I believe that friends who have repaired more circuit boards must have encountered a short circuit of public power supply. In most cases, when you use a multimeter to test the resistance value between a power supply voltage of the circuit board and the ground, the display is very small, tens of ohms, a few ohms, a few tenths of ohms, or even 0 ohms, which may be much smaller than the normal resistance value. There are many devices using this power source, and any one may be short circuited, How to locate the short circuit element quickly and effectively? In this depth, the operation methods of several methods are introduced in detail, and their advantages and disadvantages are analyzed.
First, it is necessary to judge whether there is a real short circuit. Inexperienced friends often tangle about whether a power supply voltage point on the circuit board is short circuited, because the resistance value range at both ends of the power supply of different circuit boards is uncertain. Some boards may be short circuited by hundreds of ohms, and some ohms are also normal. This requires us to generally test the ground resistance of the power supply end of some circuit boards to establish some perceptual knowledge. Generally speaking, most circuit boards without high-power CPU chips have a resistance value of more than hundreds of ohms at the power end. The more chips, the smaller the resistance value. For example, for the industrial control motherboard with high integration, the resistance of the power end of the CPU chip is very small, even below 10 Ω. Of course, if you have the same board on hand, it is easy to judge whether there is a short circuit.
1、 Power pin cutting method
Tools used: diagonal pliers (for plug-ins) or hot-air gun (for patch components)
This method is only used under extremely simple conditions. Disconnect the power pins of components in turn, and then test whether the resistance between the positive and negative of the power supply becomes much larger. If this happens, the short-circuit components can be determined. But this depends entirely on luck. Bad luck will cut many components, even the last one, which is time-consuming and troublesome, and will also damage good components. Generally, it is not used as a last resort, and it is only applicable to boards with fewer components.
2、 Electric burning method
This is the first method. You should prepare a power supply with adjustable voltage and current and voltage and current display. After determining the rated power supply voltage on the board (such as 5V voltage), when the power supply is not connected to the load, adjust the adjustable power supply to the rated power supply voltage of the board, adjust the current knob to a smaller position, and then add the positive and negative poles to the circuit board. After power on, the voltage will be pulled down due to short circuit. At this time, do not adjust the voltage knob. Just adjust the current knob and slowly increase the current. At the same time, observe the displayed current and voltage and calculate the product of current and voltage. This product is the power obtained by the power circuit board. When the power exceeds 1W (no more than 2W at most), start touching the corresponding components with your hands, When you touch a component that is particularly hot (it should be above 60 ℃, not below 40 ℃), remove the component, and then test the resistance to ground at the power end of the board, or test the resistance between the power pins of the component after removing the circuit board. There are very obvious changes before and after the whole resistance value. For example, after removing the component, the resistance of the circuit board changes from a few ohms to hundreds of ohms, Then it can be determined that the short circuit is caused by the element.
3、 Impedance method
Tools: digital bridge or milliohmmeter
For some short circuits, the resistance of the test power supply terminal is almost 0 ohm. After connecting the adjustable power supply, because the short circuit is too severe, even if the current is adjusted to the maximum value of the adjustable power supply, the voltage cannot rise, so the voltage current product (power) will not go up naturally, and the components will not have obvious heating. Moreover, because the wires connecting the adjustable power supply and the circuit board also have resistance, the current is large, The heat is inside the wire. In this case, the short-circuit point cannot be accurately judged by the electric burning method, so the digital bridge or milliohmmeter can be used to locate the short-circuit point. We know that the pins of the components on the circuit board are connected together through the copper foil wiring of the PCB. The copper foil wiring is not a superconductor, but also has resistance value. For example, the thickness of copper foil on PCB is 35 μ m. If the printed line width is 1mm, the resistance value is about 5m Ω every 10mm long. This resistance value cannot be tested by ordinary multimeter, but can be tested by using the DCR (DC resistance) test function of digital bridge or milliohmmeter.
