AC bridge Sources and Detectors

 AC bridge Sources and Detectors

Measurement at low frequency: power line may act as a source of supply to the bridge circuits.

Measurement at higher frequency: Electronic oscillators are universally used as bridge source supplies.

Electronic oscillators and higher frequency

1. Frequency is constant, easily adjustable, and determinable with accuracy.

2. The waveform is very close to a sine wave.

3. Their power output is sufficient for most bridge measurements 

(A typical oscillator has a frequency range of 40Hz to 125Hz with a power output of 7 W.)

Detectors

Commonly used detectors for ac bridges are Headphones, Vibration galvanometers, and Tuneable amplifier detectors. 

Headphones are widely used as detectors at frequencies of 250Hz and over up to 3 or 4kHz.They are the most sensitive detectors for this frequency range.

At a single frequency: Tuned detectors normally give the greatest sensitivity and discrimination against harmonics in the supply.

Vibration galvanometers are extremely useful at power and low audion\ frequency ranges. Vibration galvanometers are manufactured to work at various frequencies ranging from 5Hz to 1000Hz but are most commonly used below 200Hz as below this frequency they are more sensitive the headphones.

Tuneable amplifier detectors are the most versatile of the detectors.

The transistor amplifier can be tuned electrically and thus can be made to respond to a pointer-type instrument.

This detector can be used, over a frequency range of 10 Hz to 100KHz

For ordinary a.c.bridge measurements of Inductance and Capacitance.

 A fixed frequency oscillator of 1000Hz and output of about 1W is adequate. But for more specialized work variable oscillators are preferable with output up to 5W. In practice power is supplied is within limits to the bridge but on some occasions, a higher power may be necessary. 

Usually, an untuned amplifier is used as a detector. The balance detection is sensed both orally by headphones and visually by a pointer galvanometer having a logarithmic deflection (to avoid damage to the galvanometer which may be caused by unbalancing).