Test Modes for Induction Measurements

Dear Sampathkumar:

Okay, here's the low down on inductors. The equivalent series circuit is used for low 'Q' (or lossy) inductors, i.e. the inductor has a fairly high DC resistance value. This type of inductor can be difficult to read and get accurate results. The equivalent parallel circuit is used for high 'Q' inductors, those with very low losses (resistance). This type is generally the easiest to measure and with very good accuracy.

Usually toroidal inductors have Qs above 10 and would qualify as high Q so use the parallel equivalent circuit for best results. 'Q's below 10 are generally considered high loss and an inductor with a 'Q' below 1 or 2 is very difficult to measure.

There are several different bridge configurations used for inductance, each having their own pros and cons but I have not seen any bridges that can measure inductors with very low 'Q's <1 with any success directly.

For values of 'Q' less than 100, the series and parallel inductance differ measurably by more than .01% The 'Q' of the equivalent parallel circuit always equals the 'Q' of the equivalent series circuit. The formulae are usually given in the bridge's manual but I can give them is you don't have them. The phase and magnitude of the resulting impedance are identical for both circuits.

Hope this helps.

Regards,

Ed