# What do you mean by quality factor of a coil?

Why can't a Maxwell bridge be used for measurement of coil whose quality factor is more than 10?

Question

Why can't a Maxwell bridge be used for measurement of coil whose quality factor is more than 10?

- The quality factor, or Q factor, is expressed by omega*L/R, omega = 2pi fr, fr being the frequency at resonance. It indicates the influence of coil losses (R) with respect to the equivalent reactance at resonance and is a measure of the sharpness of the resonance peak, when the coil is made resonating with an external capacitor (for which losses are normally much lower, not to say negligible, depending on teh technology and dielectric type ... thing of tan-delta).

AC bridges are all based on reaching the balance by what is called Null Detector, normally an amperometer (or voltmeter). WHen balance is reached there are some equations characteristic of teh specific bridge that you solve to find the unknown element, in this case your coil in therms of inductance L and series resistance R. WIth Maxwell, if the Q factor is large, that is the R is small, then on the other arm diagonally opposite to LR the balancing of the real part by means of the R' in // to the C' is reached only if this R' is very large and the loss factor of the C' that is parallel to it causes a not negligible error.

That's it, it is a matter of measurement accuracy to reach the balance while taking into account the non-idealities of teh used components, in this case the losses of the capacitor that is used for balancing the reactive part, that is the L. - I have forgotten to say that teh Hay bridge is sometimes suggested to overcome this problem, because there the balancing resistor is in series to the capacitor, not in parallel.

Already a member? Log in

## All Answers (2)