The power supply produces more volts than specified. Computer power supply repair
We looked at what action to take if we have a short circuited ATX power supply fuse. This means that the problem is somewhere in the high-voltage part, and we need to check the diode bridge, output transistors, power transistor or mosfet, depending on the power supply model. If the fuse is intact, we can try to connect the power cord to the power supply, and turn it on with the power switch located on the back of the power supply.
And here a surprise may await us, as soon as we flip the switch, we can hear a high-frequency whistle, sometimes loud, sometimes quiet. So, if you hear this whistle, do not even try to connect the power supply for tests to the motherboard, assembly, or install such a power supply in the system unit!
The fact is that in the standby voltage circuits there are the same electrolytic capacitors familiar to us from the last article, which lose capacity when heated, and from old age, their ESR increases, (abbreviated in Russian as ESR) equivalent series resistance . At the same time, visually, these capacitors may not differ in any way from working ones, especially for small values.
The fact is that on small denominations, manufacturers very rarely make notches in the upper part of the electrolytic capacitor, and they do not swell or open. Without measuring such a capacitor with a special device, it is impossible to determine its suitability for operation in the circuit. Although sometimes, after desoldering, we see that the gray stripe on the capacitor, which marks the minus on the capacitor body, becomes dark, almost black from heating. As repair statistics show, next to such a capacitor there is always a power semiconductor, or an output transistor, or a duty diode, or a mosfet. All these parts emit heat during operation, which has a detrimental effect on the life of electrolytic capacitors. I think it would be superfluous to explain further about the performance of such a darkened capacitor.
If the cooler of the power supply has stopped due to grease drying out and being clogged with dust, such a power supply will most likely require replacing almost ALL electrolytic capacitors with new ones due to the increased temperature inside the power supply. Repairs will be quite tedious and not always advisable. Below is one of the common schemes on which Powerman 300-350 watt power supplies are based, it is clickable:
ATX Powerman power supply circuit
Let's look at which capacitors need to be changed in this circuit in case of problems with the duty room:
So why can't we plug the power supply whizzing into the assembly for testing? The fact is that in the duty circuits there is one electrolytic capacitor (highlighted in blue), with an increase in the ESR of which, the duty voltage supplied by the power supply to the motherboard increases, even before we press the power button system unit. In other words, as soon as we clicked the key switch on the back wall of the power supply, this voltage, which should be equal to +5 volts, goes to our power supply connector, the purple wire of the 20 Pin connector, and from there to the computer’s motherboard.
In my practice, there were cases when the standby voltage was equal (after removing the protective zener diode, which was in the short circuit) to +8 volts, and at the same time the PWM controller was alive. Fortunately, the power supply was of high quality, Powerman brand, and there was a 6.2 volt protective zener diode on the +5VSB line (as the duty room output is indicated in the diagrams).
Why is the zener diode protective, how does it work in our case? When our voltage is less than 6.2 volts, the zener diode does not affect the operation of the circuit, but if the voltage becomes higher than 6.2 volts, our zener diode goes into short circuit ( short circuit), and connects the duty circuit to ground. What does this give us? The fact is that by connecting the control panel to ground, we thereby save our motherboard from supplying it with the same 8 volts, or another high voltage rating, through the control panel line to the motherboard, and protect the motherboard from burnout.
But this is not a 100% probability that in case of problems with the capacitors the zener diode will burn out; there is a possibility, although not very high, that it will go into a break and thereby not protect our motherboard. In cheap power supplies, this zener diode is usually simply not installed. By the way, if you see traces of burnt PCB on the board, you should know that most likely some semiconductor went into a short circuit, and a very large current flowed through it, such a detail is very often the cause (although sometimes it also happens to be the effect) breakdowns.
After the voltage at the control room returns to normal, be sure to change both capacitors at the output of the control room. They can become unusable due to the supply of excessive voltage to them, exceeding their rated voltage. Usually there are capacitors with a nominal value of 470-1000 microfarads. If, after replacing the capacitors, a voltage of +5 volts appears on the purple wire relative to ground, you can short the green wire with the black one, PS-ON and GND, starting the power supply, without the motherboard.
If the cooler starts to rotate, this means with a high degree of probability that all voltages are within normal limits, because our power supply has started. The next step is to verify this by measuring the voltage on the gray wire, Power Good (PG), relative to ground. If +5 volts are present there, you are in luck, and all that remains is to measure the voltage at the 20 Pin power supply connector with a multimeter to make sure that none of them are too low.
As can be seen from the table, the tolerance for +3.3, +5, +12 volts is 5%, for -5, -12 volts - 10%. If the control panel is normal, but the power supply does not start, we do not have Power Good (PG) +5 volts, and there is zero volt on the gray wire relative to ground, then the problem was deeper than just with the control panel. Various options We will consider breakdowns and diagnostics in such cases in the following articles. Happy repairs everyone! AKV was with you.
So, they gave us a 350-watt Power Man power supply for repair.
