Why is the internal resistance of the DC current range of the multimeter not minimized?
For example, the meter head is 1 mA and 100 ohms, and the theoretical calculation of the internal resistance of the 1A range is 0, 1 ohm, but the internal resistance of the corresponding range of the multimeter is higher than the calculated value, why?
This is determined by how the multimeter is used. The basic structure of the multimeter is a microammeter head, which is connected in series with a resistor to form a voltmeter; and connected in parallel with a resistor to form an ammeter.
When measuring the voltage, it is connected in parallel with the circuit under test. When shifting gears, the switch will be disconnected instantaneously or have poor contact. At this time, there is no current flowing through the meter head, so there is no problem.
When measuring current, the meter is connected in series with the circuit under test. If the parallel resistance of each gear is directly connected in parallel with the meter head, it is necessary to change gears during the measurement. When the switch is momentarily disconnected or the contact is poor, a large current will flow through the meter head, which will easily burn the meter head. , the contact resistance of the switch can also affect the accuracy of serious measurements.
Therefore, in the current stage, instead of connecting different resistors in parallel with the meter head directly, a resistor string is formed in a fixed parallel connection with the meter head, and then different current files (as shown in the figure) are drawn from the resistor string, which is a sacrifice The sensitivity of some meter heads ensures the safety of the measurement; the influence of the contact resistance of the shift switch on the measurement is greatly reduced.
