Electronic Design

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, Lx , in the presence of a parallel capacitance, Cx , is corrupted by Cx on many inductance meters. Moreover, direct measurement of Cx in the presence of Lx is virtually impossible. But with this AGC-controlled oscillator circuit, both Lx and Cx 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. Lx and Cx 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 Lx value that will remain essentially constant under variable levels of excitation.

Connect that coil in parallel with some capacitor of accurately known value, say 0.01 mF. Measure the Lx and Cx values of this combination as described earlier. Adjust the assumed values of C1 and C4 for the narrowest range of calculated values for Lx and the most accurate value of Cx possible. A simple Basic program can be used to automate the calculations for the test results (see the program listing).

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