Detailed explanation of how to use a multimeter as an ohmmeter
1. Zero adjustment first when measuring
Put the two test leads into direct contact (short circuit), and adjust the zero ohm adjuster below the dial so that the pointer points correctly at 0 ohms. This is because the power supply voltage provided by the internal dry battery will decrease as the use time increases. When Rx = 0, the pointer may not reach full deflection. At this time, Rw must be adjusted to reduce the shunt current of the meter. To achieve the requirement of full bias current Ig.
2. Choose the appropriate measuring range
In order to improve the accuracy of the test and ensure the safety of the measured object, the appropriate range must be selected correctly. 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 different measurement ranges, the test current flowing through Rx is also different. The smaller the measuring range, the greater the test current, otherwise the opposite is true. Therefore, if you use the small range ohm range RX1 or RX10 of the multimeter to measure the small resistance Rx (such as the internal resistance of the milliamp 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 mA meter pointer.
Therefore, when measuring resistance that does not allow large currents to pass, the multimeter should be set to the ohm range of a large range. At the same time, the larger the measuring range, the higher the voltage of the dry battery connected to the internal resistance. When measuring resistance that cannot withstand high voltage, the multimeter should not be set to the large-range ohm range. For example, when measuring the inter-electrode resistance of a diode or triode, the ohm setting cannot be set to the Rxl0k position, otherwise the inter-electrode breakdown of the tube will easily occur. You can only lower the range and let the pointer point at the high resistance end. It has been pointed out before that the resistance scale is non-linear, and the scale at the high resistance end is very dense, which can easily lead to an increase in errors.
3. Things to note
(1) When used as an ohmmeter, the negative electrode of the dry battery is connected internally, and the black test lead is connected to the positive electrode of the dry battery. For the external circuit, the red test lead is connected to the dry battery.
(2) When measuring a larger resistance, hands should not touch both ends of the resistance being measured at the same time to prevent the human body resistance from being connected in parallel with the resistance being measured, causing incorrect measurement results and greatly reducing the test value. In addition, when measuring the resistance on the circuit, the power supply to the circuit should be cut off. Otherwise, not only will the measurement result be inaccurate (equivalent to connecting an external voltage), but a large current will pass through the microampere meter and the meter will be burned out. At the same time, one end of the resistance being measured should be welded away from the circuit before measurement, otherwise what is measured is the total resistance of the circuit at these two points.
(3) After use, do not place the range switch on the ohm position. In order to protect the microampere meter from accidentally burning the meter the next time you start measuring. After the measurement is completed, set the range switch to the maximum range of DC voltage or AC voltage. Never place it on the ohm range to prevent the internal dry battery from being completely exhausted when the two test leads are short-circuited.
