Multimeter used as ohmmeter
Zero adjustment should be performed first when measuring. That is, directly touch the two test leads (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.
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 a 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 through, the multimeter should be set to the ohm range of a large range. At the same time, the larger the range, the higher the voltage of the dry cell connected to the internal resistance. Therefore, 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, you cannot set the ohm gear to the Rxl0k position, otherwise it will easily break down the inter-electrode resistance of the tube. You can only lower the range and let the pointer point at the high resistance end. However, it has been pointed out earlier that the resistance scale is nonlinear, and the scale at the high resistance end is very dense, which can easily lead to increased errors.
When using the ohmmeter made by the factory, the internal dry cell is connected to the negative pole of the battery, and the black test lead is connected to the positive pole of the dry cell. For the external circuit, the red test lead is connected to the dry battery
When measuring a larger resistance, do not touch both ends of the resistance being measured at the same time. Otherwise, the resistance of the human body will be connected in parallel with the resistance being measured, causing the measurement results to be incorrect and the test value to be greatly reduced. In addition, when measuring the resistance on a circuit, the power supply to the circuit should be cut off. Otherwise, not only will the measurement results be inaccurate (equivalent to connecting an external voltage), but a large current will flow through the microampere meter and burn out the meter. At the same time, one end of the resistor 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.
Do not put the range switch on the ohm position after use. In order to protect the microampere meter from accidentally burning the meter the next time you start measuring. After the measurement is completed, you should be careful to set the range switch to the maximum range of DC voltage or AC voltage. Never place it on the ohm level to prevent the internal dry battery from being completely exhausted when the two test leads are short-circuited.
