All electronic engineers involved in the design of power supply devices sooner or later face the problem of the lack of a load equivalent or the functional limitations of the existing loads, as well as their dimensions. Fortunately, the appearance of cheap and powerful field-effect transistors on the Russian market has somewhat corrected the situation.
Amateur designs of electronic loads based on field-effect transistors began to appear, more suitable for use as electronic resistance than their bipolar counterparts: better temperature stability, almost zero channel resistance in the open state, low control currents - the main advantages that determine the preference for their use as regulating component in powerful devices. Moreover, a wide variety of offers have appeared from device manufacturers, whose price lists are replete with a wide variety of models of electronic loads. But, since manufacturers focus their very complex and multifunctional products called “electronic loads” mainly on production, the prices for these products are so high that only a very wealthy person can afford the purchase. True, it is not entirely clear why a wealthy person needs an electronic load.
I have not noticed any commercially manufactured EN aimed at the amateur engineering sector. This means that you will have to do everything yourself again. Eh... Let's begin.
Advantages of Electronic Load Equivalent
Why, in principle, are electronic load equivalents preferable to traditional means (powerful resistors, incandescent lamps, thermal heaters and other devices) often used by designers when setting up various power devices?Citizens of the portal who are involved in the design and repair of power supplies undoubtedly know the answer to this question. Personally, I see two factors that are sufficient to have an electronic load in your “laboratory”: small dimensions, the ability to control the load power within large limits using simple means (the same way we regulate the sound volume or the output voltage of the power supply - with a regular variable resistor and not by powerful switch contacts, rheostat motor, etc.).
In addition, the “actions” of the electronic load can be easily automated, thus making it easier and more sophisticated to test a power device using an electronic load. At the same time, of course, the engineer’s eyes and hands are freed, and the work becomes more productive. But the delights of all possible bells and whistles and perfections are not in this article, and, perhaps, from another author. In the meantime, let's talk about just one more type of electronic load - pulsed.
Regarding resistor R16. When a current of 10A passes through it, the power dissipated by the resistor will be 5W (with the resistance indicated on the diagram). In the actual design, a resistor with a resistance of 0.1 Ohm is used (the required value was not found) and the power dissipated in its body at the same current will be 10 W. In this case, the temperature of the resistor is much higher than the temperature of the EN keys, which (when using the radiator shown in the photo) do not heat up much. Therefore, it is better to install the temperature sensor on resistor R16 (or in the immediate vicinity), and not on the radiator with EN keys.
A few more photos
This device is designed and used to test DC power supplies with voltages 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 W.
General description of the scheme
Figure 1 - Schematic diagram of the electronic load.
The diagram shown in Figure 1 allows you to smoothly regulate the load of the power supply under test. Power field-effect transistors T1-T6 connected in parallel are used as an equivalent load resistance. To accurately set and stabilize the load current, the circuit uses a precision operational amplifier op-amp1 as a comparator. The reference voltage from the divider R16, R17, R21, R22 is supplied to the non-inverting input of op-amp1, and the comparison voltage from the current-measuring resistor R1 is supplied to the inverting input. The amplified error from the output of op-amp1 affects the gates of the field-effect transistors, thereby stabilizing the specified current. Variable resistors R17 and R22 are located 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 chip 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 power source being tested. Switching between modes occurs with the S1 button.
The proposed circuit implements three types of protection: overcurrent protection, thermal protection and reverse polarity protection.
The maximum current protection provides the ability to set the cutoff current. The MTZ circuit consists of a comparator on OU3 and a switch that switches the load circuit. The T7 field-effect transistor with a low open-channel resistance is used as a key. The reference voltage (equivalent to the cut-off current) is supplied from the divider R24-R26 to the inverting input of op-amp3. Variable resistor R26 is located on the front panel of the device with a graduated scale. Trimmer resistor R25 sets the minimum protection current. The comparison signal comes from the output of the measuring op-amp2 to the non-inverting input of op-amp3. If the load current exceeds the specified value, a voltage close to the supply voltage appears at the output of op-amp3, thereby turning on the MOC3023 dynistor relay, which in turn turns on transistor T7 and supplies power to LED1, which signals the operation of the current protection. The reset occurs after completely disconnecting the device from the network and turning it back on.
