Troubleshooting and self-repair of a computer power supply

The performance of a personal computer (PC) not least depends on the quality of the power supply unit (PSU). If it fails, the device will not be able to turn on, which means that you will have to replace or repair the computer's power supply. Whether it's a modern gaming computer or a weak office computer, all PSUs work on a similar principle, and the troubleshooting technique is the same for them.

Principle of operation and main components

Before you start repairing a power supply unit, you need to understand how it works, know its main components. Repair of power supplies should be carried out extremely careful and remember about electrical safety during work. The main units of the power supply unit include:

• input (mains) filter;
• additional driver of a stabilized 5 volt signal;
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• main shaper +3.3 V, +5 V, +12 V, as well as -5 V and -12 V;
• line voltage regulator +3.3 volts;
• high-frequency rectifier;
• voltage shaping line filters;
• control and protection unit;
• block for the presence of the PS_ON signal from the computer;
• voltage driver PW_OK.

The inlet filter is used for interference suppressiongenerated by the power supply in​ electrical circuit. At the same time, it performs a protective function during abnormal operating modes of the power supply unit: protection against excess current value, protection against voltage surges.

When the power supply unit is switched on to a 220 volt network, a stabilized signal with a value of 5 volts is sent to the motherboard through an additional driver. The operation of the main driver at this moment is blocked by the PS_ON signal generated by the motherboard and equal to 3 volts.

After pressing the power button on the PC, the PS_ON value becomes zero and the starting the main converter. The power supply begins to generate basic signals to the computer board and protection circuits. In the event of a significant excess of the voltage level, the protection circuit interrupts the operation of the main driver.

To start the motherboard, a voltage of +3.3 volts and +5 volts are applied to it simultaneously, from the power supply device, to form the PW_OK level, which means food is normal. Each wire color in the power supply corresponds to its voltage level:

• black, common wire;
• white, -5 volts;
• blue, -12 volts;
• yellow, +12 volts;
• red, +5 volts;
• orange, +3.3 volts;
• green, signal PS_ON;
• gray, PW_OK signal;
• purple, food on duty.

The power supply device is based on the principle of pulse width modulation (PWM). The mains voltage, converted by the diode bridge, is fed to the power unit. Its value is 300 volts. The operation of the transistors in the power unit is controlled by a specialized microcircuit PWM controller. When a signal arrives at the transistor, it opens, and a current arises on the primary winding of the pulse transformer. As a result of electromagnetic induction, voltage appears on the secondary winding. By changing the pulse duration, the opening time of the key transistor is regulated, and hence the signal magnitude.

The controller included in the main inverter starts up from the enabling signal motherboard. The voltage goes to the power transformer, and from its secondary windings goes to the other nodes of the power source, which form a number of required voltages.

PWM controller provides stabilization of the output voltage by using it in a feedback circuit. With an increase in the signal level on the secondary winding, the feedback circuit decreases the voltage value at the control pin of the microcircuit. In this case, the microcircuit increases the duration of the signal sent to the transistor switch.

A filter is placed at the end of each power supply line. Its purpose is to remove parasitic pulsations formed by transient processes of transistors. It consists, like any mains filter, of an electrolytic capacitor and inductance.

Power device diagnostics

Before proceeding directly to diagnostics of the computer power supply device, you need to make sure that the problem is in it. The easiest way to do this is to connect known good block to the system unit. Troubleshooting a computer power supply can be carried out according to the following method:

1. In case of damage to the power supply unit, you must try to find a manual for its repair, a circuit diagram, data on typical malfunctions.
2. Analyze the conditions under what conditions the power source worked, whether the electrical network is working properly.
3. Using your senses to determine whether there is a smell of burning parts and elements, whether there was sparking or flash, listen to whether extraneous sounds are heard.
4. Assume one malfunction, highlight the defective item. This is usually the most time consuming and painstaking process. This process is even more time consuming if there is no electrical circuit, which is simply necessary when searching for "floating" faults. Using measuring devices, trace the path of the fault signal to the element on which there is a working signal. As a result, to conclude that the signal disappears at the previous element, which is inoperative and requires replacement.
5. After repair, it is necessary to test the power supply with the maximum possible load.

