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.

I. NECHAYEV, Moscow

When setting up and testing high-current power supplies, the need arises for a powerful load equivalent, the resistance of which can be varied within a wide range. Using powerful variable resistors for these purposes is not always possible due to the difficulty of purchasing them, and using a set of constant resistors is inconvenient, since it is not possible to smoothly regulate the load resistance.

A way out of this situation may be to use a universal equivalent load collected on powerful transistors. The operating principle of this device is based on the fact that by changing the control voltage at the gate (base) of the transistor, you can change the drain (collector) current and set its required value. If you use powerful field-effect transistors, the power of such a load equivalent can reach several hundred watts.

In most of the previously described similar designs, for example, the current consumed by the load is stabilized, which weakly depends on the applied voltage. The proposed load equivalent is similar in properties to a variable resistor.

The device diagram is shown in Fig. 1.

The device contains an input voltage divider R1-R3 and two voltage-controlled current sources (VTUN). The first ITUN is assembled on op-amp DA1.1 and transistor VT1, the second - on op-amp DA1.2 and transistor VT2. Resistors R5 and R7 - current sensors, resistors R4, R6 and capacitors C3-C6 ensure stable operation of the ITUN.

The input of each ITUN is supplied with voltage UR3 from resistor R3, which is proportional to the input voltage and equal to Uin * R3/(R1+R2+R3). The current of the first ITUN flowing through transistor VT1 is equal to IVT1= UR3/R5, the current of the second flowing through transistor VT2 is IVT2= UR3/R7. Since the resistance of resistors R5 and R7 is the same, the input resistance of the load equivalent is equal to Rin = U in/(IVT1+IVT2) = R5(R1+R2+R3)/2R3. For the resistor ratings Rin indicated in the diagram, you can change the resistor R1 from approximately 1 to 11 Ohms.

Powerful IRF3205 field-effect switching transistors are used as control elements, on which almost all the power is dissipated. The transistor of this series has a minimum channel resistance of 0.008 Ohm, permissible drain current of 110 A, power dissipation of up to 200 W, drain-source voltage of 55 V. These parameters correspond to a case temperature of 25 ° C. When the case heats up to 100 °C, the maximum power is halved. The maximum case temperature is 175 °C. To increase the maximum power, both ITUNs are connected in parallel.

Most of the parts are placed on a printed circuit board made of one-sided foil-coated fiberglass (Fig. 2).

A photograph of the board with parts is shown in Fig. 3.

Elements used for surface mount: resistors P1-12 or similar imported ones, with R5 and R7 made up of five 0.1 Ohm resistors connected in parallel. Capacitors are also for surface mounting, but K10-17 or similar can be used. Variable resistor R1 is SPO, it can be replaced with SP4-1.

The transistors are installed on a common heat sink with the obligatory use of heat-conducting paste. It should be remembered that it is electrically connected to the drains of the field effect transistors.

To blow the heat sink, a fan (M1) from computer unit nutrition. To power op-amp DA1 and fan M1, a separate stabilized source with a voltage of 12 V is required. If, with a total power dissipation of 150...200 W, the temperature of the transistor housings exceeds 80...90 °C, then it is necessary to install another fan or use a more efficient heat sink .

Using the expression for the equivalent input resistance, you can select the values ​​of the elements to obtain the required interval of its change. To simplify the device, you can use only one ITUN, but in this case the maximum power dissipation will be halved. When testing transformers and other sources alternating current A diode bridge of appropriate power should be installed at the input of the device, as shown by the dotted line in Fig. 1 in the article.

LITERATURE
1. Nechaev I. Universal load equivalent. - Radio, 2002, No. 2, p. 40.41.
2. Nechaev I. Universal load equivalent. - Radio, 2005, No. 1, p. 35.

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 on Russian market cheap and powerful field-effect transistors 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 wide limits by simple means(the same way we adjust sound volume or output voltage power supply - a regular variable resistor and not powerful switch contacts, a 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

Since the trend now is to reduce the cost of production as much as possible, low-quality goods quickly reach the repairman’s door. When buying a computer (especially the first one), many choose the “most beautiful of the cheap” case with a built-in power supply - and many do not even know that such a device is there. This is a “hidden device” on which sellers save a lot. But the buyer will pay for the problems.

The main thing

Today we will touch on the topic of repairing computer power supplies, or rather their initial diagnostics. If there is a problematic or suspicious power supply, then it is advisable to carry out diagnostics separately from the computer (just in case). And this unit will help us with this:

The block consists of loads on lines +3.3, +5, +12, +5vSB (standby power). It is needed to simulate a computer load and measure output voltages. Since without load the power supply can show normal results, but under load many problems can appear.

Preparatory theory

We will load with anything (whatever you find on the farm) - powerful resistors and lamps.

I had 2 car lamps 12V 55W/50W lying around - two spirals (high/low beam). One spiral is damaged - we will use the second one. There is no need to buy them - ask your fellow motorists.

