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 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. 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 that the thermal protection has tripped. 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 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 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.
This simple circuit electronic load can be used to test various types of 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. The operating voltage of our electronic load will depend on its characteristics. 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 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.
DIY kits. The schemes on which they are made were not created by the Chinese or even by Soviet engineers. Any radio amateur will confirm that during everyday research it is very often necessary to load certain circuits to identify the output characteristics of the latter. The load can be a regular lamp, a resistor or a nichrome heating element.
Often, those radio amateurs who study power electronics are faced with the problem of finding the right load. When checking the output characteristics of a particular power supply, be it homemade or industrial, a load is required, and a load that can be adjusted. The simplest solution to this problem is to use training rheostats as a load.
But finding powerful rheostats these days is problematic, and besides, rheostats are also not rubber, their resistance is limited. There is only 1 solution to the problem - electronic load. In an electronic load, all the power is allocated to power elements - transistors. In fact, electronic loads can be made to any power, and they are much more versatile than a regular rheostat. Professional laboratory electronic loads cost a ton of money.
The Chinese, as always, offer analogues and there are countless of these analogues. One of the options for such a 150W load costs only 9-10 dollars, which is not much for a device that is probably comparable in importance to a laboratory power supply.
In general, the author of this homemade product, AKA KASYAN, preferred to make his own version. Finding a diagram of the device was not difficult.
This circuit uses an operational amplifier chip lm324, which consists of 4 separate elements.
If you look carefully at the circuit, it immediately becomes clear that it consists of 4 separate loads that are connected in parallel, due to which the total load capacity of the circuit is many times greater.
This is a regular current stabilizer based on field-effect transistors, which can be easily replaced with reverse bipolar transistors. Let's look at the operating principle using one of the blocks as an example. The operational amplifier has 2 inputs: direct and inverse, and 1 output, which in this circuit controls a powerful n-channel field-effect transistor.
We use a low-resistance resistor as a current sensor. To operate the load, a low-current power supply of 12-15V is required; more precisely, it is needed to operate the operational amplifier.
An op-amp always strives to ensure that the voltage difference between its inputs is zero, and it does this by varying the output voltage. When connecting a power source to a load, a voltage drop will form across the current sensor; the greater the current in the circuit, the greater the drop across the sensor.
Thus, at the inputs of the operational amplifier we will receive a voltage difference, and the operational amplifier will try to compensate for this difference by changing its output voltage by smoothly opening or closing the transistor, which leads to a decrease or increase in the resistance of the transistor channel, and, consequently, the current flowing in the circuit will change .
In the circuit we have a reference voltage source and a variable resistor, by rotating which we have the opportunity to forcibly change the voltage at one of the inputs of the operational amplifier, and then the above-mentioned process occurs, and as a result, the current in the circuit changes.
The load operates in linear mode. Unlike pulse mode, in which the transistor is either completely open or closed, in our case we can force the transistor to open as much as we need. In other words, smoothly change the resistance of its channel, and, therefore, change the circuit current literally from 1 mA. It is important to note that the current value set by the variable resistor does not change depending on the input voltage, that is, the current is stabilized.
In the diagram we have 4 such blocks. The reference voltage is generated from the same source, which means all 4 transistors will open evenly. As you noticed, the author used powerful field keys IRFP260N.
These are very good transistors with 45A, 300W power. In the circuit we have 4 such transistors and, in theory, such a load should dissipate up to 1200 W, but alas. Our circuit operates in linear mode. No matter how powerful the transistor is, in linear mode everything is different. The power dissipation is limited by the transistor body, all the power is released in the form of heat on the transistor, and it must have time to transfer this heat to the radiator. Therefore, even the coolest transistor in linear mode is not so cool. In this case, the maximum that a transistor in a TO247 package can dissipate is about 75W of power, that’s it.
We've sorted out the theory, now let's move on to practice.
Printed circuit board was developed in just a couple of hours, the wiring is good.
