Introduction to Using a Multimeter as an Ohmmeter
① When measuring, zero should be adjusted first. That is, directly touch the two probes (short circuit), adjust the dial - the zero ohm adjuster below makes the pointer point correctly at 0 ohms. This is because as the internal dry battery is used for a longer period of time, the power supply voltage it provides will decrease. When Rx=0, the pointer may not reach full bias. At this time, Rw must be adjusted to reduce the shunt current of the meter head and meet the requirement of full bias current Ig.
② In order to improve the accuracy of testing and ensure the safety of the tested object, it is necessary to correctly select the appropriate range gear. When measuring resistance, it is generally required that the pointer be within the range of 20% to 80% of the full scale, so that the testing accuracy can meet the requirements.
Due to different range settings, the magnitude of the test current flowing through only Rx varies. The smaller the range, the larger the test current, otherwise the opposite is true. So, if the small range ohm range RX1 and RX10 of a multimeter are used to measure a small resistance Rx (such as the internal resistance of a milliampere meter), a large current will flow through Rx. If this current exceeds the current allowed by Rx, Rx will burn out or the milliampere meter pointer will bend. So when measuring resistors that do not allow high currents to pass through, the multimeter should be set to the high range ohm range. The larger the range range, the higher the voltage of the dry battery connected to the internal resistance. Therefore, when measuring resistors that cannot withstand high voltage, the multimeter should not be placed in the large range ohm range. When measuring the inter electrode resistance of a diode or transistor, the ohm range should not be set to Rx10k, otherwise it is easy to break down the inter electrode resistance of the transistor. We can only lower the range gear and let the pointer point to the high resistance end. But as pointed out earlier, the resistance scale is non-linear, and the scale at the high resistance end is very dense, which can easily cause errors to increase. When using an ohmmeter made by the factory, a dry battery is connected internally to the negative pole of the battery, and the black probe is connected to the positive pole of the dry battery. For external circuits, when the red probe is connected to a dry battery to measure a large resistance, hands should not touch both ends of the measured resistance at the same time. Otherwise, the human resistance will be connected in parallel with the measured resistance, resulting in incorrect measurement results and a significant decrease in the test value. In addition, when measuring the resistance on the circuit, the power supply of the circuit should be cut off, otherwise not only will the measurement result be inaccurate (equivalent to an external voltage), but it will also cause a large current to pass through the microampere meter and burn out the meter. At the same time, one end of the measured resistor should be soldered off from the circuit before measurement, otherwise the total resistance of the circuit at that two points will be measured.