Of course, incandescent lamps have a very low resistance when cold - and when starting up they will create a large load for a short time - and cheap Chinese ones may not be able to withstand this - and will not start. But the advantage of lamps is accessibility. If I can get powerful resistors, I’ll install them instead of lamps.

Resistors can be looked for in old devices (tube TVs, radios) with resistance (1-15 Ohms).

You can also use a nichrome spiral. Use a multimeter to select the length with the required resistance.

We will not load it to full capacity, otherwise we will end up with 450W in the air as a heater. But 150 watts will be fine. If practice shows that more is needed, we’ll add it. By the way, this is the approximate consumption of an office PC. And the extra watts are calculated along the +3.3 and +5 volt lines - which are little used - approximately 5 amperes each. And the label boldly says 30A, which is 200 watts that the PC cannot use. And the +12 line is often not enough.

For the load I have in stock:

3pcs resistors 8.2ohm 7.5w

3pcs resistors 5.1ohm 7.5w

Resistor 8.2ohm 5w

12v lamps: 55w, 55w, 45w, 21w

For calculations we will use formulas in a very convenient form (I have it hanging on the wall - I recommend it to everyone)

So let's choose the load:

Line +3.3V– used mainly for food random access memory– approximately 5 watts per bar. We will load at ~10 watts. Calculate the required resistor resistance

R=V 2 /P=3.3 2 /10=1.1 Ohm we don’t have these, the minimum is 5.1 ohm. We calculate how much it will consume P=V 2 /R=3.3 2 /5.1=2.1W - not enough, you can put 3 in parallel - but we get only 6W for three - not the most successful use of such powerful resistors (by 25%) - and the place will take a lot. I’m not installing anything yet - I’ll look for 1-2 Ohms.

Line +5V– little used these days. I looked at the tests - on average he eats 5A.

We will load at ~20 watts. R=V 2 /P=5 2 /20=1.25 Ohm - also a low resistance, BUT we already have 5 volts - and even squared - we get a much larger load on the same 5 ohm resistors. P=V 2 /R=5 2 /5.1=4.9W – put 3 and we will have 15 W. You can add 2-3 on the 8th (they will consume 3W), or you can leave it like that.

Line +12V- the most popular. There is a processor, a video card, and some little gadgets (coolers, drives, DVDs).

We will load at as much as 155 watts. But separately: 55 per power connector motherboard, and 55 (+45 through the switch) to the processor power connector. We will use car lamps.

Line +5 VSB- emergency meals.

We will load at ~5 watts. There is an 8.2 ohm 5w resistor, let's try it.

Calculate powerP=V 2 /R=5 2 /8.2= 3 W Well, that's enough.

Line -12V- Let's connect the fan here.

Chips

We will also add a small-sized 220V 60W lamp to the housing in the 220V network break. During repairs, it is often used to identify short circuits (after replacing some parts).

Assembling the device

Ironically, we will also use the case from a computer power supply (non-working).

We unsolder the sockets for the power connector of the motherboard and processor from the faulty motherboard. We solder the cables to them. It is advisable to choose colors as for the connectors from the power supply.

We are preparing resistors, lamps, ice indicators, switches and a connector for measurements.

We connect everything according to the diagram... more precisely, according to the VIP scheme :)

We twist, drill, solder - and you're done:

Everything should be clear by appearance.

Bonus

Initially I didn’t plan it, but for convenience I decided to add a voltmeter. This will make the device more autonomous - although during repairs the multimeter is still somewhere nearby. I looked at cheap 2-wire ones (which are powered by the measured voltage) - 3-30 V - just the right range. Simply by connecting to the measurement connector. But I had 4.5-30 V and I decided to install a 3-wire 0-100 V - and power it from charging mobile phone(also added to the case). So it will be independent and show voltages from zero.

This voltmeter can also be used to measure external sources(battery or something else...) – by connecting it to the measuring connector (if the multimeter has disappeared somewhere).

A few words about switches.

S1 – select the connection method: through a 220V lamp (Off) or directly (On). At the first start and after each soldering, we check it through a lamp.

S2 – 220V power is supplied to the power supply. The standby power should start working and the LED +5VSB should light up.

S3 – PS-ON is shorted to ground, the power supply should start.

S4 – 50W addition on the processor line. (50 is already there, there will be a 100W load)

SW1 – Use the switch to select the power line and check one by one if all voltages are normal.

Since our measurements are shown by a built-in voltmeter, you can connect an oscilloscope to the connectors for a more in-depth analysis.

By the way

A couple of months ago I bought about 25 PSUs (from a PC repair company that was closing). Half working, 250-450 watts. I bought them as guinea pigs for studying and attempting repairs. The load block is just for them.

That's all. I hope it was interesting and useful. I went to test my power supplies and wish you good luck!

This device is designed and used to test power supplies direct current, 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 dinistor relay, which in turn turns on transistor T7 and supplies power to LED1, signaling operation 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 switch 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 bipolar power supply 15V and the ability to adjust the bias voltage. 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 - Home printed circuit board, fastening the transformer from 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 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.