DC Regulated Power Supply – How to use it to find shorted components on main board
Very often, when electronic equipment does not work or does not work, we immediately suspect a faulty switching power supply. But do you know that faulty or shorted components on the motherboard or main board could also cause the power supply to stop working?
The Switch Mode Power Supply (SMPS) is designed so efficiently that whenever a short circuit occurs on the main board, the power supply shuts down and stops working altogether. If you are inexperienced with switching power supply troubleshooting, you may think that the power supply has a problem when, in fact, the main board is the real cause of no power problem.
The switching power supply has a current sensing circuit (if you look at pin 3 of the UC3842 PWM IC, it says I-sense, which means current sensing) and if there is a short circuit on the secondary side (either the secondary diodes or on the main board), the current consumed would increase and this would cause the PWM IC to stop outputting to the power supply and thus the power supply would shut down. This all happens in a fraction of seconds and you have no chance of knowing if there are any output voltages on the secondary side.
Some older SMPS power supply designs do not use the PWM IC, but have the circuitry to detect excessive current draw and shut down when they detect a shorted component on the secondary side. A good example was the power supply used in the printer. Printers typically have two boards; one was the power supply while the other was the main board. If there is any short circuit on the main board, the power supply would not work. To isolate where the problem is, the power board connector must be removed. With the power supply connector removed from the main board, you can now power on the printer and check if there is voltage present at the power supply connector.
If there are zero voltages measured on all supply pins (VCC), then we can conclude that the power supply has a problem and you can put all your concentration on this power supply board. What if there are voltages measured across the connector? This means that the main board is causing the no power issue probably due to some shorted components on the main board.
FYI, dot matrix printers generally require two voltages to work. One is 5 volts (for logic IC, eeprom and CPU) and the other is 30+ volts for the motors. The question now is how do we know if the main board is the main cause of the power supply shutting down? Very simple, just use your analog multimeter set to X 1 Ohm and measure between the supply pin (let’s say 5 volt pin) and the main board ground then reverse the probes. A good board should not show two similar readings and if you get two similar ohm readings this means the 5 volt line has been shorted to ground through some bad components.
If you have confirmed that the 5 volt line has a problem, how can we find the culprit since 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. Shorting any of these components could cause the printer to lose power. You can remove each component wire (5 volt supply) on the main board and hope the short goes away. Assuming that if you remove one of the filter capacitor pins and the short goes away, we can say that the real culprit is the filter capacitor.
The real issue is what if the board has many components and this will be very time consuming to isolate the problem by removing one pin at a time. It’s not easy to identify the 5 volt supply pin to a spider IC that has 100 pins or more. Many spider ICs have more than one 5 volt supply pin. Some even have 4 and others have 6 to 8 power pins. Does this mean you have to check one pin at a time until you finally locate the fault? Not only that, removing the supply pin from the spider ICs and checking for any shorts between the grounds also requires very good skill. If you messed up the track on the circuit board, the main board can be considered irreparable. Although you can repair the broken circuit track, this does not mean that you have solved the actual fault!
Do you see the additional problem arise when you try to find the fault? There must be a better way to approach this type of problem. Yes, it is true that one can use the ESR meter to slowly trace the source of the short circuit. You can use the ESR meter to locate short circuits on electronic boards by checking the actual resistance of the track. One probe connects to ground and the other to the circuit line and if the measurement increases as you go along the track, you know you’re headed in the wrong direction! If the ESR value decreases as you go down the track, then you are on the right track. The problem is what happens if the main board has double layers, 4 or even 8 layers? You may be wasting a lot of your precious time trying to track down the actual fault using the ESR meter method! After explaining so much about the issues, now is the time you are waiting for to get the answer on how to resolve this type of failure quickly.
Do you know that the DC regulated power supply In addition to using it to power up an electronic circuit, it can also be used to troubleshoot and find a shorted component on a motherboard. Assuming you’ve confirmed that the VCC supply lines were shorted to ground through some bad components, you can easily spot this with the use of a variable digital DC regulated power supply. If you don’t have the digital one you can always use one with the cheaper analog panel meter.
Now connect the alligator clip of the positive output supply to the 5 volt VCC pin on the main board and the negative output to the main board ground as shown in the picture. Once everything is done, it’s time to turn on the DC regulated power supply. Slowly move the knob clockwise and watch the voltage increase. At the same time, the current consumption can be seen on the current digital meter reading. From experience, if there is a short circuit on the main board, as long as the voltage increases, the current will increase dramatically, and if there is no short circuit, there will be less current consumption.
Ok, back to this problem, if you notice that the current drawn also increases (rapidly) in proportion to the increase in the voltage setting, you are now sure that there is a shorted component on the main board. What does this mean? This is good news for us, because if the current draws too much we can tell who is to blame by touching the hottest components on the main board. You can touch any component that is very hot with your finger. Touch integrated circuits, diodes, transistors, capacitors, etc. Once you located that one component that is extremely hot to the touch, that was the culprit! That is the real cause you are trying to find! I have used this method to solve many main board problems and usually the culprit turned out to be a faulty gate array and ram IC.
Why don’t I immediately change the output voltage of the DC regulated power supply to 5 volts, since the faulty main board also uses 5 volts to work? If I do this and the faulty components heat up too quickly, I’m afraid this will cause the faulty component to open the circuit. If the faulty component opens the circuit, the current draw will drop and the only clue you have to identify the culprit has been destroyed. If this happens, you may not have a higher rate to repair the board. Of course, you can still replace the SMD spider ICs one by one, but this will surely consume a lot of repair time.
The above method that I just mentioned is not limited to just the main board of the printer; It can be applied to other types of boards, such as computer motherboards, hard drives, DVDs, VCDs, industrial boards, etc. Get a DC regulated power supply and I’m sure you can spot a shorted component on the main board in no time! By the way, the specification of the DC regulated power supply that I am using was 5A 30 volts (variable rate). Have a nice day friend!