The core technical principle of QJ84 digital double arm bridge:
Core technical principle: Why can "low resistance" be measured?

The core breakthrough of QJ84 digital double arm bridge is to eliminate the interference of lead resistance and contact resistance. Its principle is based on the "double arm bridge balance condition" and "four terminal wiring method". Compared with the shortcomings of traditional single arm bridge (Wheatstone bridge), it highlights the advantages of low resistance measurement:

1. The limitations of traditional single arm bridge (large measurement error for low resistance)

A single arm bridge is composed of a circuit consisting of "power supply, bridge arm resistance, and ammeter". When measuring medium to high resistance (1 Ω -1M Ω), the accuracy is high. However, when measuring low resistance (<1 Ω), the lead resistance (the resistance of the test line itself) and the contact resistance (the contact resistance between the terminal and the measured resistance) will be directly connected in series in the bridge arm, resulting in an error of over 10% (for example, when measuring a 0.01 Ω resistance, if the lead resistance is 0.005 Ω, the error can reach 50%).

2. Improvement of QJ84 digital double arm bridge: four terminal connection method+double arm balance

Four terminal wiring method (core design):

Divide the two ends of the tested resistor Rx into "current terminals (C1, C2)" and "voltage terminals (P1, P2)", and connect them through four test wires:

Current terminals (C1, C2): connected to high current (usually 1-10A), responsible for injecting test current I into Rx, with lead resistors r1 and r2 connected in series in the current loop, which does not affect voltage measurement;

Voltage terminals (P1, P2): Connect the high-precision voltage sampling module to measure only the voltage U at both ends of Rx (avoiding voltage drops at r1 and r2), calculate the resistance using Ohm's law Rx=U/I, and eliminate interference from lead and contact resistance.

Double arm balance principle:

The instrument is equipped with "proportional arm resistors (R1, R2)" and "standard resistor arm (Rn)" to form a "dual arm circuit":

The proportional arm R1/R2 can be adjusted (such as 100:1, 10:1, 1:1) to adapt to different ranges of Rx;

The standard resistance Rn is a high-precision known resistance (accuracy ± 0.001%). By adjusting Rn to make the ammeter current 0 (bridge balance), Rx=(R1/R2) × Rn is achieved to achieve low resistance precision measurement.

3. Digital advantages (compared to traditional analog double arm bridge QJ44)

Automatic balancing: No need to manually adjust the knob to find balance, the CPU automatically calculates the proportional arm and standard resistance, and the result is obtained in 1-3 seconds;

Digital display: The LCD screen directly displays the resistance value (with a resolution of 0.1 μ Ω) to avoid reading errors in analog meters;

Strong anti-interference: Built in digital filtering circuit, suppresses power frequency interference (50Hz), stable reading in laboratory, workshop and other environments;

Function extension: Supports data storage (1000 sets), USB export, automatic calibration, and adapts to modern detection needs.

Core technical parameters of QJ84 digital double arm bridge: "hard indicators" for selection

The parameters of QJ84 digital double arm bridge directly determine the low resistance measurement capability, and it is necessary to focus on the four dimensions of "measurement range, accuracy, resolution, and test current". Typical parameters are as follows:

core parameters

Common scope

Technical significance and selection suggestions

The measurement range is 0.0001 Ω (100 μ Ω) -11 Ω (mainstream), which should cover the measured resistance value: ① For measuring the conductor resistance of the wire (such as copper wire 0.01 Ω/100m), select 0.0001-1 Ω; ② Measure the resistance of the motor winding (such as 0.1-10 Ω for small motors) and select 0.01-11 Ω

Measurement accuracy: 0.0001-0.01 Ω: ± 0.5% FS; 0.01-11 Ω: ± 0.1% FS Accuracy is the core indicator: ① For laboratory precision measurement (such as material resistivity), select ± 0.05% FS; ② For workshop rapid testing (such as motor delivery), select ± 0.5% FS

Resolution 0.1 μ Ω (range 0.0001-0.01 Ω); The resolution of 1 μ Ω (0.01-11 Ω range) determines the "minimum distinguishable resistance difference": ① Choose a resolution of 0.1 μ Ω for measuring fine wires (such as enameled wires); ② Choose 1 μ Ω resolution for measuring thick wires

Test currents of 1A, 2A, 5A, 10A (adjustable in multiple levels): The larger the test current, the greater the U=IRx, and the more accurate the voltage sampling: ① Choose 5-10A for measuring<0.01 Ω resistance (such as grounding electrode contact resistance); ② Choose 1-2A for measuring easily heating elements (such as windings) (to avoid overheating damage)

Current stability ≤ 0.1%/h (small current fluctuation) Current fluctuation can cause U fluctuation, affecting accuracy: ① Precision measurement requires ≤ 0.05%/h; ② Ordinary detection can accept ≤ 0.1%/h

The working environment temperature is 0-40 ℃, and the humidity is ≤ 80% RH. Outdoor or workshop use requires a wide temperature design (-10-50 ℃) and an IP54 protection level (dustproof and waterproof); Choose a regular environment for laboratory use

1000 sets of data storage (with timestamp) are required to record testing data (such as product factory reports) and select models with storage function; No need to record the selection of basic items (low cost)