DC Regulated Power Supply – How To Use It To Find Out Shorted Components In Main Board
Very often whenever electronic equipment don’t functionality or work, we would immediately suspect a faulty switch mode power. But do you know that defective or shorted components in the motherboard or primary board could cause the power supply to stop working too?
Switch mode power (SMPS) are designed so efficiently that whenever there is any short circuit happen in the main board the power supply might shut itself off and completely stop working. If you have no experience regarding troubleshooting switch mode power supply, you may think that the power supply have problem where in fact the main board is the source of no power problem.
Change mode power supply have a current feeling circuit (if you look at UC3842 PWM IC pin 3, this stated I-sense which mean current sense) and if there is short circuit in the secondary side (either in secondary diodes or main board), the existing drawn would be increase and this may lead the PWM IC to stop generating output to the power fet and thus the power supply would turn off. All this happen in a split of seconds and you do not have the chance to know if there are output voltages on the secondary side.
Some older design of SMPS power supply do not use the PWM IC, but it do have the signal to detect over current drawn and shut itself down anytime it detects a shorted component in the secondary side. One good instance was the power supply used in printer. Printers usually have two boards; one was the power supply while the other was main board. If there is any short circuit in the main board, the power supply would not function. In order to isolate at where the is actually, one must remove the connector from the power supply board. Once the supply connection to the main board was eliminated, you can now switch on the printer plus check if there is any voltages existing at the power supply connector.
If you will find zero voltages measured across all of the supply (VCC) pins then we are able to conclude that the power supply have problem and you can put your whole concentration in this particular power supply board. What if there are voltages measured across the connector? This means that the main board is causing the no strength problem most probably due to some shorted components in the main board.
For your info, dot-matrix printers usually required 2 voltages to function.
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One is the five Volts (for logic IC, eeprom and CPU) and the other can be 30+ volts for the motors. The question now is how do we know if the main board is the main cause that will shutting down the power supply? Very simple, simply use your analog multimeter set to By 1 Ohm and measure between your supply pin (say 5 volts pin) and the main board terrain and then reverse the probes. An excellent board should not show two comparable reading and if you get two similar ohms reading then this means that the particular 5 volts line had shorted to ground through some faulty components.
If you have confirmed that the 5 volts line have problem after that how do we find out the culprit since there are so many components connected to this collection? TTL IC’s, CPU, EEPROM, transistors, diodes and even small filter capacitors are all connected to the 5 volts series. Either one of these components shorted could cause no power to the printer. You might remove each components lead (5 volt supply) in the main board and hope that the short circuit will be long gone. Assuming if 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 is actually what if the board has many elements on it and this will consume lots of your time to isolate the problem simply by removing one pin at a time. It is not easy to identify the supply 5 volts pin to a spider IC that has 100 pins or more. Many spider IC’s have more than one 5 volts supply pin. Some even possess 4 and some have 6 to 8 supply pins. Does this mean you need to check one pin at a time unless you finally locate the fault? Not just that, to remove the supply pin from your spider IC’s and check for any kind of short circuit between the grounds required a very good skill too. If you messed out there the circuit board track, the main board can then be considered beyond restoration. Even though you can repair the damaged circuit track, this does not imply you have solved the actual fault!