Thermal protection is carried out on the comparator OU4, temperature sensor RK1 and executive relay RES55A. A thermistor with negative TCR is used as a temperature sensor. The response threshold is set by trimming resistor R33. Trimmer resistor R38 sets the hysteresis value. The temperature sensor is installed on an aluminum plate, which is the base for mounting the radiators (Figure 2). If the temperature of the radiators exceeds the specified value, the RES55A relay with its contacts closes the non-inverting input of OU1 to ground, as a result, transistors T1-T6 are turned off and the load current tends to zero, while LED2 signals the activation of thermal protection. After the device cools down, the load current resumes.
Protection against polarity reversal is made using a dual Schottky diode D1.
The circuit is powered from a separate network transformer TP1. Operational amplifiers OU1, OU2 and the ADC chip are connected from a bipolar power supply assembled using stabilizers L7810, L7805 and an inverter ICL7660.
For forced cooling of radiators, a 220V fan is used in continuous mode (not indicated in the diagram), which is connected via a common switch and fuse directly to the 220V network.
Setting up the scheme
The circuit is configured in the following order.
A reference milliammeter is connected to the input of the electronic load in series with the power supply being tested, for example a multimeter in current measurement mode with a minimum range (mA), and a reference voltmeter is connected in parallel. The handles of variable resistors R17, R22 are twisted to the extreme left position corresponding to zero load current. The device is receiving power. Next, the tuning resistor R12 sets the bias voltage of op-amp1 so that the readings of the reference milliammeter become zero.
The next step is to configure the measuring part of the device (indication). Button S1 is moved to the current measurement position, and the dot on the display panel should move to the hundredths position. Using trimming resistor R18, it is necessary to ensure that all segments of the indicator, except the leftmost one (it should be inactive), display zeros. After this, the reference milliammeter switches to the maximum measurement range mode (A). Next, the regulators on the front panel of the device set the load current, and using the trimming resistor R15 we achieve the same readings as 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 tenths position. Next, using the trimming resistor R28, we achieve the same readings as the reference voltmeter.
Setting up the MTZ is not required if all ratings are met.
Thermal protection is adjusted experimentally; the operating temperature of power transistors should not exceed the regulated range. Also, the heating of an individual transistor may not be the same. The response threshold is adjusted by trimming resistor R33 as the temperature of the hottest transistor approaches the maximum documented value.
Element base
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 as power transistors T1-T6 (IRFP450). Field-effect transistor T7 (IRFP90N20D) operates in switching 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. Any similar operational amplifiers with a bipolar 15V power supply and the ability to regulate the bias voltage can be used as precision operational amplifiers op-amp 1.2 (OP177G). A fairly common LM358 microcircuit is used as op-amp 3.4 operational amplifiers.
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 placed on a separate indicator board.
Trimmer resistors R12, R15, R18, R25, R28, R33, R38 are multi-turn from BOURNS, type 3296. Variable resistors R17, R22 and R26 are domestic single-turn, type SP2-2, SP4-1. A shunt soldered from a non-working multimeter with a resistance of 0.01 Ohm and rated for a current of 20A was used as a current-measuring resistor R1. 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 analog-to-digital converter chip ICL7107 can be replaced with a domestic analogue 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 operating 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 L7805 current stabilizer chip is installed on a small radiator, the approximate power dissipation of the chip is 0.7 W.
Design features
The base of the housing (Figure 2) is made of 3mm thick aluminum sheet and 25mm angle. 6 aluminum radiators, previously used to cool 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 power dissipation is taken at the rate of 10 cm2 per 1 W. Taking into account the use of forced cooling using a 120mm fan with a capacity of 1.7 m3/hour, the device is capable of continuously dissipating up to 600W.
Figure 3 - Radiator assembly.
Power transistors T1-T6 and dual Schottky diode D1, whose base is a common cathode, are attached directly to the radiators without an insulating gasket using thermal paste. Current protection transistor T7 is attached to the heatsink through a thermally conductive dielectric substrate (Figure 4).
Figure 4 - Attaching transistors to the radiator.
The installation of the power part of the circuit is made with heat-resistant wire RKGM, the switching of the low-current and signal parts is made with ordinary wire in PVC insulation using heat-resistant braiding and heat-shrinkable tubing. Printed circuit boards are manufactured using the LUT method on foil PCB, 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 - Assembly view without casing.
Figure 8 - Top view of the assembly without the casing.
