Test and Measurement Electrical Equipments Section

One Stop For Your Test and Measurement Need

Q meter basics | How to design Q meter circuit

This page covers Q meter basics. It mentions how to design Q meter circuit.

The Q meter instrument is designed and developed to measure electrical properties of coils and capacitors. The operation of Q meter is same as series resonant circuit.

As we know, voltage across coil or capacitor is equal to applied voltage times Q of the circuit. If applied voltage is fixed, voltmeter across capacitor can easily be calibrated to read Q.

series resonant circuit

The figure-1 depicts series resonant circuit. At resonance, following conditions are fulfilled.
Xc = XL
Ec = I*Xc = I * XL
E =I*R
Here, E = Applied voltage, I = Circuit Current, Ec = voltage across capacitor, Xc = capacitive reactance, XL = inductive reactance, R = Coil resistance

The magnification of the circuit is known as Q.
Where Q = XL/R = Xc/R = Ec/E

If E is maintained at constant level then voltmeter connected across capacitor can be calibrated directly in terms of circuit Q.

How to design Q meter circuit

Q meter circuit
Figure-1: Q meter circuit

The figure-2 depicts practical Q meter circuit. To make measurement, unknown coil is connected to test terminals. The curcuit can be tuned to resonance in either of the following ways.
• By setting oscillator to given frequency and varying internal resonating capacitor.
• By pre-setting capacitor to desired value and adjusting oscillator frequency.
➨The Q reading on output meter must be multiplied by index setting of "Multiply Q by" meter to obtain actual Q value.

Inductance of coil can be calculated from known values of frequency(f) and resonating capacitor (C) using following equation.
➨ L = 1 / (2*π*f)2 * C

There are three methods to connect unknown components to test terminals of the Q meter viz. direct connection, series connection and parallel connection.

Other Electrical test and measurement equipments

Power Analyzer
Benchtop Power Supply
Benchtop Power Supply
Benchtop Power Supply
Benchtop Power Supply

Share this page

Translate this page