Operation process of QJ84E DC double arm bridge:

Core working principle

This bridge is based on the "DC double arm bridge principle" (Kelvin bridge principle), which separates the "current circuit" and "potential circuit" to offset the interference of additional resistance on measurement. The specific process is as follows:

Four terminal wiring method: The measured resistance (Rx) adopts a four terminal wiring, namely two "current terminals" (C1, C2) and two "potential terminals" (P1, P2). Connect the current end to the current circuit of the bridge to provide test current; Connect the potential end to the potential circuit, only collect the voltage at both ends of Rx, and avoid wiring resistance (such as wire resistance and terminal contact resistance) from entering the measurement circuit.

Dual arm balance adjustment: The bridge consists of two sets of "arms": the "ratio arm" (R1, R2, fixed or adjustable standard resistors) and the "comparison arm" (R3, R4, adjustable standard resistors). Adjust the resistance values of the ratio arm and the comparison arm to make the current at the output terminal (ammeter) of the bridge zero, at which point the bridge reaches equilibrium.

Resistance calculation: When balanced, the measured resistance Rx is calculated using the formula Rx=(R1/R2) × R3 (or adjusted according to the actual configuration of the ratio arm and comparison arm). Due to the exclusion of wiring resistance, the measurement accuracy is much higher than that of a regular single arm bridge.

2、 Main functions and features

Wide range and high-precision measurement: The measurement range is usually 0.0001 Ω -22 Ω, with an accuracy level of up to 0.05, which can meet the precise detection requirements of low value resistance of equipment such as motors and cables (such as motor winding resistance usually between 0.01 Ω -1 Ω).

Additional resistance elimination: Through four terminal wiring and dual arm structure, it effectively eliminates the influence of wiring resistance and contact resistance of ≤ 0.1 Ω, which is its core advantage compared to ordinary DC single arm bridge (such as QJ23 type).

Easy to operate: The traditional QJ84E is mostly a pointer type galvanometer, equipped with a zero adjustment knob and a ratio arm switch. Some improved models have added a digital display function, which can directly read the Rx value and reduce reading errors; Built in power supply (such as 1.5V dry battery), no need for external high voltage, safe to use.

Strong stability: The ratio arm and comparison arm use high-precision manganese copper alloy resistors with a small temperature coefficient (≤ 5 × 10 ⁻⁶/℃), which are less affected by environmental temperature and ensure long-term measurement stability; The galvanometer has high sensitivity (usually ≤ 1 × 10 ⁻⁶ A/mm) and can quickly determine the balance state of the bridge.

3、 Typical application scenarios of QJ84E DC double arm bridge:

Motor and transformer testing: Measure the DC resistance of the motor stator and rotor windings to determine whether there is a turn to turn short circuit (abnormally low resistance value) or poor contact (excessively high resistance value) in the windings; Detect the DC resistance of the transformer winding and evaluate the welding quality of the winding.

Cable and wire testing: Measure the DC resistance of conductors in power cables and communication cables, calculate the resistivity of conductors, and verify whether the cable cross-section meets the standard (such as 2.5mm ² copper conductor resistance ≤ 10 Ω/km).

Grounding system detection: Measure the DC resistance of the grounding electrode and grounding grid (usually required to be ≤ 4 Ω), evaluate the current dissipation capacity of the grounding system, and avoid equipment damage caused by lightning strikes or faults due to excessive grounding resistance.

Standard resistance calibration: As a calibration tool for low resistance standards, compare and calibrate the standard resistance of 0.001 Ω -10 Ω to ensure that the accuracy of the measuring instrument meets the requirements.

4、 Precautions for use

Wiring specifications (key):

Strictly adopt a four terminal connection, with the current terminals (C1, C2) connected to the outer side and the potential terminals (P1, P2) connected to the inner side, and the potential terminals should be close to both ends of the measured resistor to avoid crossing with the current terminals; Coarse and short copper wires (with a cross-sectional area of ≥ 2.5mm ²) should be selected to reduce the impact of wire resistance.

Before wiring, clean the terminals of the tested resistor (polish the oxide layer with sandpaper) to ensure good contact and avoid excessive contact resistance.

Balance adjustment steps:

First, adjust the ratio arm knob to a gear that matches the approximate value of the measured resistance (for example, when measuring 0.01 Ω, select 10 ⁻ ³ for the ratio arm), and then adjust the comparison arm knob to gradually reset the ammeter pointer to zero; When adjusting, the action should be slow to avoid severe deflection and damage to the ammeter pointer.

Environmental and power requirements:

Avoid using in environments with high temperature (>40 ℃), high humidity (>85% RH), or strong electromagnetic fields (such as near transformers) to prevent accuracy drift; The power supply voltage must meet the requirements of the instrument (such as a built-in 6V battery, which needs to be replaced in a timely manner when the voltage is insufficient to avoid measurement errors).

Pre processing of measured resistance:

If the measured resistance is an inductive load (such as a motor winding), discharge (short-circuit the two terminals for 30 seconds) before measurement to avoid damaging the ammeter due to the induced electromotive force generated by residual inductance; When measuring, the test current should not be too high (usually ≤ 1A) to prevent the measured resistor from heating up and causing changes in resistance.

In order to help you operate more standardly and reduce measurement errors, I can help you organize a "QJ84E DC Double arm Bridge Operation Process and Error Control Guide", including wiring diagrams, balance adjustment steps, common error causes and solutions. You can directly compare and use it during actual measurement. Do you need it?