. 6 aluminum radiators, previously used for cooling thyristors, are screwed to the base. To improve thermal conductivity, Alsil-3 thermal paste is used.
Figure 2 - Base.
The total surface area of the radiator assembled in this way (Figure 3) is about 4000 cm2. An approximate estimate of the dissipation power is taken from the calculation of 10cm2 per 1W. Taking into account the use of forced cooling using a 120mm fan with a capacity of 1.7 m3 / h, the device is capable of dissipating up to 600W for a long time.
Figure 3 - Radiator assembly.
Power transistors T1-T6 and a dual Schottky diode D1, whose base is a common cathode, are attached to the heatsinks directly without an insulating gasket using thermal paste. The current protection transistor T7 is attached to the heatsink through a heat-conducting dielectric substrate (Figure 4).
Figure 4 - Mounting transistors to the heatsink.
The installation of the power part of the circuit is made with a heat-resistant wire RKGM, the switching of the low-current and signal parts is made with a conventional wire in PVC insulation using a heat-resistant braid and a heat-shrink tube. Printed circuit boards are made by the LUT method on foil textolite, 1.5 mm thick. The layout inside the device is shown in Figures 5-8.
Figure 5 - General layout.
Figure 6 - Main printed circuit board, transformer mounting on the reverse side.
Figure 7 - View of the assembly without a casing.
Figure 8 - View of the assembly from above without casing.
The basis of the front panel is made of electrotechnical sheet getinax 6 mm thick, milled for fastening variable resistors and darkened indicator glass (Figure 9).
Figure 9 - Front panel base.
The decorative appearance (Figure 10) is made using an aluminum corner, a stainless steel ventilation grill, plexiglass, a paper backing with inscriptions and graduated scales compiled in the FrontDesigner3.0 program. The casing of the device is made of millimetric stainless steel sheet.
Figure 10 - Appearance of the finished device.
Figure 11 - Connection diagram.
Archive for article
If you have any questions about the design of the electronic load, ask them on the forum, I will try to help and answer.
In order to test the power supplies, there is an electronic load. This device works on the principle of signal generation. The main parameters of the modifications include the threshold voltage, the allowable overload, and the dissipation factor. There are several types of devices. In order to understand the loads, it is first of all recommended to familiarize yourself with the device circuit.
Modification scheme
The standard load circuit includes resistors, a rectifier, and modulator ports. If we consider low frequency devices, then they use transceivers. These elements work on open contacts. Comparators are used to transmit the signal. Recently, loads on stabilizers have been considered popular. First of all, they are allowed to be used in a DC network. They have a fast transformation process. It is also worth noting that an amplifier and a regulator are considered an integral element of any load. These devices are closed on the plate. They have a fairly high conductivity. It is the modulator that is responsible for the generation process of the models.
Types of modifications
There are pulse and programmable devices. In a separate category, laboratory ones are allocated, which are suitable for powerful power supplies. Also, modifications differ in the frequency with which they work. Low-frequency loads are equipped with transistors with a channel adapter. They are used on AC power. Models of high-frequency type are made on the basis of an open thyristor.
Impulse devices
How is a pulsed electronic load made? First of all, experts recommend choosing a good thyristor for assembly. In this case, the modulator is suitable only for two phases. Experts say that the expander should work alternately. Its operating frequency should be approximately 4000 kHz. The transceiver is installed into the load through a modulator. After soldering the capacitors, it is worth taking up the amplifier.
For stable operation of the load, three channel directivity filters are required. A tester is used to test the device. The resistance should be approximately 55 ohms. With an average load, the load produces around 200 watts. Comparators are used to increase the sensitivity. When the system closes, it is worth checking the circuit from the capacitor. If the resistance on the contacts is underestimated, then the transceiver needs to be changed to a capacitive analogue. Many experts point to the possibility of using wave filters, which have good conductivity. Regulators for these purposes are used on a triode.
