How to check the power supply

It is advisable to check the power supply before installing it into the computer, especially if you are buying a used power supply. And new power supplies, despite being tested in production, are often faulty. Where to look, how to take measurements and where, what voltage deviations are permissible for the power source? In this text we will try to answer these questions.

We will consider two situations. In the first case, we have only the power supply itself; in the second, it is possible to install it into a test system – a ready-made computer. To measure voltages, we need a multimeter. You can take an inexpensive option, but it is still better to spend money, since the measurements will be more accurate. Soft voltage measurements in most cases are very inaccurate, and using programs like HWMonitor or AIDA64 to make measurements is a completely useless exercise.

RGK DM40 multimeter readings: 12V – 12.43V; 5V – 5.108V; 3.3V – 3.305V.

Even in the simplest model of a multimeter, when measuring a constant voltage, deviations from real values ​​will be small, and unlike software readings, they will give an almost real picture of the nature of voltage stabilization in the PSU.

We check the power supply without connecting to a computer
First of all, you need to conduct an external examination for damage to both the power supply case itself and the cables. When the power supply unit is connected to the network and the switch on the rear panel of the unit is in the correct position (on), a standby voltage of 5 V should appear on the 24-pin connector. The permissible deviation from the nominal value is ± 5%, that is, from 4.75 V to 5 , 25 V.

The standby voltage is applied to the motherboard and allows its logic to give a signal to turn on the power supply. That is, when we press the button on the system unit, we give a signal to the motherboard, and already it signals the power supply unit that it would be nice to start. You can measure it here:
98-380

98-375

98-369

98-368

98-367

98-366

98-365

98-364

98-361

98-349

77-888

77-887

77-885

77-884

77-881

If not, check that the power cable is in good working order, the presence of voltage in the network and the position of the switch on the rear panel of the unit. Is everything correct, but there is no tension? Check again whether you are measuring on the correct contact, and if everything is done correctly, but there is no voltage, most likely the power supply unit is faulty. Failure of the standby power source is not such a rare cause of breakdown.

If there is a standby voltage, as in the picture above, then you can start the power supply by closing two contacts on the 24-pin connector block. In this case, we need PS_ON and any ground contact. It is convenient to do this with an ordinary paper clip, if you bent it in the right way, but any piece of wire will do.

This operation must be done carefully. Although when the unit is not started, but the unit is turned on, we have voltage only on a pair of contacts – the standby voltage source and PS_ON, and if you short them somewhere else, nothing terrible will happen. Modern power supplies usually have protection against short circuit on the standby power source.

The power supply unit should start, and the fan should spin, if it works at all at low loads, that is, your power supply unit is not with semi-passive cooling. The main voltages can now be measured. There are three of them: 3.3 V; 5 V and 12 V. There is also a voltage of -12 V, but it can be ignored. It is not needed in modern systems. First of all, where to measure. The most affordable connectors in this case are the four-pin Molex connectors.

Previously, in all ATX power supplies, the wires were of a certain color for each voltage, and there were explanations about this for a couple of pages in the Power Supply Design Guide, but recently black wires have become fashionable. Yes, they look definitely more aesthetically pleasing, but it became more difficult to navigate where the voltage on the connector is. Therefore, I made a couple of pinout pictures for you. It is convenient to navigate where which side of the connector is by the latch.

Connector for additional power supply for video cards.

We measure all voltages, and if they are within acceptable limits, the power supply unit can be considered conditionally serviceable. Why conditionally? Full information about its state can be obtained only by testing under load.

Checking the power supply unit as part of the system unit
If you bought a used unit, then it is better to first check it using the above method, and then install it into your computer. Then we just run the benchmarks that simultaneously load the main consumers, the video card, the processor, and repeat the measurements.

It is best to measure under load on the most loaded connector. That is, 12 V on the connector for powering the processor and video card. For other voltages, this is not so important, because the currents there are small. Because a current flows through the wires going to these connectors, and the more it is, the greater the voltage drop across the wires.

The voltage measured on a connector that is not connected to anything will differ from the voltage on a video card connector, for example. And we are interested in exactly how much comes to the consumer, and not how much at the output inside the power supply itself.

How to measure the voltage at the connector connected to the motherboard or video card? You can use this method: gently (!) Stick a thin needle into the desired connector contact from the side of the wires, and we are already connected to it with a multimeter probe.

In this case, in the photo, instead of the needle, the output of the MLT resistor is used.

Naturally, it will most likely not be possible to load the maximum power supply using a computer. Unless you are installing a 300W unit on a system with a GeForce RTX 3080. To maximize the power supply, you need special hardware. There are special loads for testing computer power supplies, and there are universal electronic loads.

However, all this is quite expensive. A specialized stand stands like a good used foreign car. If you do not want to engage in block testing, then spending that kind of money is pointless.

Short circuit test

According to the Power Supply Design Guide, an output short is defined as any output impedance less than 0.1 ohm. The power supply must be capable of sustaining an extended output short circuit without damaging components, PCB tracks, or connectors. When the short circuit is removed, the power should be restored automatically or by re-shorting PS_ON to ground.

It makes little sense to check the presence and operation of the short-circuit protection system. Today it is found in all modern power supplies. The only exception is the most budgetary power supplies. They can save money on the protection of low-voltage lines. For 3.3V, this is not so bad. We do not have available connectors with such a voltage, it is present only on the 24-pin connector, and problems can only arise if the insulation of the 3.3 V wires is damaged, which is extremely rare.

But the 5V line is available on both Molex and SATA connectors. You can check the operation of the short circuit protection with a thin wire. Thin, because if there is no protection, or its response time is long, let this wire burn better than the power supply wires or something on the board. In this case, it is advisable to hold it not with your fingers. Melting metal is not the most pleasant thing to touch 🙂

And finally, a few answers to simple questions:

When connecting the power cable to the PSU, a click occurs, similar to arcing. This is normal, the capacitors are charging.
When the power supply is turned on (and turned off), a click occurs inside the power supply. This is normal, the relay is energized, switching the thermistor to protect against current surges. Not available in all PSUs.
Why do you say not to use software for verification? My multimeter shows about the same values ​​as the program. Because the program can show quite sane values ​​for some time, and then suddenly give something completely unacceptable and has nothing to do with reality.
In such a simple way, you can check the health of a computer power supply unit and protect your components from poor-quality power supply.