Suppose there are 10 chips on the circuit board. Because a chip has a severe short circuit, the resistance value between VCC and GND tested by the multimeter is only 0.2 Ω. Not to mention that the general cheap multimeter can not display the accuracy of 0.2 Ω, and the displayed resistance value will change with the contact degree of the multimeter pen (the contact resistance is unstable), so the multimeter can not locate this short circuit point, The digital bridge or milliohmmeter can sense the change of 1m Ω. We only need to test and record the resistance value on the VCC and GND pins of each element and locate the element with the lowest resistance value.
Of course, this method also has limitations. For example, there are too many components or there is a short circuit in the middle layer of PCB, so it is difficult to eliminate it.
4、 Constant current voltage measurement method
Tools: adjustable power supply or special constant current source.
This method is essentially the same as that of digital bridge milliohmmeter, aiming at the situation of serious short circuit. The method has the advantages of simple operation and low cost, and is more suitable for the situation with a large number of chip components. The specific operation is as follows: adjust the adjustable power supply voltage below the rated voltage of circuit board components, such as 5V power supply, set the adjustable power output below 5V when there is no load, first adjust the current knob to the minimum, and then short circuit the output. At this time, the constant current indicator lights up, and then slowly increase the current to display a fixed value, such as 1a, Then do not move the current knob again, add the current between VCC and GND on the circuit board, and pay attention not to reverse the voltage direction. Next, we can directly test the voltage of the VCC and GND pins of the components on the circuit board. If a component is really short circuited, the voltage of the VCC and GND pins of the component will be the minimum. After the component is positioned, we will remove the component, and the circuit board short circuit disappears, so we can completely determine that the short circuit is caused by this component.
Remember two points when using this method: 1. After connecting to the power supply, be sure to keep the constant current indicator on. 2. When comparing the voltage of VCC pin of each component to its GND pin, do not adjust the current knob.
5、 Magnetic field tracing method
Tool: short circuit fault tracker
According to Ampere’s law, the wire with current will produce a magnetic field. The greater the current, the stronger the magnetic field. If the magnetic field strength sensor (Hall sensor) is used to test the magnetic field strength, it can non-contact sense whether there is a short circuit somewhere. The special short-circuit tracker generally has this function. If a component is short circuited, most of the current flows through the component, and the stronger the magnetic field generated inside the component. If the Hall probe is used to detect the output corresponding to the sound frequency from low to high, the magnetic field intensity from low to high, and finally the place where the sound is more sharp is the short-circuit point.
The magnetic field tracking method should pay attention to two points: one is to constantly change the orientation of the probe. The maximum magnetic field can be detected only when the magnetic field of the conductor passes vertically through the hall sensor; Second, this method is not suitable for occasions with magnetic components on the circuit board, such as inductance, transformer, relay, etc. the magnetic gathering effect of these devices will interfere with the detector.
6、 Coolant rapid cooling method
Tool material: electronic component fault detection rapid refrigerant
The operation method is to power up the circuit board with reference to the electric burning method, and then spray the refrigerant where the short circuit is suspected. If there is a short circuit heating of the element, the frost like refrigerant will volatilize rapidly and show a different color from the surrounding, so that the damaged element can be located quickly. This method is powerless when the short-circuit resistance is very small, because the power consumption of the corresponding short-circuit point is too small and the heating is not obvious.
7、 Thermography
Tool: infrared thermal imager
The infrared thermal imager can accurately detect the temperature by sensing the infrared emitted by each part of the object. After the circuit board is powered on, where there is short-circuit heating, the more obvious the corresponding infrared characteristics will be, the more prominent the disappeared image color will be, and the maximum temperature and minimum temperature points will be marked. It is very convenient to detect short-circuit. The thermal imager detection is very accurate in locating the faults of diodes, capacitors and ICs. It has repeatedly built miracles in our maintenance work. The disadvantage is that the current high-pixel infrared thermal imager is expensive.