Multimeter used as an ohmmeter
① When measuring, it should be zeroed first. That is, touch the two test leads directly (short circuit), and adjust the dial—the zero-ohm adjuster below to make the pointer point to 0 ohm correctly. This is because the power supply voltage provided by the internal dry battery will drop as the use time increases. 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. To meet the requirements of full bias current Ig.
②In order to improve the accuracy of the test and ensure the safety of the measured object, it is necessary to correctly select the appropriate range file. Generally, when measuring resistance, the pointer is required to be within the range of 20%-80% of the full scale, so that the test accuracy can meet the requirements.
Due to the different range files, the test current flowing only on Rx is also different. The smaller the range, the larger the test current, otherwise the opposite. Therefore, if the small-range ohm range RX1 and RX10 of the 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 the current exceeds the current allowed by Rx, Rx It will burn out, or bend the pointer of the mA meter. Therefore, when measuring a resistance that does not allow a large current to pass, the multimeter should be placed on a large range of ohms. At the same time, the larger the range, the higher the voltage of the dry battery connected to the internal resistance. Therefore, when measuring a resistance that cannot withstand high voltage, the multimeter should not be placed on the ohm range with a large range. For example, when measuring the inter-electrode resistance of a diode or triode, the ohm gear cannot be set to Rxl0k, otherwise it is easy to break down the inter-electrode of the tube. You can only lower the range and let the pointer point to the high resistance end. However, it has been pointed out above that the resistance scale is non-linear, and the scale at the high resistance end is very dense, which may easily cause an increase in error. ①When the factory uses it as an ohmmeter, it is connected to a dry battery, the negative pole of the battery, and the black test lead is connected to the positive pole of the dry battery. As far as the external circuit is concerned, when the red test pen is connected to the dry battery 2 to measure a large resistance, the hands cannot touch both ends of the measured resistance at the same time, otherwise, the human body resistance will be connected in parallel with the measured resistance, making the measurement result incorrect and the test value will be greatly reduced. Small. In addition, when measuring the resistance on the circuit, the power supply of the circuit should be cut off, otherwise not only the measurement result will be inaccurate (equivalent to an external voltage), but also a large current will flow through the microampere meter, which will burn out the meter. At the same time, one end of the resistance to be tested should be welded away from the circuit before measurement, otherwise the measured resistance is the total resistance of the circuit at the two points.
