Electronic load with continuously adjustable current. Do-it-yourself electronic load: diagram. Homemade electronic load on a field-effect transistor. Features of Sorensen series devices

This simple circuit electronic load can be used for testing various types power supplies. The system behaves as a resistive load that can be regulated.

Using a potentiometer, we can fix any load from 10mA to 20A, and this value will be maintained regardless of the voltage drop. The current value is continuously displayed on the built-in ammeter - so there is no need to use a third-party multimeter for this purpose.

Adjustable electronic load circuit

The circuit is so simple that almost anyone can assemble it, and I think it will be indispensable in the workshop of every radio amateur.

The operational amplifier LM358 makes sure that the voltage drop across R5 is equal to the voltage value set using potentiometers R1 and R2. R2 is for coarse adjustment and R1 for fine adjustment.

Resistor R5 and transistor VT3 (if necessary, VT4) must be selected corresponding to the maximum power with which we want to load our power supply.

Transistor selection

In principle, any N-channel MOSFET transistor will do. It will depend on its characteristics operating voltage our electronic load. The parameters that should interest us are large I k (collector current) and P tot (power dissipation). Collector current is the maximum current that the transistor can allow through itself, and power dissipation is the power that the transistor can dissipate as heat.

In our case, the IRF3205 transistor theoretically can withstand current up to 110A, but its maximum power dissipation is about 200 W. As is easy to calculate, we can set the maximum current of 20A at a voltage of up to 10V.

In order to improve these parameters, in in this case We use two transistors, which will allow us to dissipate 400 W. Plus, we will need a powerful radiator with forced cooling if we are really going to push the maximum.

Over time, I have accumulated a certain number of different Chinese AC-DC converters for charging batteries mobile phones, flashlights, tablets, as well as small switching power supplies for electronics and the batteries themselves. Cases often indicate electrical parameters devices, but since most often you have to deal with Chinese products, where inflating the indicators is sacred, it would not be a bad idea to check the real parameters of the device before using it for crafts. In addition, it is possible to use power supplies without a housing, which do not always contain information about their parameters.

Therefore, I assembled an electronic load for myself using an LM358 operational amplifier and a KT827B composite transistor, testing power supplies with voltages from 3 V to 35 V. In this device, the current through the load element is stabilized, so it is practically not subject to temperature drift and does not depend on the voltage of the source being tested, which is very convenient when taking load characteristics and conducting other tests, especially long-term ones.

Tools:
- soldering iron, solder, flux;
- electric drill;
- jigsaw;
- drills;
- M3 tap.

Instructions for assembling the device:

Operating principle. The device's operating principle is a voltage-controlled current source. A powerful composite bipolar transistor KT 827B with a collector current Ik = 20A, a gain h21e of more than 750 and a maximum power dissipation of 125 W is equivalent to the load. Resistor R1 with a power of 5W is a current sensor. Resistor R5 changes the current through resistor R2 or R3 depending on the position of the switch and, accordingly, the voltage on it. An amplifier with negative feedback from the emitter of the transistor to the inverting input of the operational amplifier is assembled using the LM358 operational amplifier and the KT 827B transistor. The action of the OOS is manifested in the fact that the voltage at the output of the op-amp causes such a current through the transistor VT1 that the voltage across the resistor R1 is equal to the voltage across the resistor R2 (R3). Therefore, resistor R5 regulates the voltage across resistor R2 (R3) and, accordingly, the current through the load (transistor VT1). While the op-amp is in linear mode, the indicated value of the current through transistor VT1 does not depend either on the voltage on its collector or on the drift of the transistor parameters when it warms up. The R4C4 circuit suppresses the self-excitation of the transistor and ensures its stable operation in linear mode. To power the device, a voltage of 9 V to 12 V is required, which must be stable, since the stability of the load current depends on it. The device consumes no more than 10 mA.

Perhaps the problem can be solved by installing 1-2 of the same transistors in parallel, but I haven’t practically checked - those same IRF3205 transistors are not available in the required quantity.

The housing for the electronic load was used from a faulty car radio. There is a handle for carrying the device. I installed rubber feet on the bottom to prevent slipping. I used bottle caps for medicines as legs.

To check and adjust power supplies, especially powerful ones, a low-impedance regulated load with permissible power dissipation of up to 100 W or even more is required.

The use of variable resistors for this purpose is not always possible, mainly due to limited power dissipation. for a current of several tens of amperes can be made on the basis of a current stabilizer based on a powerful field-effect switching transistor. But these equivalents are not always convenient to use, since they require a separate power source.

Its diagram is shown in Fig. 1 (click to enlarge). A current stabilizer is assembled on op-amp DA1.2 and field-effect transistor VT2. The current through the field-effect transistor (I VT2) depends on the resistance of the current sensor R I (resistors R11-R18) and the voltage on the motor of the variable resistor R8 (U R8), which regulates the current: I VT2 = U R8 /R I. Capacitor C4 suppresses high-frequency interference, and C5 and C6 in the circuit feedback Op-amp DA1.2 and a field-effect transistor respectively increase the stability of the stabilizer.

