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 very low resistance when cold - and will create heavy load for a short time - and the cheap Chinese may not be able to withstand this - and 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 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 via a 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– here we connect the fan.

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 scheme... 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. Powerful ones are used as equivalent load resistance field effect transistors T1-T6 connected in parallel. 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. 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 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 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 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 surface mount and are 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 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.

To check and adjust power supplies, especially powerful ones, a low-impedance regulated load with an allowable 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 feedback circuit of the op-amp DA1.2 and the 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

Applied fixed resistors for current sensor series RC (size 2512, maximum power dissipation 1 W), the rest - RN1-12 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-effect 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 one side 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 electric 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. Maximum input voltage in 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 alternating current 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.

The power-regulated load is part of the test equipment needed when setting up various electronic projects. For example, when building a laboratory power supply, it can "simulate" the connected current sink to see how well your circuit performs not only at idle, but also under load. Adding power resistors for the output can only be done as a last resort, but not everyone has them and they cannot last long - they get very hot. This article will show how a variable electronic load bank can be built using inexpensive components available to hobbyists.

Electronic load circuit using transistors

In this design the maximum current should be approximately 7 amps and is limited by the 5W resistor that was used and the relatively weak FET. Even higher load currents can be achieved using a 10 or 20 W resistor. The input voltage should not exceed 60 volts (maximum for these field-effect transistors). The basis is an op-amp LM324 and 4 field-effect transistors.

Two "spare" operational amplifiers of the LM324 chip are used to protect and control the cooling fan. U2C forms a simple comparator between the voltage set by the thermistor and the voltage divider R5, R6. Hysteresis controlled by positive feedback, received by R4. The thermistor is placed in direct contact with the transistors on the heatsinks and its resistance decreases as the temperature increases. When the temperature exceeds the set threshold, the U2C output will be high. You can replace R5 and R6 with an adjustable variable and manually select the response threshold. When setting up, make sure that the protection is triggered when the temperature of the MOSFET transistors is slightly below the maximum permissible value specified in the datasheet. LED D2 signals when the overload protection function is activated - it is installed on the front panel.

In the U2B element operational amplifier There is also a voltage comparator hysteresis and it is used to control a 12 V fan (can be used from old PCs). The 1N4001 diode protects the MOSFET BS170 from inductive voltage surges. The lower temperature threshold for activating the fan is controlled by resistor RV2.

Assembling the device

An old aluminum box from a switch with big amount internal space for components. In the electronic load I used old AC/DC adapters to supply 12 V for the main circuit and 9 V for the instrument panel - it has a digital ammeter to immediately see the current consumption. You can already calculate the power yourself using the well-known formula.

Here's a photo of the test setup. The laboratory power supply is set to 5 V. The load shows 0.49A. A multimeter is also connected to the load, so that the load current and voltage are monitored simultaneously. You can verify for yourself that the entire module is working smoothly.

When I started trying to repair computer blocks I had one problem with the power supply. The fact is that it is not very convenient to constantly connect the power supply to the computer (just a lot of inconvenience), and also not safe (since an incorrectly or incompletely repaired unit can damage the motherboard or other peripherals).
After searching the Internet a little for circuit diagrams, I found some circuit solutions to this problem. There were also on a microcontroller, on transistors-resistors with printed circuit board(which I’m thinking of doing for myself in the future), and on nichrome spirals. Since the nearest radio store is 150 km from me, I decided to collect the load from what was lying around in the garage and a nichrome spiral, which is sold for electric stoves in almost any electrical store.

I chose the case from the same power supply, soldered the main connections, and took some onto clamping blocks, made an LED indication of the channels: +12, +5, +3.3, +5VSB, PG. There is no load on channels -5, -12 yet. I installed a switch from the power supply that connects PS_ON and GND. Displayed on back panel wires from all power ratings, to check the voltage with a tester. The connector is soldered away from the motherboard, and there is also a fan left for blowing the coils and resistors. For the +12V load, two resistors from old 5.1 Ohm TVs were used.

A few words about how to measure a spiral. We take a tester and measure all the resistance, then measure the length of the entire spiral. Knowing the length of the spiral to a millimeter, we divide the resistance in Ohms by millimeters and find out how many Ohms per 1 mm. Next we calculate the length of the spiral segment.
Example.

Let's look at the diagram (it is very simple and easy to repeat):

And now a few photos of the completed device.