ZY9624 Double Bridge Test AccessoryZY9624 Double Bridge Test Accessory

productIt is an additional device designed to improve the testing speed of double bridges in power systems for transformer DC resistance detection, which is difficult due to the large inductance of transformers (especially large capacity ultra-high voltage transformers) and time-consuming measurement. It is connected to double bridges (such as QJ44)）Cooperation not only maintains the various advantages of the original double bridge, but also has fast testing speed, high efficiency, reliability and practicality.

Functional features:

1Do not change the double bridge structure, do not change the technical specifications of the double bridge. The testing speed is greatly improved when measuring transformer windings.

2The product adopts fully sealed, maintenance free lead-acid batteries with small volume, large capacity, and long service life. Internal backup of the instrument

Charger, can be charged by mains power.

3 selectLCDThe digital display meter can understand the battery capacity, the steady-state condition of the tested circuit, and detect faults early.

4Protective measures are taken for the measurement circuit to effectively prevent back electromotive force from damaging the instrument.

5The working current of the measurement circuit is1A(Constant current).

Bridge and Road Form

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As shown in Figure 1.4.1, the most commonly used resistance bridge consists of four resistors forming the bridge arm, one diagonally connected to the power supply and the other as the output.

As shown in the figure, the resistance of each arm of the bridge is R1, R2, R3, R4, and U is the DC power supply voltage of the bridge. When the four arm resistor R1=R2=R3=R4=R, it is called an equal arm bridge; When R1=R2=R, R3=R4=R '≠ R, it is called an output symmetric bridge;

When R1=R4=R, R2=R3=R '≠ R, it is called a power symmetrical bridge.

Figure 1.4.1 Bridge Circuit Diagram 1.4.2 Current Output Type

way of working

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Single arm operation: Only one arm of the bridge is connected to the measured object, while the other three arms use fixed resistors; Dual arm working: If two arms of the bridge are connected to the measured object and the other two are fixed resistors, it is called a dual arm working bridge, also known as a half bridge form; Full bridge mode: If all four bridge arms are connected to the measured object, it is called full bridge mode.

Output

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The output modes of the bridge include current type and voltage type, which are mainly determined by the load situation.

1) Current output type

When the output signal of the bridge is large and a load with a smaller resistance value, such as a galvanometer or a light oscilloscope, is connected to the output terminal for measurement, the bridge will output in the form of current, as shown in Figure 1.4.2a. The load resistance is Rg, which can be obtained from the figure

;

So the open circuit voltage UAB at the output of the bridge is

(1-4-1)

By applying the active port network theorem, the current output bridge can be simplified into the circuit shown in Figure 1.4.2a. In the figure, E 'is equivalent to the open circuit voltage Uab at the output of the bridge, and R' is the input resistance of the network

(1-4-2)

As can be seen from Figure 1.4.2b. The current flowing through the load Rg is (1-4-3)

When Ig=0, the bridge is balanced. Therefore, the balance condition of the bridge is

R1R3=R2R4 or

When the load resistance Rg of the bridge is equal to the output resistance of the bridge, that is, when impedance matching occurs, there is

At this point, the output power of the bridge is maximized, and the output current of the bridge is

(1-4-4)

The output voltage is

(1-4-5)

When the bridge arm R1 is a variable resistor related to the measured variable and has a resistance increment Δ R, omitting the Δ R term in the denominator results in a symmetrical output bridge, R1=R2=R,R3=R4=R

For a symmetrical power bridge, R1=R4=R,R2=R3=R'≠R

For the equal arm bridge, R1=R2=R3=R4=R

From the above results, it can be seen that for the three forms of bridge, when Δ R<<R, their output currents are proportional to the resistance change rate of the strain gauges, and there is a linear relationship between them.

2) Voltage output type

When an amplifier is connected to the output terminal of a bridge, due to the high input impedance of the amplifier, the load resistance of the bridge can be considered infinite, and the bridge outputs in the form of voltage. The output voltage is the open circuit voltage at the output terminal of the bridge, expressed as (1-4-6)

Let the bridge be in a single arm working state, where R1 is the strain gauge and the remaining bridge arms are fixed resistors. When R1 feels the measured resistance increment Δ R1, it can be obtained from the initial equilibrium condition R1R3=R2R4. Substituting it into equation (1-4-6), the output voltage of the bridge caused by the imbalance of Δ R1 is

(1-4-7)

For an output symmetric bridge, R1=R2=R, R3=R4=R?/ SUP>， When the resistance of arm R1 changes, Δ R1=Δ R, according to (1-4-7), the output voltage can be obtained as follows:

(1-4-8)

For a symmetrical power bridge, R1=R4=R,R2=R3=R'≠R。 When the R1 arm generates a resistance increment Δ R1=Δ R, it can be obtained from equation (1-4-7)

(1-4-9)

For the equal arm bridge R1=R2=R3=R4=R, when the resistance increment of R1 is Δ R1=Δ R, the output voltage can be obtained from equation (1-4-7) as follows:

(1-4-10)

From the above three results, it can be seen that when the resistance of the bridge arm strain gauges changes, the output voltage of the bridge also changes accordingly. When Δ R is equal to R, the output voltage of the bridge is linearly related to the strain. It can also be seen that under the same change in bridge arm resistance, the output voltage of the equal arm bridge and the output symmetrical bridge is higher than that of the power symmetrical bridge, that is, their sensitivity is higher. Therefore, equal arm bridges or output symmetric bridges are often used in use.