Programmable Models
The electronic programmable load is quite easy to assemble. For this purpose, an expansion transceiver for 230 V is used. Three contactors are used to transmit the signal, which depart from the transistor. Regulators are used to control the conversion process. Linear analogs are most often used. The triode is used with an insulator. In this case, you will need a blowtorch. The resistor is directly fixed on the transceiver.
For the model, conventional comparators, which have a low dissipation factor, are definitely not suitable. It is also worth noting that many make a mistake when they install one filter. For normal operation of the prior, only capacitive analogues are used. The nominal voltage at the output should be approximately 200 V with a resistance of 40 ohms. If you assemble devices on a single-junction expander, then linear models are not suitable.
First of all, the device will not work due to a large overload of the thyristor. It is also worth noting that the model will require a line modulator with low sensitivity. Some specialists use stabilizers when assembling. If we consider a simple modification, then an adjustable type is suitable. However, inverting elements are most often used.
Laboratory Modifications
A do-it-yourself laboratory electronic load is being assembled with a powerful thyristor. Resistors are used with a capacitance of 40 pF. Experts say that only expansion type capacitors can be used. Particular attention during assembly should be paid to the modulator. If you use a wired analog, then the load will require three filters. A simple electronic load has a phase-type modulator with a conductivity of 30 microns. The resistance is approximately 55 ohms. It is also worth noting that loads are often stacked based on a switched transceiver. The main feature of such devices lies in the high pulsation. In this case, conductivity is provided at around 30 microns.
FET device
The electronic load is not made only on the basis of the comparator, and the thyristor is used of an adjustable type. When assembling, first of all, it is worth choosing a capacitor unit, which plays a role. In total, three filters are required for modification. The resistor is installed behind the plates. Experts say that the electronic load on the field effect transistor produces a resistance of 40 ohms.
If the conductivity increases greatly, then a capacitive capacitor is installed. It is recommended to use the transceiver directly with two contacts. The relay is installed as standard with the regulator. The rated voltage for loads of this type is not more than 400 watts. Experts say that the lining should be fixed behind the resistor. If we consider a high-frequency model for 300 V power supplies, then the modulator will need a wave type. In this case, a tetrode is installed behind the thyristor.
Variable Current Model
The smooth electronic load circuit includes one thyristor. Capacitors for the model will require an expansion type with low conductivity. It is also worth noting that one amplifier is put into the load. The most commonly used wave analogues, which have a phase adapter. The regulator is directly installed behind the modulator, and the rated voltage should be about 300 watts.
A simple electronic load with continuously adjustable current has two contactors for connection. Thyristors can sometimes be used on plates. Comparators in devices are installed with and without stabilizers. In this case, much depends on the operating frequency. If this parameter exceeds 300 kHz, then it is better not to install a stabilizer. Otherwise, the scattering coefficient will increase significantly.
TL494 based device
The electronic load based on the TL494 is quite easy to assemble. Resistors for modifications are selected in line type. As a rule, they have a high capacity. And they are able to work in a DC network. When assembling the model, the thyristor is used on two plates. Electronic impulse load based on TL494 works with an expander of phase or impulse type.
The first option is the most common. The rated voltage of the loads starts from 220 watts. Filters are used of the full type, and the conductivity is no more than 4 microns. When installing a regulator, it is important to evaluate the output impedance. If this parameter is not constant, then an amplifier is used for the model. Contactors are installed with and without adapters. The output voltage in the circuit is approximately 300 watts for loads. When devices are turned on, the current often rises. This happens due to the heating of the modulator. The user can avoid this problem by lowering the sensitivity.
100W Models
An electronic load (diagram shown below) of 100 W involves the use of two channel thyristors. The transistor in models is quite often used on an expansion basis. It has a conductivity of about 5 microns. It is also worth noting that there are loads on the relay. They are best suited for powerful power supplies. For self-assembly, wave comparators are additionally used. Home-made devices give out a voltage of no more than 300 V, and the operating frequency starts from 120 kHz.
200 W devices
The 200 W electronic load includes two pairs of thyristors, which are connected in pairs. Many models use wired low frequency comparators. It is also worth noting that a modulator is required to assemble the modification. Amplifiers are used to speed up the process. These elements can only work from wired filters.
