Download Measurement CKT

Simple Meter circuits
1.
How to extend the range of a current meter?
Answer: A shunt resistor can be added to the circuit to bypass the excess current.
The value of the shunt resistor is computed from the current divider approach:
Rshunt 
Rm
IT
1
Im
Im
Ishunt
IT
Example: Find the shunt resistance if Im = 50 A, Rm = 2 K, and IT = 2 mA.
Rshunt 
2.
Rm
IT
1
Im

2 K
2 K  2 K


 51.3
2mA
39
 1 40  1
50A
How to extend the range of a voltage meter?
Answer: A series resistor can be added to the circuit to handle the excess voltage.
The value of the series resistor is computed from the voltage divider approach:
V
Rseries  T  Rm
Im
V
Example: Find the series resistance if Im = 50 A, Rm = 2 K, and VT = 2 V.
V
2V
Rseries  T  Rm 
 2 K  40 K  2 K  38 K
Im
50 A
3. Capacitance measurements: Capacitance can be measured accurately with an AC
bridge (p. 130-135). However, if an AC bridge is not available, the following simple
circuit can be used to estimate the capacitance.
Connect the above circuit and measure the AC voltages across the resistor and the
capacitor. You may need to switch the positions between the two components if
the AC voltmeter does not have a floating ground.
C
4.
VR
2 f R VC
Inductance measurements:
The inductance is more complicated because an inductor usually has internal
resistance. Connect the above circuit and vary the variable resistance until the two
voltages are equal.
R2  r 2
L
2 f
DC Bridge circuits
5.
What is a Wheatstone bridge?
Answer: Wheatstone bridge is often used in high precision measurements because
it is a balance type instrument. In the following circuit the unknown resistance can
be calculated from the three known resistors if the bridge circuit is under the
balanced condition—zero current through the meter.
R2
R1
G
R3
Rx
R x R3

R1 R 2
Example: Find Rx if the following resistances will balance the Wheatstone bridge:
R1 = 4 K, R2 = 6 K , and R3 = 9 K.
R x R3
R
9 K

 Rx  R1  3  4 K 
 6 K
R1 R 2
R2
6 K
Note: The unbalanced Wheatstone bridge cannot be solved using the basic
series/parallel circuit techniques.