Practical recommendations for repair

If you decide to repair the power supply yourself, first of all it is removed from the case of the system unit. After that, the fastening screws are unscrewed and the protective cover is removed. Having blown and cleaned from dust, they begin to study it. Practical repair do-it-yourself computer power supply can be presented step by step as follows:

1. Visual inspection. With it, special attention is paid to blackened places on the board and elements, the appearance of capacitors. The top of the capacitors should be flat, the bulge indicates its unusability, there should be no smudges at the bottom at the base. If there is a power button, it will not be superfluous to check it.
2. If the inspection did not cause suspicion, then the next step will be to dial the input and output circuits for the presence of a short circuit (SC). In the presence of a short circuit, a broken semiconductor element is detected, which is in a circuit with a short circuit.
3. The mains voltage across the rectifier capacitor is measured and the fuse is checked. If there is a voltage of 300 V, go to the next step.
4. If there is no voltage, the fuse blows out, the diode bridge and key transistors are checked for a short circuit. Resistors and protective thermistor for open circuit.
5. The presence of a standby voltage, stabilized by five volts, is checked. Statistics show that when the power supply device does not turn on, one of the most common reasons is a malfunction of the standby power circuit, with operable power elements.
6. If stabilized five volts are present, the presence of PS_ON is checked. When the value is less than four volts, the reason for the understatement of the signal level is sought. Typically PS_ON is generated from standby voltage through a 1 kΩ pull-up resistor. The supervisor's circuit is checked, first of all, for compliance in the circuit with the capacitance values ​​of the capacitors and the resistor values.

If the cause is not found, the PWM controller is checked. To do this, you need a stabilized 12 volt power supply device. On board the leg of the microcircuit is disconnectedresponsible for the delay (DTC), and the source power is applied to the VCC foot. The oscilloscope looks at the presence of signal generation at the terminals connected to the collectors of transistors, and the presence of a reference voltage. If there are no pulses, the intermediate stage is checked, most often collected on low-power bipolar transistors.

Typical malfunctions and check of elements

When restoring the PC power supply, you will need to use various kinds of devices first of all, it is a multimeter and preferably an oscilloscope. With the help of the tester, it is possible to carry out measurements for a short circuit or an open circuit of both passive and active radioelements. The performance of the microcircuit, if there are no visual signs of its failure, is checked using an oscilloscope. In addition to measuring equipment for repairing a PC power supply, you will need: a soldering iron, a suction for solder, flushing alcohol, cotton wool, tin and rosin.

If the computer's power supply does not start, possible malfunctions can be presented in the form of typical cases:

1. Primary fuse blown. Diodes in the rectifier bridge are broken. The elements of the crossover filter will ring for a short circuit: B1-B4, C1, C2, R1, R2. Breakage of varistors and thermistor TR1, short-circuit transitions of power transistors and auxiliary Q1-Q4.
2. A constant voltage of five volts or three volts is too low or too high. Violations in the operation of the stabilizing circuit, microcircuits U1, U2 are checked. If it is not possible to check the PWM controller, then the microcircuit is replaced with an identical or analogous one.
3. The output signal level is different from the working one. Fault in the feedback circuit. The PWM microcircuit and radioelements in its strapping are to blame, special attention is paid to capacitors C and low-power resistors R.
4. No PW_OK signal. The presence of the main voltages and the PS_ON signal is checked. The supervisor responsible for monitoring the output signal is being replaced.
5. There is no PS_ON signal. The supervisor's microcircuit, the elements of the strapping of its circuit burned out. Check by replacing the microcircuit.
6. The fan does not turn. Measure the voltage supplied to it, it is 12 volts. Ring the thermistor THR2. Measure the resistance of the fan leads for a short circuit. Perform mechanical cleaning and lubricate the seat for the fan blades.

Principles of measuring radioelements

The PSU case is connected to the common wire of the printed circuit board. The measurement of the power section of the power supply is carried out with respect to the common wire. The limit on the multimeter is set to more than 300 volts. In the secondary part there is only a constant voltage not exceeding 25 volts.

The resistors are checked by comparing the tester readings and the markings applied to the resistance case or indicated on the diagram. The diodes are checked by a tester, if it shows zero resistance in both directions, then a conclusion is made about its malfunction. If it is possible to check the voltage drop across the diode in the device, then you can not solder it, the value is 0.5-0.7 volts.

Capacitors are checked by measuring their capacity and internal resistance, which requires a specialized ESR meter. When replacing, note that low internal resistance (ESR) capacitors are used. Transistors ring up the performance of p-n junctions or, in the case of outfield, the ability to open and close.

Checking the repaired power supply

After the ATX unit has been repaired, it is important to correctly turn it on for the first time. At the same time, if not all the problems have been eliminated, the failure of the repaired and new units of the device is possible.

The power supply device can be started autonomously, without using a computer unit. For this, contact PS_ON is bridged to the common wire. Before turning on, a 60 W light bulb is soldered in place of the fuse, and the fuse is removed. If, when turned on, the light starts to shine brightly, then there is a short circuit in the unit. In the event that the lamp flashes and goes out, the lamp can be desoldered and a fuse installed.

The next stage of checking the power supply occurs under load. First, the presence of a standby voltage is checked for this, the output is loaded with a load of the order of two amperes. If the duty officer is in order, the power supply is turned on by shorting PS_ON, after which the output signal levels are measured. If there is an oscilloscope, it looks like a ripple.