The finished board must be tinned, the power paths reinforced with single-core copper wire, and everything should be generously filled with solder to minimize losses due to the resistance of the conductors.
The board provides seats for installing transistors, both in the TO247 and TO220 housings.
If you use the latter, you need to remember that the maximum that the TO220 case is capable of is a modest 40W of power in linear mode. Current sensors are low-resistance 5W resistors, with a resistance from 0.1 to 0.22 Ohms.
It is advisable to install operational amplifiers on a socket for solderless mounting. To more accurately regulate currents, it is worth adding 1 more low-resistance variable resistor to the circuit. The first will allow for rough adjustment, the second more smooth.
Precautionary measures. The load has no protection, so you need to use it wisely. For example, if the load contains 50V transistors, then it is prohibited to connect the tested power supplies with a voltage higher than 45V. Well, to have a small reserve. It is not recommended to set the current value to more than 20A if the transistors are in a TO247 package and 10-12A if the transistors are in a TO220 package. And, perhaps, the most important point is not to exceed the permissible power of 300W, if transistors in a TO247 package are used. To do this, it is necessary to build a wattmeter into the load in order to monitor the power dissipation and not exceed the maximum value.
The author also strongly recommends using transistors from the same batch to minimize the variation in characteristics.
Cooling. I hope everyone understands that 300W of power will stupidly be used to heat transistors, it’s like a 300W heater. If heat is not effectively removed, then the transistors will fail, so we install the transistors on a massive solid radiator.
The area where the key backing is pressed against the radiator must be thoroughly cleaned, degreased and polished. Even small bumps in our case can ruin everything. If you decide to apply thermal paste, then do it in a thin layer, using only good thermal paste. There is no need to use thermal pads, there is also no need to insulate the substrates of the radiator keys, all this worsens heat transfer.
Well, now, finally, let's check the operation of our load. We will load this laboratory power supply, which produces a maximum of 30V at a current of up to 7A, that is, an output power of about 210W.
First, let's look at the diagram. I do not claim originality, since I looked at the constituent elements and adapted them to what I had from the parts.
The protection circuit is made up of fuse FU1 and diode VD1 (it may be redundant). The load is performed on four 818 transistors VT1...VT4. They have acceptable current and power dissipation characteristics, and are not expensive or in short supply. VT5 control is on an 815 transistor, and stabilization is on an LM358 operational amplifier. I installed an ammeter that shows the current passing through the load separately. Because if you replace resistors R3 R4 with an ammeter (as in the diagram at the link above), then, in my opinion, part of the current that will flow through VT5 will be lost and the readings will be underestimated. And judging by how the 815 heats up, a decent amount of current flows through it. I’m even thinking that between the VT5 emitter and ground it is necessary to put another Ohm resistance of 50...200.
Separately, we need to talk about the R10…R13 circuit. Since the adjustment is not linear, it is necessary to take one variable resistance of 200...220 kOhm with a logarithmic scale, or install two variable resistors, which provide smooth regulation over the entire range. Moreover, R10 (200 kOhm) regulates the current from 0 to 2.5A, and R11 (10 kOhm), with R10 turned to zero, regulates the current from 2.5 to 8 A. The upper current limit is set by resistor R13. When setting up, be careful, if the supply voltage accidentally gets to the third leg of the op-amp, the 815 will open completely, which will most likely lead to failure of all 818 transistors.
Now a little about power supplies for the load.
No, this is not a perversion. I just didn’t have a small-sized 12-volt transformer at hand. I had to make a multiplier and increase the voltage from 6 volts to 12 for the fan and install a stabilizer to power the load itself and the alarm.
Yes, I installed a simple temperature alarm into this device. I looked at the diagram. When the radiator heats up above 90 degrees, a red LED turns on and a buzzer with an integrated generator turns on, which makes a very unpleasant sound. This indicates that it is time to reduce the current in the load, otherwise you may lose the device due to overheating.