What do we do first? External and internal inspection. Let's look at the "offal". Are there any burnt radio elements? Maybe the board is charred somewhere, or a capacitor has exploded, or it smells like burnt silicon? We take all this into account during the inspection. Be sure to look at the fuse. If it burns out, then replace it with a temporary jumper for about the same amount of Amperes, and then measure through two network wires. This can be done on the power supply plug with the “ON” button turned on. It should NOT be too small, otherwise when you turn on the power supply it will happen again.
We measure voltages
If everything is OK, turn on our power supply to the network using network cable, which comes with the power supply, and don’t forget about the power button if you had it off.
My patient showed 0 volts on the purple wire. I take it and connect the purple wire to ground. Ground is black wires with the inscription COM. COM – short for “common”, which means “general”. There are also some types of “lands”:
As soon as I touched the ground and the purple wire, my multimeter made a meticulous “ppiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiight” sound and showed zeros on the display. Short circuit, definitely.
Well, let's look for a circuit for this power supply. After googling the Internet, I found a diagram. But I found it only on Power Man 300 Watt. They will still be similar. The only differences in the scheme were serial numbers radio components on the board. If you can analyze printed circuit board for compliance with the circuit, then this will not be a big problem.
And here is the circuit for Power Man 300W. Click on it to enlarge to full size.
We are looking for the culprit
As we see in the diagram, standby power, hereinafter referred to as standby power, is designated as +5VSB:
Directly from it goes a zener diode with a nominal value of 6.3 Volts to the ground. And as you remember, a zener diode is the same diode, but is connected in reverse in circuits. The zener diode uses the reverse branch of the current-voltage characteristic. If the zener diode were live, then our +5VSB wire would not short to ground. Most likely the zener diode is burnt out and destroyed.
What happens when various radio components burn from a physical point of view? Firstly, their resistance changes. For resistors, it becomes infinite, or in other words, goes into a break. With capacitors it sometimes becomes very small, or in other words, goes into a short circuit. With semiconductors, both of these options are possible, both a short circuit and an open circuit.
In our case, we can check this in only one way, by unsoldering one or both legs of the zener diode, as the most likely culprit of the short circuit. Next, we will check whether the short circuit between the duty switch and ground has disappeared or not. Why is this happening?
Let's remember some simple tips:
1)When serial connection The rule of greater than greater works, in other words, the total resistance of the circuit is greater than the resistance of the larger resistor.
2) With a parallel connection, the opposite rule works, less than the smaller, in other words, the final resistance will be less than the resistance of the resistor of the smaller value.
You can take arbitrary resistor resistance values, calculate them yourself and see for yourself. Let's try to think logically, if one of the resistances of parallel connected radio components is equal to zero, what readings will we see on the multimeter screen? That's right, also equal to zero...
And until we eliminate this short circuit by desoldering one of the legs of the part that we consider to be problematic, we will not be able to determine in which part we have a short circuit. The point is that during audio testing, ALL parts connected in parallel to the part that is in a short circuit will ring short with the common wire!
We try to remove the zener diode. As soon as I touched it, it fell apart in two. No comments…
It's not the zener diode
We check whether the short circuit in the duty and ground circuits has been eliminated or not. Indeed, the short circuit has disappeared. I went to the radio store to get a new zener diode and soldered it. I turn on the power supply, and... I see how my new, just purchased zener diode emits magical smoke)...
And then I immediately remembered one of the main rules of a repairman:
If something burns out, first find the reason for it, and only then replace the part with a new one or risk getting another burnt out part.
Cursing to myself, I bite the burnt zener diode with side cutters and turn on the power supply again.
That’s right, the duty is too high: 8.5 Volts. My head is spinning main question: “Is the PWM controller still alive, or have I already burned it out?” I download the datasheet for the microcircuit and see the maximum supply voltage for the PWM controller, equal to 16 Volts. Phew, looks like it should pass...
Checking the capacitors
I start googling about my problem on special sites dedicated to repairing ATX power supplies. And of course, the problem of overestimated standby voltage turns out to be a banal increase in the ESR of electrolytic capacitors in the standby circuits. We look for these capacitors in the diagram and check them.
I remember my assembled ESR meter
It's time to check what he is capable of.
I check the first capacitor in the duty circuit.
ESR is within normal limits.
Finding the culprit of the problem
I'm checking the second one
I wait for a value to appear on the multimeter screen, but nothing has changed.
I understand that the culprit, or at least one of the culprits of the problem, has been found. I resolder the capacitor to exactly the same one, in terms of nominal value and operating voltage, taken from the donor power supply board. I want to go into more detail here:
If you decide to put an electrolytic capacitor into an ATX power supply not from a donor, but a new one from a store, be sure to buy LOW ESR capacitors and not regular ones.Conventional capacitors do not work well in high-frequency circuits, but in the power supply, these are precisely the circuits.
So, I turn on the power supply and measure the voltage at the control room again. Having learned from bitter experience, I am no longer in a hurry to install a new protective zener diode and measure the voltage at the control room, relative to the ground. The voltage is 12 volts and a high-frequency whistle is heard.
