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Thursday, 7 February 2019

Bridges in Electronic Instrumentation

Bridges in Electronic Instrumentation:

Values of components like resistors, inductors and capacitors and of other circuit parameters directly derived from component values like frequency phase angle and temperature are measured using bridge circuits. In the bridge method of measurement we compare the value of an unknown component with that of a standard component. Therefore the accuracy of measurement will be very high. The reason is that we use a null indicator to balance the bridge and the null indication is independent of the characteristics of the null indicator. The advantage is that the accuracy of measurement is directly related to the accuracy of the components of the bridge, but not to that of the null detector. 

Bridges in Electronic Instrumentation can be classified as DC bridges and AC bridges. The difference is in the source of excitation. A DC bridge uses a DC source and an AC bridge uses AC source for excitation. 

An example of a DC bridge is the Wheatstone bridge. This bridge consists of four arms, consisting of resistances. A battery is used as a source of excitation. Usually a galvanometer or a sensitive ammeter is used as a null detector. 

The AC Bridge is a development of the Wheatstone bridge. Alternating current bridge circuits can be used for measurement of inductance, capacitance, storage factor, loss angle etc., very accurately. 

An AC bridge consists of four arms, an alternating voltage to excite the bridge and a detector to indicate the balance condition. Unlike the DC Bridge the AC Bridge has in its arms impedances. The detector used is sensitive to small magnitudes of alternating potential difference.

The exciting source can be the mains voltage stepped down to the required magnitude. This works out to be a source of low frequency. When higher frequencies are required oscillators may be employed. Oscillators have their frequency constant, waveform pure and extremely close to sine wave. They can be designed to supply the required power to excite the bridge circuits, the frequency range can be around 40 Hz to 150 kHz. 

The null detectors used with A.C. bridges can be any of the following types:

(a) Headphones
(b) Vibrating galvanometer
(c) Tunable amplifier detector

Headphones can be used as null detector, when the frequency of the excitation is within audio frequency range. Audible detection is inaccurate as the response of the ear is logarithmic. Visual null detection is more accurate.

Vibrating galvanometers are very useful at power frequencies. They can be used at low audio frequencies. Though they are capable of working in a frequency range from 5 Hz to 1 kHz, they are more sensitive below 200 Hz.

For use at the frequency range of 10 Hz to 100 kHz tunable oscillators are preferred. The tuned amplifier's output will operate an indicating instrument indicating the null or balance condition. In some cases instead of using a tuned amplifier, untuned amplifier may be used. In such cases simultaneous observation is done using indicating instrument connected to the amplifier and also by audible means employing headphones.

When using a galvanometer care is to be taken to see that it has logarithmic deflection, to prevent damage to the galvanometer.
The Universal Impedance Bridges in Electronic Instrumentation can be used for measurement of values of resistance, inductance and capacitance. This bridge uses different configurations for measurement of R, L and C.

With the advent of digital measuring techniques the bridge methods are becoming obsolete. Digital L, C, R meters are now available with simultaneous Tan display, offering different ranges for measurement. The "Q" meter is an instrument specially designed for the measurement of properties of coils and capacitors. It works on the principle of series resonance. That is the voltage across the coil or the capacitor is equal to the applied voltage times the "Q" of the circuit. If a voltmeter is connected across the capacitor and supplying a fixed voltage to the circuit the voltmeter can be calibrated in terms of "Q".

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