2025-12-23
Intelligent Digital Resistance Meter
Measuring the DC resistance of conductors using the bridge method is cumbersome and inefficient. However, using a DMM (digital multimeter) or an intelligent digital resistance meter to measure DC resistance is fast, has a wide measurement range, and is easy to operate, making it widely used. The measurement range is 10⁻⁷ to 10⁶ Ω, with accuracy levels of 0.25, 0.05, and 0.02.
The diagram shows the principle block diagram of a smart digital resistance measuring instrument.
The figure shows the measurement principle circuit of one type of instrument, also known as the voltage drop method. Because the resistance being measured is small, the influence of lead and contact resistance must be considered. Therefore, this instrument uses a four-wire measurement method. In the diagram, P1 and P2 are the voltage terminals, and C1 and C2 are the current terminals.
To improve the instrument's resolution and increase its measurement range, an adjustable constant current source is typically used. For small resistances, the current I is increased as much as possible, and a 16-bit or 24-bit high-precision A/D converter is employed. Considering the instability of the constant current source, the circuit samples and analyzes the input current and voltage signals almost simultaneously using the A/D converter. The entire instrument operates under the control of a CPU, and the resistance value is finally displayed in digital form. The figure shows the external appearance of several intelligent digital resistance measuring instruments.
Testing Precautions:
When using the four-terminal method for measurement, be sure to follow the wiring method shown in the diagram (internal measurement method): the two voltage terminals should be on the inside, and the two current terminals on the outside. The other end of the cable, connecting to the three phases and ground, must be reliably connected using thick, short copper wires.