It would seem that with such powerful transistors that can withstand up to 80 volts and 10 A, the total power should be at least 3 kW. But, since we are making a “boiler” and all the power of the source goes into heat, the limitation is imposed by the power dissipation of the transistors. According to the datasheet, it is only 60 W per transistor, and taking into account the fact that the thermal conductivity between the transistor and the heatsink is not ideal, the actual power dissipation is even less. And therefore, in order to somehow improve the heat dissipation, I screwed transistors VT1...VT4 directly to the radiator without gaskets using thermal paste. At the same time, I had to organize special covers for the radiator so that it would not short-circuit to the body.
Unfortunately, I did not have the opportunity to test the operation of the device over the entire voltage range, but at 22V 5A the load works without overheating. But as always, there is a fly in the ointment. Due to the insufficient area of the radiator I took, with a load of more than 130 watts, after some time (3...5 minutes) the transistors begin to overheat. What does the alarm indicate? Hence the conclusion. If you are going to do a load, take a radiator with as large an area as possible and provide it with reliable forced cooling.
Also, a small drift towards reducing the load current by 100...200 mA can be considered a fly in the ointment. I think this drift occurs due to heating of resistors R3, R4. So, if you can find 0.15 Ohm resistors for 20 W or more, then it is better to use them.
In general, the circuit, as far as I understand, is not critical to replacing parts. Four 818 transistors can be replaced with two KT896A, KT815G can, and perhaps should, be replaced with KT817G. I think you can also take a different operational amplifier.
I would like to especially emphasize that when setting up, be sure to install a resistor R13 of at least 10 kOhm, then as you understand what current you need, reduce this resistance. I am not posting the printed circuit board, because the installation of the main part of the load is hinged.
Addition.
As it turned out, I have to use the load regularly, and in the process of using it I came to the understanding that, in addition to the ammeter, I also need a voltmeter to monitor the source voltage. On Ali I came across a small device that combines a voltmeter and an ammeter. The device is 100 V / 10. And it cost me 150 rubles including postage. As for me, this is a penny because... Half a glass of beer costs about the same. Without thinking twice, I ordered two.
Why you need such a device as an electronic load, probably everyone knows - it allows you to create an imitation of a very powerful resistor at the output of power supplies, chargers, amplifiers, UPS and other circuits when setting them up. This electronic load can handle more than 100 Amps of current, dissipating more than 500 W continuously and handling 1 kW of power in burst mode.
The circuit is, in principle, simple and uses two field-effect transistors with regulating op-amps. Each of the two channels is the same and they are connected in parallel. The control voltages are interconnected and the load is divided equally between two powerful field-effect transistors. Here, 2 50 A resistors are used for the shunt, forming a feedback voltage of 75 mV. The obvious advantage of choosing such a low resistance value (each shunt is only 1.5 milliohms) is that the voltage drop is virtually negligible. Even when operating with a 100 A load, the voltage drop across each shunt resistor will be less than 0.1 V.
The disadvantage of using this circuit is that it requires an op-amp with a very low input offset, since even a small change in offset can lead to a large error in the controlled current. For example, in laboratory tests, just 100 µV of offset voltage will result in a 0.1 A change in load current. Moreover, it is difficult to create such stable control voltages without the use of DACs and precision op-amps. If you plan to use a microcontroller to drive the load, you will either need to use a precision shunt voltage amplification op amp compatible with the DAC output (eg 0-5V) or use a precision voltage divider to create the control signal.
The entire circuit was assembled on a piece of PCB using a simplified installation method and placed on the top of a large aluminum block. The metal surface is polished to ensure good thermal conductivity between the transistors and the heatsink. All connections with high current - at least 5 wires of thick stranded wire, then they can withstand at least 100 A without significant heating or voltage drop.
Above is a photo of a breadboard on which two high-precision LT1636 operational amplifiers are soldered. And the DC-DC converter module is used to convert the input voltage to a stable 12 V for the cooling fan controller. Here they are - 3 fans on the side of the radiator.