Measure L And C At The Same Time

Ferrite-core transformers used in switching power supplies can have large capacitances effectively in parallel with a winding, which make it difficult to measure the winding's inductance. Typically, if the capacitance is the result of a reflection from some secondary, it's unremovable, and direct measurement of an inductance, L_x , in the presence of a parallel capacitance, C_x , is corrupted by C_x on many inductance meters. Moreover, direct measurement of C_x in the presence of L_x is virtually impossible. But with this AGC-controlled oscillator circuit, both L_x and C_x can be measured, each without any corrupting influence from the other.

In the test circuit (see the figure), switch S₁ connects any one of four capacitors (C₁ through C₄) into the circuit. Then, with S₂ open, rotate R₃ clockwise from its full CCW position until a 2-V pk-pk output signal is obtained. If oscillation can't be achieved, return R₃ to full CCW, close S₂ , and try again.

Next, the frequency is measured and recorded. Then switch S₁ is set for any one of the remaining capacitors and the aforementioned process is repeated.

As an example, say that capacitors C₁ and C₂ were used to obtain frequencies f₁ and f₂. L_x and C_x are found from the following equations:

j = 1, 2, 3, or 4

Four capacitors are provided so that this process can be repeated for up to six combinations of capacitor values as a double check of the results. If any of the six calculations differ significantly from the others, a frequency measurement error may have to be corrected. Accurate knowledge of C₁ through C₄ is a must—they must be measured carefully. It's recommended that their given values be further refined to account for circuit capacitance, as follows: Construct a test coil of approximately 200 turns of wire on a diameter of approximately nine inches. This creates an inductance with an L_x value that will remain essentially constant under variable levels of excitation.