Again I sit down to google the problem of overvoltage in the duty room, and on the website rom.by, dedicated to repairing both ATX power supplies and motherboards and everything in general computer hardware. I find my fault by searching for typical faults of this power supply. It is recommended to replace the capacitor with a capacity of 10 µF.
I measure ESR on the capacitor.... Ass.
The result is the same as in the first case: the device goes off scale. Some say, why collect some devices, such as swollen non-working capacitors, you can see that they are swollen, or have opened like a rose
Yes, I agree with this. But this only applies to large capacitors. Capacitors of relatively small values do not swell. There are no notches in their upper part through which they could open. Therefore, it is simply impossible to determine their performance visually. All that remains is to replace them with ones that are known to work.
So, after going through my boards, I found the second capacitor I needed on one of the donor boards. Just in case, its ESR was measured. It turned out to be normal. After soldering the second capacitor into the board, I turn on the power supply using the key switch and measure the standby voltage. Exactly what was required, 5.02 volts... Hurray!
I measure all other voltages at the power supply connector. Everything corresponds to the norm. Operating voltage deviations are less than 5%. It remains to solder a 6.3 Volt zener diode. I thought for a long time why the zener diode is 6.3 Volts when the voltage on duty is +5 Volts? It would be more logical to set it to 5.5 volts or similar if it was used to stabilize the voltage on the duty room. Most likely, this zener diode is placed here as a protective one, so that if the voltage on the control panel increases above 6.3 Volts, it will burn out and short-circuit the control panel circuit, thereby turning off the power supply and saving our motherboard from burning out when entering over-voltage through the control room.
The second function of this zener diode is, apparently, to protect the PWM controller from receiving too much voltage. Since the control room is connected to the power supply of the microcircuit through a fairly low-resistance resistor, almost the same voltage is supplied to pin 20 of the PWM microcircuit that is present in our control room.
Conclusion
So, what conclusions can be drawn from this repair:
1) All parallel connected parts influence each other during measurement. Their active resistance values are calculated according to the rule of parallel connection of resistors. In the event of a short circuit on one of the parallel-connected radio components, the same short circuit will occur on all other parts that are connected in parallel to this one.
2) To identify faulty capacitors of one visual inspection is small and it is necessary to either replace all faulty electrolytic capacitors in the circuits of the problem unit of the device with known working ones, or reject them by measuring them with an ESR meter.
3) Having found any burnt part, we are not in a hurry to replace it with a new one, but look for the reason that led to its combustion, otherwise we risk getting another burnt part.
The power supply fails quite often, especially for units with “experience” in operation. The worst thing is that sometimes it breaks of this device entails the failure of almost all installed components, especially if the motherboard lacks the necessary protection - power stabilizers.
The most common faults that affect the power supply are:
- Unstable AC voltage. The source of alternating voltage for the power supply is an external network with alternating voltage. Unfortunately, the quality of this voltage in the CIS countries is extremely low. The “normal” phenomenon is a voltage value of 180, 200, and even 260 V, while the desired voltage is in the range of 210-230 V. The entire impact is taken by the input circuits of the power supply, and if the quality of the components of these circuits is at a low level, the power supply either overheats or fails altogether.
- Low quality electronic components. The number of manufacturers of electronic components is growing every day, but, unfortunately, this does not in any way affect the quality of these components. As a result, the power supply is extremely dependent on the operation of these components, which, in turn, affects its service life.
- User actions. Often the cause of a malfunction is a “well-read” user who, despite common sense tries to reduce the noise of the power supply fan using the existing speed controller or self-supply to it undervoltage, while the temperature inside the power supply is at a critical level. In addition, few people think about purchasing a source uninterruptible power supply and protect yourself from problems associated with sharp jumps voltages that the power supply bears very painfully.
- Increased humidity levels. Condensation penetrates into electronic circuit power supply, from which transformers, chokes and other components containing wire windings suffer the most. Humidity makes adjustments to the resistance of such components, which in the event of sufficiently frequent power surges leads to excessive stress on them. Accordingly, as a result, their operating time is sharply reduced, which can lead to partial or complete failure.
- Time and service life. Do not forget that any electronic components have a certain service life, which is also directly dependent on the conditions of their use. So, if you always demand this kind of power, and sometimes even more, from a power supply with a maximum power of 300 W, the resource of the components will quickly be exhausted and the power supply, at best, will simply no longer be able to produce even an average power rating.
- Depletion of internal resources. The most common and inevitable malfunction is the gradual depletion of power supply resources and a drop in its power. The result of this effect is unstable operation of the computer, frequent reboots or refusal to turn on.
The power supply is not a device that cannot be repaired with your own hands: many of the faults can be eliminated yourself. However, before you do this, it is worth understanding that the operation of all other devices depends on the power supply, so irresponsible actions when troubleshooting a malfunction expose these devices to great risk.
ADVICE!!! In most cases, repairing the power supply does not give the expected effect, or it does, but for a very short time. Therefore, I advise you to buy it immediately new block power supply, choosing a time-tested model.