The op-amp is powered by a step-up stabilized voltage converter with an output voltage of 5 V, assembled on the DA2 chip. The same voltage is supplied to the current regulator through resistor R7. Thanks to the voltage converter, the device can be powered from the power source being tested. In this case, the minimum input voltage is 0.8…1 V, which allows the proposed equivalent to be used for testing and measuring the parameters of Ni-Cd and Ni-MH batteries of AA or AAA size.

A converter supply voltage limiter is assembled on op-amp DA1.1 and transistor VT1. When the input voltage is less than 3.8 V, a voltage of about 4 V is present at the output of op-amp DA1.1, transistor VT1 is fully open and the supply voltage is supplied to the converter. When the input voltage exceeds 3.8 V, the voltage at the output of op-amp DA1.1 decreases, so the increase in voltage at the emitter of transistor VT1 stops and it remains stable. A voltage limiter is necessary since the maximum supply voltage of the converter chip (DA2) is 6 V.

Design and details of equivalent load

Fixed resistors were used for the current sensor of the RC series (size 2512, maximum power dissipation 1 W), the rest - RN1-12 of standard size 1206 or 0805, variable - SP4-1, SPO. All capacitors are surface-mounted, oxide - tantalum, size B or C, the rest are ceramic, and capacitor C6 is mounted directly on the terminals of the transistor. Connector X1 is a screw terminal block designed for the required current. Transistor BC846 can be replaced with a transistor of the KT3130 series, and IRL2910 with a transistor 1RL3705N, IRL1404Z or other powerful field switching with a threshold voltage of no more than 2.5 V. The inductor is for surface mounting SDR0703 or with EC24 wire leads.

All elements, except for the variable resistor, field-effect transistor, connector, fan and capacitor C6, are mounted on a single-sided printed circuit board made of fiberglass with a thickness of 1... 1.5 mm, its drawing is shown in Fig. 2. A heat sink with a fan is used for a voltage of 12 V from the processor personal computer. The transistor and connector are attached to the heat sink with screws, and the board is glued. The use of thermally conductive paste for the transistor is mandatory. The fan motor starts rotating at an input voltage of 3...4 V and at 8...10 V it blows the heat sink quite effectively. For this design option, a current sensor with a total resistance of 0.05 Ohm and a power dissipation of 8 W is used, so the maximum equivalent current is 12...13 A, and the maximum power dissipation does not exceed 100 W. By using larger current sensing resistors and a more efficient heat sink, both current and power dissipation can be increased accordingly. The maximum input voltage in this case depends on the permissible fan supply voltage.

The device is placed in a case of a suitable size (a case from a personal computer power supply is suitable), input jacks connected to connector X1 and a variable resistor, which can be equipped with a graduated scale, are installed on the front panel. The heat sink should be isolated from the metal case, since it has a galvanic connection with the drain of the field-effect transistor.

The maximum current value is set by selecting resistor R7, while the slider of variable resistor R8 should be in the upper position in the circuit. Since the fan motor is connected directly to the input connector, the current consumed by it is added to the stabilizer current, so when the input voltage changes, the total current also changes. In order for this current to be stable, the lower terminal of the electric motor in the circuit is connected not to the negative power line, but to the source of the field-effect transistor, as shown in Fig. 1 with a dashed line.

Can be used to test power supplies AC frequency 50 Hz, for example, step-down transformers. In this case, the device is connected (maintaining polarity) to the output of the rectifier bridge, in which it is advisable to use Schottky diodes. Between the positive terminal of capacitor C1 and the connection point between resistor R3 and the collector of transistor VT1, a diode of the same type as VD1 is installed, and the capacitance of capacitor C2 should be increased to 100 μF. In a diode bridge, the diodes must be rated for equivalent current. It should be taken into account that in this case the minimum and maximum permissible voltage will increase by the amount of the voltage drop across the bridge diodes and the additional diode.

LITERATURE
1. Nechaev I. Equivalent load. - Radio, 2007, No. 3, p. 34.
2. Nechaev I. Universal load equivalent. - Radio, 2005, No. 1, p. 35.
3. Nechaev I. Universal load equivalent. - Radio, 2002, No. 2, p. 40, 41.

This device is designed and used to test power supplies DC, 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 W.

General description of the scheme

Figure 1 - Basic electrical diagram 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 operation 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. Reset occurs after complete shutdown device from the network and restart.

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 basis 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, OA2 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 such 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 zeros are displayed on all segments of the indicator, except the leftmost one (it should be inactive). 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 switch mode and is selected based on minimum value 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 LED indicators BS-A51DRD can be used with any single or dual seven-segment indicators with a common anode without dynamic control.

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 metal sheet 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 finished device.

Figure 11 - Connection diagram.

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