The transceiver should be installed behind the plates. In this case, the load voltage is approximately 400 V. The specialist says that devices on wired transceivers do not work well. They have low conductivity, there are problems with overheating. If voltage surges are observed, it is worth changing the comparator. Another problem could be the resistor.
How to make a 300W device?
An electronic load of 300 W involves the use of two phase-type thyristors. The rated voltage of the devices is approximately 230 watts. The overload factor in this case depends on the conductivity of the comparator. If you assemble this device yourself, you will need a channel-type modulator. A blowtorch is used to install the element.
Regulators are often used with an adapter. The relay is installed low-resistance type. The dispersion coefficient for a homemade modification is approximately 80%. It is also worth noting that the contactors used are of low sensitivity. How to check the load before switching on? You can do this with a tester. The output voltage for homemade devices is usually 50 ohms. If we consider models with one comparator, then this parameter may be underestimated.
Models for 10 A blocks
An electronic load for a 10 A power supply is collected using an expansion thyristor. Transistors are quite often used at 5 pF, which have low conductivity. It is also worth noting that experts do not recommend using linear analogues. They have little sensitivity. They greatly increase the dissipation factor. Contactors are used to connect to the unit. Modulators are quite often used with adapters.
If we consider the circuit on the capacitor unit, then their frequency is on average 400 kHz. In this case, the sensitivity may change. Contactors are quite often fixed behind the modulator. Stabilizers should be used on two plates. It is also worth noting that a pole resistor is required to assemble the modification. It greatly helps to increase the speed of pulse generation.
Devices for 15 A blocks
The most common loads are for 15 A blocks. They use open resistors. In this case, the transceivers are used with different polarity. In addition, they differ in sensitivity. On average, the voltage of the devices is 320 V. The models differ in conductivity from each other. For the purpose of self-assembly, comparators are used on regulators. Stabilizers are attached before starting their installation.
Experts say that expanders can only be installed through the lining. The input conductivity must be no more than 6 microns. When installing the regulator, the comparator is carefully cleaned. If you assemble a simple model, then the modulator can be used as an inverter type. This will greatly increase the dispersion coefficient. The threshold voltage is on average 200 V. The permissible power parameter is no more than 240 W. It is also worth noting that filters of different types are used for the load. In this case, much depends on the conductivity of the comparator.
Device diagram for 20 A blocks
The electronic load (diagram shown below) for 20 A units is based on binary resistors. They maintain stable high conductivity. The sensitivity in this case is approximately 6 mV. Some modifications are distinguished by a high overload parameter. Relays for models are used on wave transistors. Comparators are used to solve conversion problems. Expanders are often of the phase type. And they can have several adapters. If necessary, the device can be assembled independently. For this, a capacitor unit is used.
The rated voltage for self-made loads starts from 300 W, and the average frequency is 400 kHz. Experts do not advise the use of transient comparators. Regulators are used with plates. An insulator is required to install the comparator. If we consider loads on two thyristors, then filters are used there. On average, the capacitance of the module is 3 pF. The dispersion index for homemade models starts from 50%. When assembling the device, special attention should be paid to the adapter for connecting to the power supply. Contactors are selected pole type. They must withstand large overloads and not overheat.
AMETEK devices
Loads of this brand are distinguished by low conductivity. They are great for 15 A power supplies. Among the models of this company, there are many pulse modifications. Their individual overload is not high, but a high pulse generation rate is provided. Experts first of all note the good security of the elements. They use several filters. They cope with phase noise that distorts signals.
If we consider high-frequency models, then they have several thyristors. It is also worth noting that there are modifications on wired comparators on the market. Based on the usual load of this brand, you can assemble an excellent device for different power supplies. The models have excellent stabilizers and very sensitive transistors.