The base of the front panel is made of electrical sheet getinax 6mm thick, milled for mounting variable resistors and tinted indicator glass (Figure 9).
Figure 9 - Front panel base.
The decorative appearance (Figure 10) is made using an aluminum corner, a stainless steel ventilation grille, plexiglass, a paper backing with inscriptions and graduated scales compiled in the FrontDesigner3.0 program. The device casing is made of millimeter-thick stainless steel sheet.
Figure 10 - Appearance of the finished device.
Figure 11 - Connection diagram.
Archive for the 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.
Brief Introduction
When testing secondary power sources (voltage converters, power supplies, etc.) and some types of primary power sources (batteries, solar panels, etc.) electronic loads. This material will help you obtain basic information about modern electronic loads, their varieties and the tasks they can solve.
General information about electronic loads
An electronic load is a device designed to simulate various operating modes of a real electrical load. In this case, the electronic load can operate in several consumption modes. The most common include: mode constant resistance, mode DC current consumption, mode constant power and mode voltage stabilization. Also, most models of electronic loads support a mode for changing their state according to a list of user-specified values, which makes it possible to implement complex test algorithms that best match the operation of the devices under test in real conditions.
What are electronic loads used for?
The main task of electronic loads is testing various power sources: accumulators, batteries, power supplies, voltage converters, voltage regulators and stabilizers, solar panels, generators and other similar devices. To conduct testing, the electronic load is connected to the power supply being tested and one or more tests are run. At the same time, the electronic load behaves like a real load: for example, it changes its resistance according to a given algorithm, simulates large starting currents, short circuits and other conditions you specify. During the test, the electronic load continuously measures voltage, current and power consumption.
Most electronic loads contain an accurate multimeter that measures the voltage, current, and power drawn by the load. Some models can perform a standardized discharge of batteries and accumulators, measuring the actual capacity of the battery in Amp-hours. Many models can also be controlled by a computer, which allows them to be used as part of automated control and measuring systems.
What are the types of electronic loads?
Most series of electronic loads are designed for testing DC power supplies (batteries, power supplies, solar panels, etc.), typical examples are the ITECH IT8500+ series and the ITECH IT8800 series. To test AC power supplies (inverters, uninterruptible power supplies, transformers, etc.), specialized AC/DC electronic AC and DC loads are produced, a typical example: ITECH IT8615 series.
Structurally, serial electronic loads are manufactured in instrument housings. The size and weight of the case are directly related to the maximum power that the load can dissipate. The lowest power models can dissipate about 100 W and are housed in small, compact cases, such as the IT8211 model rated at 150 W.
Typical low power electronic load
(ITECH IT8211 model, maximum power 150 W).
Typical high power electronic load
(ITECH IT8818B model, maximum power 5 kW).
Models are also available that can dissipate tens and even hundreds of kilowatts. To see design options for different power electronic loads, check out the ITECH IT8800 series.
Sometimes, to reduce the cost, a rheostat (a powerful variable resistor) is used instead of an electronic load. The use of a rheostat when testing power devices is associated with the following limitations:
- lack of constant current consumption mode;
- lack of constant power mode;
- lack of voltage stabilization mode;
- absence of a state change mode according to a list of specified values;
- lack of work automation;
- significant inductance of the rheostat;
- the need to use an additional voltmeter and ammeter.
Therefore, instead of outdated testing methods, it is more effective and ultimately cheaper to use modern instrumentation, specially designed for a specific task.
Using a good electronic load can significantly simplify and speed up the process of testing any power supply, as well as make this process safe and efficient.
Video review of electronic loads
In this video we will look at general information about what electronic loads are, what they are used for and what they are.
Basic information about electronic loads and problems solved with their help.
If you need detailed pricing information or technical advice on choosing the optimal electronic load for your application, just call us or write to us and we will be happy to answer your questions.
I needed to load a switching power supply, but I had nothing to use, I went through my bins, found nichrome and all sorts of nonsense in the form of ancient saprotes.... I tried to load the source as it was not flexible and decided to solder the electronic load, as they say, for centuries... Circuits on the Internet there turned out to be a lot of simple ones and some more complex ones... As a result of a little torment, this miracle was born... During the first tests, it turned out that the radiator was heating up and quite significantly... And then the idea came to use a temperature control and cooling control device that I had previously made and thermal protection on PIC12F629...I once did it for a laboratory worker... The diagram is on our website... And everything started working...
