Measuring a Capacitor

How do I correctly measure capacitance and dissipation factor?

The key to measure the capacitance and dissipation factor correctly is the meter settings.

The voltage settings are critical for high capacitance capacitors. For some cap meters, the applied voltage to the test component is not enough and the capacitance reads low.

The frequency settings are also important. Since capacitance changes with frequency, industry standards specify the test frequency at 1MHz, 1kHz, or 120Hz (see Table 1).

The recognition of the aging phenomenon for EIA class II capacitors is also important. For class II materials, capacitance decreases with Time. Therefore, an industry convention has been accepted that states the capacitance shall be within tolerance at 1000 hours since the time of the last heat (TOLH).

Why is it necessary to measure capacitance at different test frequency/voltages depending on the capacitance range?

The frequency settings for the meter are depended on the parasitic of the component. To achieve more accurate reading of the component, measuring frequencies are directed away from the Self-Resonance Frequency (SRF) of the component. Industry users set the standards at different frequency points depending on the capacitance value (see Table 1). The capacitance over 10uF was considered in the range of Tantalum capacitors. Therefore, as the ceramic capacitance range began to increase into the Tantalum capacitor range, the industry adopted the frequency standard for Tantalum measurements into ceramic capacitors.

The applied voltage is also depended on the capacitance of the capacitor. Generally, the 10uF and under have applied voltage of 1.0 ± 0.2 Vrms. But over 10uF, the applied voltage is 0.5 ± 0.1 Vrms. High capacitance capacitors have very low impedance, so to supply enough current to make the measurement, the power supply needs more current than that supplied at 1.0 ± 0.2 Vrms. By lowering the applied voltage, the power supply will be able to supply enough current to measure the high capacitance capacitor accurately.

What is the difference between Cp and Cs of capacitance?

An impedance analyzer can measure the capacitance in parallel known as Cp or in series known as Cs. The circuit model will be depending on the value of capacitance of the capacitor (see Figure 1).

When C is small and impedance is high, parallel impedance between C and Rp will become significantly higher than Rs. Thus the meter setting for measuring capacitance should be Cp. When C is large and impedance is small, parallel impedance for C and Rp is not as significant. Therefore, Cs should be used for the meter setting to measure capacitance. A good rule of thumb to select the impedance setting is to use Cp for capacitor impedance values greater than 10kΩ and Cs for less than 10Ω.

How can I accurately measure the quality factor (Q)?

The quality factor is a measure of the extent to which a capacitor acts like a theoretically pure capacitor. It is the inverse of the dissipation factor (DF). Q is typically reported for capacitance values ≦ 330pF, DF > 330pF.

An accurate Q value can be obtained from Q-meter that utilizes precise inductance coils that correspond to a specific capacitance range. Often multiple coils are necessary to adequately measure across the 0.5 ~ 330pF range. For capacitors > 330pF, the quality factor can be calculated by taking the inverse of the dissipation factor (see Formula 1).

What is the allowable ripple current of capacitors?

When there is a fluctuation of voltage in a capacitor, a charge or discharge current enters or leaves the capacitor in response to this. The current that enters or leaves the capacitor is known as a ripple current. This current is normally indicated with an effective value because it is not a direct current in principle. The capacitor generates heat with the ripple current so an upper limit must be set, and the value of this upper limit is what is known as the allowable ripple current.

For more information you can watch Another Teaching Moment on MLCCs:
An article from our BLOG on MLCCs:
Classes of Capacitors: Understanding Ceramic Capacitor Temp-Coefficients

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