Features of Sorensen series devices
The standard electronic load of this series includes a thyristor and a linear comparator. Many models are made with pole filters that are capable of operating at high frequencies. It is also worth noting that laboratory modifications are on the market. They have a fairly low dispersion coefficient. Models are quite often used of the switched type. The overload indicator is on average 20 A. Protection systems are used in different classes. There are impulse models on store shelves. They are well suited for testing computer power supplies. Expanders in devices are used with plates.
ITECH series models
Loads of this series are distinguished by high conductivity. They have good security. In this case, multiple transceivers are used. The electronic load for the power supply operates on average at a frequency of 200 kHz. In this case, the overload is 4 A. Amplifiers in devices are used with contact adapters. Thyristors are used of phase or code type. Among the models of this series there are programmable modifications. They are well suited for testing computer power supplies. Transceivers can be found with or without expanders.
Loads based on IRGS4062DPBF
Making an electronic load with your own hands based on this transistor is quite simple. The standard scheme of the model includes two capacitor units and one expander. It should be noted right away that models of this class are well suited for 10 A power supplies. The voltage parameter for loads is 200 W. Filters for devices are selected low frequency. They are able to work under heavy loads.
First of all, a thyristor is installed during assembly, and a different type of comparator can be used. The transistor is directly installed using a soldering iron. If its conductivity exceeds 5 microns, then it is worth installing a dipole filter at the beginning of the circuit. Experts say that the electronic load on the IRGS4062DPBF transistor can be done with transient comparators. However, they have a high dissipation factor.
It is also worth noting that the models of this series are only suitable for DC circuits. The permissible overload parameter of devices is 5 A. If we consider devices on pulse comparators, then they have a lot of advantages. First of all, the high frequency catches the eye. In this case, the resistance devices show at the level of 50 ohms.
They have no problems with conductivity and sudden voltage surges. Stabilizers are allowed to use different types. However, they must operate in a DC circuit. There are also modifications without capacitors on the market. Their dispersion coefficient is approximately 55%. For devices of this class, this is very small.
Devices based on KTC8550
Transistor database loads are highly valued among professionals. Models are great for testing small power units. The allowable overload indicator is usually 5 A. Models may use different protection systems. When assembling the modification, it is allowed to use binary modulators with a conductivity of 4 microns. Thus, the devices will output a higher frequency at the level of 300 kHz.
If we talk about the shortcomings, it is worth noting that the modifications are not able to work with 10 A power supplies. First of all, there are problems with impulse surges. Overheating of the capacitor will also make itself felt. To solve this problem, expanders are installed on the loads. Triodes are usually used with two plates and an insulator.
Usually, in the manufacture (as well as in the repair) of power supplies or voltage converters, it is required to check their performance under load. And then the search begins. Everything that is at hand is used: various incandescent lamps, old electronic lamps, powerful resistors and the like. Selecting the right load in this way is an incredibly costly (both in time and nerves) exercise. Instead, it is very convenient to use an electronic adjustable load. No, no, you don't have to buy anything. Even a schoolboy can do such a load. All you need is a powerful field breaker, an op-amp, a few resistors, and a bigger heatsink. The circuit is more than simple and yet works great.
The idea is to use an opamp to stabilize the voltage drop across a special current-sense resistor. This is done as follows: a certain reference voltage is applied to the non-inverting input of the opamp, and a voltage drop across the current-measuring resistor is applied to the inverting input. The operating unit has such a property that in the steady state, the voltage difference at the inverting and non-inverting inputs is zero (unless, of course, it is in saturation mode, but for this we have a brain with a calculator to calculate and select everything). The output of the op-amp is fed to the gate of the MOSFET and thus controls the turn-on ratio of the FET, and therefore the current through it. And the greater the current through the field, the greater the voltage drop across the current-measuring resistor. This results in negative feedback.
