Summary of Operation Issues for Multimeter Ohm Range
1. The measurement error of the multimeter is the smallest in the DC voltage range, followed by the AC voltage range, and the worst in the Ohm range.
The minimum error in measuring DC voltage with a multimeter is due to its simple measurement circuit, as shown in the figure. Although the circuit for measuring AC voltage is basically the same as that for measuring DC voltage, it mostly uses nonlinear components such as rectifying diodes, so the error is larger than the former. When measuring resistance, a dry battery must be used as a power source, and the voltage of the battery will change over time. Adjusting the resistor R in a timely manner will also produce errors. Therefore, the resistance (RA+R) on the instrument scale is different from that during use, so the Ohmic range error is the largest in three-way measurement.
2. What do a small grid and a large grid on the ohm scale of a multimeter represent?
On the ohm scale line of a multimeter, a small grid represents how much, and a large grid represents how much, which cannot be generalized. It depends on where the selected gear is located to determine. As shown in the figure, if the conversion switch is turned to the "R * 1K" gear, there are 5 large grids between 5 and 10 on the ohm scale line. Each large grid represents: (10-5)/5 * 1K=1K, and there are 10 small grids between 5 and 10. Therefore, each small grid represents: (10-5)/10 * 1K=0.5K. If the conversion switch is set to the "R * 10" position, each large grid represents: (10-5)/5 * 10 Ω=10 Ω, and each small grid represents: (10-5)/10 * 10 Ω=5 Ω.
3. There is a tingling sensation when measuring the resistance of the secondary winding of a transformer that is not connected to a power source using the ohm range.
Magnetic fields, like electric fields, have energy. When a transformer is in an open circuit for the first time and unloaded for the second time, it is equivalent to an inductive coil with a core. When a person holds the two probes of a multimeter with both hands to contact the terminals of the transformer's secondary winding, the multimeter power supply (1.5V) charges the winding coil and converts it into magnetic field energy stored in the coil. When a person holds the probe and makes good contact with the terminal, the current flowing through the coil is about 11mA (x1 Ω range) or 6.5mA (xlO Ω range) due to the low resistance of the coil and the multimeter head. If there is poor contact or at the moment when either hand leaves the terminal post, the magnetic field energy stored in the coil will be released through the human body due to the disconnection of the multimeter power supply. Due to the fact that the resistance of the discharge circuit is much greater than that of the charging circuit, in order to prevent the sudden decrease in current in the coil, a self induced back electromotive force (about 70-100V) is generated in the coil, causing a tingling sensation in both hands. However, due to the low energy of the magnetic field and the small discharge current, there is no harm to the human body (some measurement personnel hold the exposed parts of the two multimeter probes with both hands and press them against the terminal of the transformer secondary winding, believing that this can provide good contact). If a multimeter is used correctly, this type of electrical phenomenon can be completely avoided.
4. There is a tingling sensation when measuring the resistance of the two phase wires of a power-off motor using an Ohm gear.
The stator winding of an electric motor, like the winding of a transformer, generally has a large inductance. When measuring it with the ohm range of a multimeter, direct current is applied to the winding of the motor, so when it is disconnected, the measured winding has a high induced voltage. If both hands touch the two ends of the winding at the same time, they will feel numb.
