What Are The Possible Reasons That A PCB Doesn't Work?

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How to find faults in electronic circuits?

An electronic circuit is a combination of individual electronic components such as resistors, capacitors, inductors, diodes, transistors, and integrated circuits connected together.

Most often, people are interested in electronics in order to be able to repair any device. Only a small part of amateurs is engaged in independent development. Theoretical knowledge, although it gives a general understanding of the principle of operation of the components, it is much more important for repairs to know the methods for checking them. We will tell you how to find a malfunction in an electronic circuit with your own hands, eyes and a simple tool.

Basic Troubleshooting Methods

Before repairing, it is important to determine what the problem is - this process is called diagnostics. So, there are two stages of checking electronic devices:

1. Checking the performance of the device. It does not always happen that the device is completely “dead”, you need to check that the device does not turn on at all, or turns on and immediately turns off, or some specific buttons or functions do not work.

For example, when repairing LCD monitors, there is such a problem as a failure of the backlight. In this case, the monitor may either not turn on at all, then its indicator blinks, or the indicator indicates the on state, but there is no image. In this case, if you shine a flashlight on the screen, you can see that the image is still there and the monitor seems to be working, but it is dark - and this is just one of the examples where a preliminary check simplifies diagnostics.

2. Visual inspection. Externally, you can identify most problems with an electrical appliance. These can be either simply burnt components - diodes, resistors, transistors and capacitors, or soldering defects or mechanical damage to the elements and the printed circuit board itself.

3. Measurements. If the board and parts look normal, then you should proceed to the measurements. They are carried out mainly using a multimeter and an oscilloscope. In some cases, specialized devices are used, such as frequency meters, logic analyzers, and so on.

PCB board

So, the generalized troubleshooting algorithm is:

  • Inspection of the board
  • Determination of excessive heating of the electronic components of the board
  • Measurements and dialing with a multimeter
  • Use of an oscilloscope and other instruments
  • Replacement of a failed part or block.

Visual inspection

Visual inspection should be carried out from the general to the specific. Or in simple words - inspect the general view of the electronic device, immediately check the integrity of the cables and power wires. Their cover should be even and intact, without kinks and sharp bends, cones and other irregularities on the shell should not be.

After you are convinced of the integrity of the device, you need to disassemble it and get to the printed circuit board. Inspection of the insides should begin with checking the integrity of the loops, wires and other interconnections. It is important not to tear them even during disassembly, since often the cables go from the boards to the key blocks and displays mounted on the case.

Next, the integrity of the fuse in the power circuit is checked, often if it is blown, it can be determined with the naked eye. It stands near the place where the power cord is connected to the board.

After that, inspect for traces of heat or soot on the board and damaged components. Consider what faulty electronic components look like. For example, cases of faulty transistors and burnt diodes break or crack.

A crack or small dot appears on integrated circuits

 In some cases, both of them burn out, leaving traces of burning on the board as a result. Pay attention to whether there is a characteristic smell of burnt insulation. So you can localize from which element or section of the board this smell comes from. You can see below how to identify burnt transistors and microcircuits.

Resistors usually burn out or darken, less often the resistive layer breaks and the part looks good.

PCB board

How to identify burnt capacitors? 

They basically break through "short" between the plates and, if they are in the power circuit, then the board tracks or the capacitor case are damaged. If the circuit was low-current, a broken capacitor will simply short-circuit it without visible traces of high currents flowing. Cases of capacitors crack less often.

While electrolytic capacitors can be calculated by a deformed case covers or traces of electrolyte leaking down. There are two diagonal grooves on the lid of the condenser; it is needed so that the case does not break in an emergency. In this case, the lid swells or cracks. Rarely squeezes the bottom.

With SMD components, the situation is somewhat more complicated. Often they are extremely difficult to consider for integrity. There is one method for finding a short circuit in an SMD board - this is thermal paper, such paper is used in a cash register, so any check can be used.

 Printing on it occurs due to heating

 This means that when you apply power to the board, the shorted part will overheat and be printed on paper. 

 But you need to remember about electrical safety and do not resort to this diagnostic method if you are not sure if there is a dangerous voltage there. This can be done safely and accurately with a thermal imager.

In most cases, you will need a laboratory power supply or other current-limiting power source to determine a thermal short. If you are diagnosing 220V circuits, you can use a test lamp, if there is a short circuit, and then the lamp will light up in full heat. In fact, it will act as a current-limiting resistor.

Why Visual Inspection is Important?

During visual inspection, it is important to determine the condition of the contacts of all detachable connections. They should be clean, without oxides with a characteristic copper or silver sheen. If the contacts are not too heavily oxidized, they can be cleaned with a stationery eraser or the wooden side of a match.

In more advanced cases, they need to be tinned, so you can restore the contact surface with tin. The worst option, when there is nothing to clean or tinker with, then you either need to change the entire board, or solder the conductors to the tracks of the board and connect them through them.

Also carefully inspect the tracks of the printed circuit board, they can burn out, crack when the board is bent, peel off and oxidize. They are restored either with a drop of tin or with a piece of wire, when the tracks are too tight - they are replaced with a piece of wire - a thin winding wire or a twisted pair core will do, soldering them to the beginning and end of the printed track.

To summarize, learn 5 tips for external diagnostics of electronics:

1. Most faults can be found by external inspection;

2. Carefully check the quality of the soldering and the presence of microcracks;

3. Pay special attention to power circuits;

4. Swollen electrolytic capacitors in most cases are both the cause of complete inoperability and the inoperability of some individual functions;

5. Not always outwardly serviceable part is such.

Measuring and ringing circuits

If the external examination did not bring results, then a series of measurements should be taken. If the device does not show signs of life and:

- His fuse burned out - then with the help of a multimeter we call the circuit and find in which section we have a short circuit. The ringing mode in most multimeters is combined with the diode test mode (in the figure below);

- If the fuse is good, check with a voltmeter whether the supply voltage is coming to the board.

If the voltage does not come, then the problem is most likely in the cable, you can determine this by ringing the cable from the plug to the connection to the printed circuit board.

Important:

Do not connect the power supply directly to the mains unless you are sure that you have corrected all problems. Connect in series the incandescent light bulb, which we mentioned in the middle of the article.

The next step is to check the power circuit, for this we turn on the device and check the presence of the output voltages of the power supply. Please note that there are times when the power supply does not turn on without load. Then we check the health of the power supply, it starts with checking the diode bridge. For more information, please do not hesitate to contact Kingfordpcb.

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