That is, if, as a result of heating, the characteristics of the field device change so that the current through it increases, then this will cause an increase in the voltage drop across the current-measuring resistor, a negative voltage difference (error) will appear at the inputs of the op-amp, and the output voltage of the op-amp will begin to decrease (in this case, the degree the opening of the field and the current through it), until the error becomes equal to zero. If the current through the field device decreases for some reason, then this will cause a decrease in the voltage drop across the current-measuring resistor, a positive voltage difference (error) will appear at the inputs of the op-amp and the output voltage of the op-amp will begin to increase (in this case, the degree of opening of the field device and the current through it will begin to increase ) until the error is zero. In short, such a circuit stabilizes the voltage drop across the current-measuring resistor - after all transients, it is set equal to the reference voltage (which is applied to the non-inverting input).
By changing the reference voltage in this circuit, it is possible to arbitrarily regulate the current through the field device, and the given current is stable, since it depends only on the value of the reference voltage and the resistance of the current-measuring resistor, and does not depend on the MOSFET parameters, which can change very much as a result of heating. The reference voltage can be set with a simple divider, and adjusted with trimmers.
Circuit elements:
Operational amplifier - any that allows single supply, I used OP220.
T1 is a powerful MOSFET, anyone, as long as it could dissipate more power, I took the CEP603AL from an old computer power supply. (here, of course, there is a limit on the opening voltage of the field device and the current through it, but more on that below)
R ti - current-measuring resistor for tenths of an ohm, there are a lot of them everywhere: in printers, monitors, etc., I took 0.22 ohm, 3 W from the printer
R nd \u003d 10 kOhm - resistor that determines the current setting range
R kd \u003d 10 kOhm - resistor that determines the initial current setting range
R gn \u003d 2 kOhm - a resistor with which the current is set within a given range
R tn \u003d 330 Ohm - resistor required to fine-tune the set current
Excellent trimmers, with comfortable handles, can be removed from the boards of old computer monitors.
Ready product:
So let's see how this is calculated.:
U 1 \u003d U p * (R gn + R tn) / (R nd + R kd + R tn + R gn), where U p is the supply voltage, U 1 is the voltage at the non-inverting input of the op-amp
U 2 \u003d I n * R ti, where I n is the load current, U 2 is the voltage drop across the current-measuring resistor (and, accordingly, the voltage at the inverting input of the op-amp)
From the condition of equality of voltages at the inputs of the op-amp, we have:
U p * (R gn + R tn) / (R dn + R kd + R tn + R gn) \u003d I n * R ti, from here we find:
In \u003d Up * (R gn + R tn) / ((R dn + R kd + R tn + R gn) * R ti)
Substituting the values of our resistors into this expression, we determine the current setting ranges:
at Rnd \u003d 10 kOhm, we get In \u003d Up * 2.33 / ((2.33 + 10 + 10) * 0.22) \u003d Up * 0.47
with Rnd=0, we get: In = Up*2.33/((2.33+10)*0.22)=Up*0.86
That is, by changing the resistance of the resistor Rnd from 10 kOhm to zero, we change the upper limit of the current setting range from 0.47 * Up to 0.86 * Up. This means that, for example, for a +10V supply, we can adjust the current in the range from 0 to 4.7 A or from 0 to 8.6 A, depending on the resistance of the resistor R nd , and for + 5V supply from 0 to 2 .35 A or from 0 to 4.3 A. In the specified range, the current is adjusted by the trimmers Rgn (coarse) and Rtn (fine).
There are three restrictions. The first limitation is related to the current sense resistor. Since this resistor is designed for maximum power dissipation P R, then the maximum current through it should not exceed the value determined by the expression: I 2 max \u003d P R /R ti. For the indicated ratings: I 2 max \u003d (3 / 0.22), I max \u003d 3.7 A. You can increase this value by choosing a resistor with a lower resistance (then the ranges will also have to be recalculated), using a radiator or connecting several such resistors in parallel.
The second two limitations are related to the transistor. Firstly, the main dissipated power is allocated on the transistor (therefore, for better heat dissipation, a larger radiator should be screwed to it). Secondly, the transistor starts to open when the voltage between the gate and the source (Vgs exceeds some threshold value, threshold voltage), so the device will not work if the supply voltage is less than this threshold. The same value will affect the maximum possible current at a given supply voltage.