Do you know about "The Schering Bridge" ? Discuss about it.
ECE
Saturday 12 May 2012
The Wien Bridge
- As shown in Figure, one arm of a Wien bridge consists of a capacitor in series with a resistor (C1 and R3) and another arm consists of a capacitor in parallel to a resistor (C2 and R4).
- The other two arms simply contain a resistor each (R1 and R2).
- The values of R1and R2 are known, and R4 and C2 are both adjustable.
- The unknown values are those of C1 and R3.
- Like other bridge circuits, the measuring ability of a Wien Bridge depends on 'balancing' the circuit.
- Balancing the circuit in Figure means adjusting R4 and C2 until the current through the ammeter between points A and B becomes zero.
- This happens when the voltages at points A and B are equal.
- When the Wien Bridge is balanced, it follows that R2/R1 = Z1/Z2 where Z1 is the impedance of the arm containing C1 and Z2 is the impedance of the arm containing C2.
- Mathematically, when the bridge is balanced,
R2/R1 = (1/ωC1 + R3) / (R4/[ωC2(R4 + 1/ωC2)])
wherein ω = 2πf; or
R2/R1 = (1/ωC1 + R3) / (R4/[ωC2R4 + 1]); or
R2/R1 = (1/ωC1 + R3) (ωC2 + 1/R4); or
R2/R1 = C2/C1 + ωC2R3 + 1/(ωC1R4) + R3/R4.
R2/R1 = (1/ωC1 + R3) / (R4/[ωC2R4 + 1]); or
R2/R1 = (1/ωC1 + R3) (ωC2 + 1/R4); or
R2/R1 = C2/C1 + ωC2R3 + 1/(ωC1R4) + R3/R4.
- When the bridge is balanced, the capacitive reactances cancel each other out, so
R2/R1 = C2/C1 + R3/R4. Thus, C2/C1 = R2/R1 - R3/R4.
- Note that the balancing of a Wien Bridge is frequency-dependent.
- The frequency f at which the Wien Bridge in Figure becomes balanced is the frequency at which ωC2R3 = 1/(ωC1R4), or 2πfC2R3 = 1/(2πfC1R4).
- Thus, the frequency f is given by the following equation: f = (1 / 2π) x (sqrt(1/[R3R4C1C2])).
Thursday 10 May 2012
The Wheatstone Bridge
- In the Wheatstone Bridge shown in Figure, the resistance values of resistors R2, and R3 are known, while the resistance value of variable resistor R1 may be adjusted
- The resistance value of R1 is adjusted until the current reading of the ammeter connected between points A and B of the circuit becomes zero.
- When this happens, the bridge is said to be 'balanced', i.e., the voltages at points A and B are already equal, so the value of the unknown resistance may easily be calculated using voltage ratios:
Runknown / R3 = R1 / R2.
-
The equivalent resistance Rb of the circuit
when it is balanced is just the resistance of the left leg (R1+R2) in parallel
with the resistance of the right leg (R3+Runknown).
Rb = [(R1+R2)(R3+Runknown)] / [R1 + R2 + R3 + Runknown].Alternatively, if the resistance values of R1, R2, and R3 are known but R1can not be adjusted, then the value of Runknown can still be calculated using Kirchhoff's Voltage Law.This set-up is often seen in strain gauges and resistance temperature detection circuits, since it is quicker to read a voltmeter than to manually adjust a resistor to balance the circuit.
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