Ground-to-chassis resistance measured with a multimeter
In ATX power supply maintenance, it is often necessary to use a multimeter to measure the ground resistance of the test point. For the AC high voltage input side: select the diode mode of the multimeter: connect the red pen to the negative terminal of the full bridge and the black pen to the test point. At this point, the multimeter will have a reading, which is called the "resistance to ground" of the ATX power supply's high voltage side. For the DC low-voltage output side: select the diode mode of the multimeter, connect the red pen to the ground in the output terminal, and connect the black pen to the test point. At this point, the multimeter will also have a reading, which is called ATX power low. In short, depending on the selected reference point, the resistance of the ATX power supply to ground is defined as the "high voltage side resistance to ground" and "low voltage side resistance to ground". Since the ground resistance value can be used as a basis for maintenance, it means that the ground resistance value must reflect some essential property of the test point in the circuit. Judging whether this essential property is normal or not can effectively clarify the fault point. Strictly speaking, the resistance to ground is essentially the equivalent internal resistance of the circuit network between the "negative pole" or "ground" of the circuit board and the "test point". The resistance to ground is called the "diode value". The manifestation of "diode value" is not obvious in motherboard maintenance, but in switch power supply maintenance, using the term "diode value" instead of "ground resistance" can better reflect the essential properties of the circuit. Firstly, the presence or absence of ground resistance reflects whether the circuit has been properly connected. Simply put, the current must be able to flow back from the test point to the ground of the motherboard, otherwise there will be a circuit break. Therefore, if a test point does not have a resistance value to ground, it means that it is not connected to ground or negative pole. For circuit boards that do not drop components, such signals are indeed present but rare. Secondly, for specific testing points; Its resistance to ground should not be too small, even to the point of short circuit to ground or negative pole, nor too large, even to the point of open circuit. It will have a normal value, which is determined by the circuit itself where the test point is located. If the resistance to ground of a certain test point deviates significantly from the normal value, it can be clearly determined that there is a faulty component in the circuit where the test point is located. This is the theoretical basis for using the resistance to ground to determine whether there may be a faulty component. The above two points are the basic value of ground resistance in maintenance. When measuring the resistance to ground in practice, the voltage drop caused by the resistance between the probes and the contact resistance between the probes and the test point cannot sometimes be ignored. For the error caused by the resistance between the probes, if it is a digital multimeter, the relative measurement function provided by the multimeter itself can be used to zero it. For the contact resistance between the probe and the test point, the tip of the probe should be used to reliably contact the test point as much as possible.
