Q-Meter


Posted on Feb 5, 2014

We know that every inductor coil has a certain amount of resistance and the coil should have lowest possible resistance. The ratio of the inductive reactance to the effective resistance of the coil is called the quality factor or Q-factor of the coil. A high value of Q is always desirable as it means high inductive reactance and low resistance. A


Q-Meter
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low value of Q indicates that the resistance component is relatively high and so there is a comparatively large loss of power. The effective resistance of the coil differs from its dc resistance because of eddy current and skin effects and varies in a highly complex manner with the frequency. For this reason Q is rarely computed by determination of R and L. One possible way for determination of Q is by using the inductance bridge but such bridge circuits are rarely capable of giving accurate measurements, when Q is high. So special meters are used for determination of Q accurately. The Q-meter is an instrument designed for the measurement of Q-factor of the coil as well as for the measurement of electrical properties of coils and capacitors. -This instru ­ment operates on the principle of series resonance i. e. at resonate condition of an ac series circuit voltage across the capacitor is equal to the applied voltage times of Q of the circuit. If the voltage applied across the circuit is kept-constant then voltmeter connected across the capacitor can be calibrated to indicate Q directly. Circuit diagram of a Q-meter is shown is figure. A wide-range os ­cillator with frequency range from 50 kHz to 50 MHz is used as a power supply to the cir ­cuit. The output of the oscillator is shorted by a low-value resistance, Rsh usually of the or ­der of 0. 02 ohm. So it in ­troduces almost no resistance into the oscillatory circuit and...




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