The Detectors Types are as follows
- For low frequency, the most convenient detector is the vibration
- For ordinary laboratory work at frequencies up to a few 100 Hz, the moving coil type of instrument is usually employed. It has a high
- In high voltage testing, the moving magnet type of vibration galvanometer with remote controlled tuning is used, (for 300 Hz —1 kHz).
- For higher AF frequencies (>800 Hz), the telephone (headphone) is the best detector. (Vibration galvanometers and headphones have no phase selectivity, i.e. they do not indicate whether it is resistance or reactance adjustments that are required.)
- The ac galvanometer and separately excited dynamometer are phase selective, and are best suited at low frequencies. They have a high
- In many cases, especially in bridges for routine use, a pointer instrument is used. It is advantageous if it can be made phase selective.
These pointer instruments are generally moving coil milliammeters operated with some arrangement of copper oxide rectifiers (frequency range 40 Hz — 1 kHz).
- Modern bridge techniques employ the amplifier as a regular feature.
- At frequencies above about 3 kHz, and particularly at high AF or RF, a heterodyne or beat-tone detector is used.
- With all Detectors Types, the impedance should be selected to suit that of the A higher sensitivity can be obtained by using an interbridge transformer. Also, when headphones are used as detectors, precautions should be taken to eliminate capacitance effects between the observer and the headphones.
- A moving magnet vibration galvanometer has a range of up to 1500 Hz.
- An electrodynamometer can also be used as an ac detector.
- Electrometers are used as detector because small capacitances possess a very large impedance when used with ac circuits at low frequency, and when measured in the bridge they form a high impedance branch. Hence, this Detectors Types is used to increase sensitivity.
Precautions to be Taken When Using a Bridge
Assuming that a suitable method of measurement has been selected and that the source and detector are given, there are some precautions which must be observed to obtain accurate readings.
The leads should be carefully laid out in such a way that no loops or long lengths enclosing magnetic flux are produced, with consequent stray inductance errors.
With a large L, the self-capacitance of the leads is more important than their inductance, so they should be spaced relatively far apart.
In measuring a capacitor, it is important to keep the lead capacitance as low as possible. For this reason the leads should not be too close together and should be made of fine wire.
In very precise inductive and capacitances measurements, leads are encased in metal tubes to shield them from mutual electromagnetic action, and are used or designed to completely shield the bridge.