|
| |
Back
Kelvin Bridge | Up
Neets Module 21-Test Methods and Practices | Next
Figure 3-5.—Typical vector-bridge configuration (amplitude) |
3-7
additional resistance in series with branch S of the bridge. This adjustment becomes necessary because
the Q of the unknown capacitor or inductor in branch X is higher than the comparable Q of the standard in
branch S.
Schering Bridge
The Schering bridge, shown in figure 3-1, is a commonly used type of bridge for the measurement of
capacitors and dielectric losses. The Q of a capacitor is defined as the reciprocal of the dissipation factor,
which is the ratio of the capacitor's dielectric constant to its conductivity at a given frequency.
Accordingly, capacitor Q is determined by the frequency used to conduct the measurement and the value
of the capacitor, CB, required to obtain bridge balance. The accuracy of this type of bridge is excellent,
about 2% for dissipation factors ranging from 0.00002 to 0.6. Typical accuracies for capacitive reactances
in the range of 100 picofarads to 1 microfarad are 0.2%.
Hay Bridge
The Hay bridge, shown in figure 3-1, is used for the measurement of inductance and the Q of the
inductor. It is interesting to note that this type of bridge measures inductance by comparing it with a
standard capacitor of known characteristics. This arrangement provides the advantage of a wide
measurement range with the minimum use of electronic parts as comparison standards. A typical range of
values that can be measured with the Hay bridge is from 1 microhenry to 100 henries. The accuracy of the
measurements made with this bridge is about 2%. The frequency used in conducting the inductance
measurement must be taken into account because of the series reactance of capacitor CB. The loss factor
of the inductor under test is balanced in terms of the Q of the inductor. The Hay bridge, then, is used for
measurement of inductances having a Q greater than 10. For instance, a Q of 10 gives a calibration error
of 1%, whereas a Q of 30 gives a calibration error of 0.1%.
Q-5.
When you are testing an inductor with a Hay bridge, the characteristics of the inductor are
compared with what type of device?
Maxwell Bridge
The Maxwell bridge, shown in figure 3-1, is used for the measurement of inductance and inductive
Q. This bridge is similar to the Hay bridge because it also measures inductance by comparison with a
standard capacitor of known characteristics. Notice, in particular, that capacitor C
B
is connected in
parallel with resistor R
B. In connection with this difference, the requirement of an accurately known
frequency is removed. This bridge circuit is employed for measuring the inductance of inductors having
large losses; i.e., low Q. The range of this type of instrument is much greater than that of the Hay bridge;
values ranging from 1 microhenry to 1,000 henries are measurable, with an error of only 2%.
VECTOR BRIDGES
The basic bridges described up to now determined the resistive and reactive components of the
unknown impedance; however, the vector bridge indicates the magnitude and phase angle. Typically,
vector bridges require two null readings. Consider the basic bridge circuit of figure 3-5. The magnitude of
the unknown impedance (Z
X) is determined by the voltages applied across R and ZX and to the bases of
emitter followers Q1 and Q2, which bias the balanced rectifiers, CR1 and CR2. Resistors A and B are
equal in value. When R is adjusted to equal ZX, the voltages between points 1 and 2 and between points 1
and 4 are equal in magnitude, and the vtvm will indicate 0 volts.
|
