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Quite often, when electronic devices do not work, or work that you immediately suspect a faulty switch mode supply. But you know that defective or shorted components on the motherboard or system board can cause the power supply shuts down too?
switch mode power supply (SMPS) is designed to be effective when there is a short circuit occurs on the motherboard and the power supply would be switched off completely stopped. If you have no experience about troubleshooting switch mode power supply, you might think that the power supply has a problem which is actually the main board is the real cause of no power problem.
switching power supply has a modern sense circuit (if you look at UC3842 PWM IC pin 3, stated I-sense, which means, in the current sense), and if there is a short circuit on the secondary side (or secondary diodes or motherboard), the recorded power would increase and this will lead the PWM IC to stop generating the power FET and thus the power supply is shut down. All this happens in a split second and you will not have the opportunity to know if any output voltage on the secondary side.
do not use some older design of SMPS power supply PWM IC, but can not be detected during the current draw of the circuit on and off whenever it detects a shorted component in the secondary side. A good example of the power supply used in printer. Printers usually have two boards; One was in power, the other main area. If there is a short circuit on the motherboard, power supply is not working. In order to isolate where the problem is, you have to unplug the power supply board. If the plug is removed from the motherboard, you can now turn on the printer and make sure that there is power present in the power supply connector.
If there are zero voltages measured at each supply (VCC) pin then we can conclude that the power supply is a problem and you can put your whole concentration in this power supply board. What if the voltages measured across the connector? This means that the motherboard is most likely caused by non-electrical short circuit problem is that some parts of the main board.
For your information, dot-matrix printers usually required two operating voltage. One was from 5 (logic IC, EEPROM and CPU) and the other is more than 30 volts to the engines. The question now is, how do we know if the motherboard is the main reason to shut down the power supply? Very simple, just use the analog multimeter set of X 1 Ohm and measure between the supply pin (say 5 volts pin) and the motherboard ground and then reverse the probes. A good board should not show two similar reading and if you get two similar ohms reading then this means that the 5 volt line had shorted to ground any defective parts.
If you have confirmed that the 5 volt line have problem then how to find the culprit, because there are so many components connected to this line? TTL IC, CPU, EEPROM, transistors, diodes and even small filter capacitors are all connected to the 5 volt line. Either one of these components can cause short-circuit does not have jurisdiction on the printer. You may remove the individual components lead (5 volt supply) on the main board and hope that the short circuit will be gone. Assuming you happen to remove one of the filter capacitor pin and the short circuit is gone, then we can say that the real culprit is the filter capacitor.
The real problem is what if the board has many components on it and it consumes a lot of time to isolate the problem by removing one pin at a time. It is not easy to identify the supply 5 volts pin to a spider IC that is 100 or more taps. Many spider IC 5V supply pin number. Some even have 4 and some supply pins 6-8. This means you have to check one pin at a time, until finally the location of the fault? Not only that, remove the supply pin from the spider IC and check for any short circuit between the grounds required a very good skill too. If you messed out the circuit board track, the main board can then be considered beyond repair. Despite the fact that you can repair the break in the track, it does not mean that they solved the actual fault!
Do you see the additional problem arises when you try to figure out the problem? There must be a better way to solve this kind of problem. Yes, it’s true that you can use ESR meter slowly to the source of the short circuit can use the ESR meter to locate short circuits in electronic tables checking the actual track resistance. A probe is connected to the ground and the other to the circuit line and if the measurement rises as you probe further along the track, you know you are heading to the wrong direction! If the ESR value decreases as you move along the track in the right direction. The problem is, what if the motherboard with double layers, 4 or even 8 layers? You may waste a lot of precious time trying to track the actual error in the ESR meter method! After explaining the problem so, now is the time that you expect that the answer to how to solve this kind of error quickly.
Do you know that the DC regulated power is used under supply that can power up electronic circuit can use to troubleshoot and find shorted component in a motherboard? Assuming you have confirmed that the VCC supply lines shorted to ground any defective parts, it is easy to detect the use of digital variable DC power supply is regulated. If there is not a digital one can always use an analog meter panel, which is cheaper.
Now connect the alligator clip to the positive output supply of 5 volt VCC pin in the main board and the negative output to the main board ground as shown in the picture. When all is ready, it’s time to turn the DC regulated power supply. Slow changes in the same clockwise and see the voltage increase. However, the current consumption can be seen in the current digital meter reading. Experienced if there is a short circuit on the motherboard, when the voltage increases, the current will rise dramatically and if no short-circuit would be less power consumption.
right back to this problem, if you notice that the power consumption will also increase (quickly) than in proportion to the increase in the voltage setting, it is now safe that is part of the main circuit board. What does it mean? This is a good news for us, because if the current drawn know the culprit by touching the hottest components on the main board too much. The finger touching any components that are very hot. Tap ICs, diodes, transistors, capacitors, etc. Once positioned so that only one component that is extremely hot to touch, it was the culprit! This is the real reason for trying to find! Has he used this method to solve many problems motherboard and is usually the culprit turned out to be a faulty gate array and ram IC.
Why did not immediately turn off the output voltage to a regulated DC power supply 5 V, since the defective board also uses 5V run? If I do this, and if the faulty components turned hot too fast, I fear this will lead to a defective part is normally open. If the faulty component open circuit, the current consumption is reduced, and the only clue we have to identify the culprit has been destroyed! If this thing happens, it will not be higher repair the board. Of course, still it can be substituted by each SMD spider IC, but it is certainly up a lot of time to repair.
The above method I just mentioned is not limited to printers motherboard; you can apply to other types of care, such as computer motherboards, hard drives, DVDs, VCDs, and industrial supplies, etc. Get a DC regulated power supply and I’m sure you can detect shorted component in the main board in the shortest time possible! Incidentally, the specifications of the DC regulated power supply that I was 5A 30 volts (variable type). Have a good day my friend!
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Source by Jestine Yong