Load diagram.
To increase the stability of the LM358 control microcircuit, it is necessary to connect microcircuit pins 6 and 7 together, and connect pin 5 to ground...
Temperature control circuit.
When the power is turned on, the fan turns on briefly and its serviceability is checked (based on the signal from the tachogenerator sensor); if the fan is working and the temperature is normal, the relay turns on, supplying power to the controlled device. As the load warms up (about 50 degrees), the fan turns on, and if the temperature drops below 45 degrees, the cooler turns off. Those. there is a hysteresis of 5 degrees. When the temperature reaches 75 degrees, the thermal protection is triggered, the load is turned off, and if a fan malfunction is detected, the thermal protection is triggered already at 60 degrees. If the thermal protection is triggered, then the load will not turn back on, no matter how cold it is. The cooler will continue to operate normally, i.e. will cool the radiators and turn off when the temperature drops below +45 degrees. To reset the thermal protection, you need to turn off and turn on the power to the controller.
Well, the photos...
The indicator used a purchased one up to 10 amperes...Events showed that the indicator was needed up to 20 amperes...
The case was taken from an old computer power supply..
Trans power supply circuit from a Chinese ancient mafon, a radiator with a cooler from a fourth hemp if I'm not mistaken...
Well, a bunch of bricks in the form of load saprotes...
When operating a load of 18 amperes, the heating of the parts was at operating temperatures... I measured it with a multimeter and an electronic thermometer...
The readings of the devices are different for everyone, in one word, China... At load, the ammeter readings are more accurate compared to the power supply, I checked with a multimeter...
If you have any questions, I will answer... The rest is all in the archive... All diagrams are taken from the Internet, I do not claim authorship, I processed the diagrams to suit my needs....
ARCHIVE:
When testing high-power power supplies, an electronic load is used, for example, to force a given current. In practice, incandescent lamps are often used (which is a bad solution due to the low resistance of the cold filament) or resistors. An electronic load module is available for purchase on online store sites (priced at about 600 rubles).
Such a module has the following parameters: maximum power 70 W, continuous power 50 W, maximum current 10 A, maximum voltage 100 V. The board has a measuring resistor (in the form of a bent wire), transistor IRFP250N, TL431, LM258, LM393. To start the artificial load module, you need to attach the transistor to the radiator (it is better to equip it with a fan), turn on the potentiometer that provides current regulation and connect a 12 V power source. Here is a simplified block diagram:
The V-V+ connector is used to connect the wires connecting the device under test; it is worth connecting an ammeter in series with this circuit to monitor the specified current.
Power is supplied to connector J3, the device itself consumes a current of 10 mA (not counting the fan current consumption). We connect the potentiometer to connector J4 (PA).
A 12V fan can be connected to connector J1 (FAN), this connector carries the supply voltage from connector J3.
On connector J2 (VA) there is voltage at the V-V+ terminals, we can connect a voltmeter here and check what the voltage is at the load output of the power source.
At a current of 10 A, limiting continuous power to 50 W leads to the fact that the input voltage should not exceed 5 V, for a power of 75 W, the voltage is 7.5 V, respectively.
After testing with the power supply, a battery with a voltage of 12 V was connected as a voltage source so as not to exceed 50 W - the current should not be more than 4 A, for a power of 75 W - 6 A.
The level of voltage fluctuations at the module input is quite acceptable (according to the oscillogram).
Schematic diagram. loads
This is not a 100% accurate diagram, but it is quite similar and has been collected many times by people. There is also a drawing of the printed circuit board.
Operating principle
The transistor is an N-channel MOSFET with higher current Id and power Pd and lower resistance RDSON. The maximum currents and operating voltages of the artificial load block will depend on its parameters.
The NTY100N10 transistor was used, its to-264 package provides good heat dissipation, and its maximum dissipation power is 200 W (depending on the radiator on which we place it).
A fan is also necessary; the thermistor RT1 is used to control it - at a temperature of 40 oC it turns off the power and turns it on again when the radiator temperature exceeds 70 oC. With a load of 20 A, the resistor should have a power of 40 W and be well cooled.
To measure current, an ammeter based on the popular ICL7106 microcircuit is used. The circuit does not require configuration; after proper assembly it works immediately. You only need to select R02 so that the minimum current is 100 mA, you can also select the value of R01 so that the maximum current does not exceed 20 A.
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