How to use the graphic calculator of capacitor impedance over frequency
To use the calculator, enter the capacitance of the capacitor and, optionally, the values of its parasitic elements. You can modify the units using the selectors. The impedance as a function of frequency will be plotted on the graph. You can add as many traces as you want. You can also calculate the parallel of the impedances plotted on the graph. You can hide the traces by clicking on their names.
Impedance of an ideal or real capacitor
An ideal capacitor is a passive component whose impedance decreases as a function of frequency. This may lead us to think that, for a very high frequency, its impedance will be almost 0. This is correct for a first approximation, but if you use real components, this is not the case. The reason is that physical components have unwanted effects due to the construction of the components. These unwanted effects are called parasitic effects, and we can model them as ideal passive components. Look at the figure below.
The ESR (Equivalent Series Resistance) acts from the frequency at which the capacitor impedance is lower than the ESR. The ESL (Equivalent Series Inductance) causes the impedance to increase with frequency. The parasitic components place a limit on the expected electrical behavior. Since parasitics depend on the physical characteristics of the components, they are not very noticeable at low frequencies. On the other hand, at high frequencies they can even be the dominant effect. In the specific case of a capacitor, at high frequencies we can find the paradox of it behaving like an inductor. In a circuit, it is a good idea to combine in parallel some capacitors that are physically different or are built with different technologies. This will cause the parasitics to act in different frequency ranges. In this way you can minimize parasitic effects. For example, electrolytic capacitors have very high capacitance, but also very high ESR and ESL. Therefore, they are effective capacitors at low frequencies. Ceramic capacitors have low capacitance but also very small parasitics. Consequently, they are useful in a higher frequency range. You can combine them in parallel to get a lot of capacitance and, at the same time, they work well at high frequencies. Do a few tests with the calculator to see the effect by clicking on the button to plot the parallel of all the impedances.
Frequently Asked Questions
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What are ESR and ESL?
ESR (Equivalent Series Resistance) and ESL (Equivalent Series Inductance) are parasitic elements found in real capacitors that affect performance. They reduce the filtering capability of the capacitors. Their effect happens at high frequencies. ESR causes power loss and makes the capacitor look like a resistor at a sufficiently high frequency, while ESL makes the component to behave like an inductor, with impedance increasing with frequency. Consequently, they limit high-frequency filtering. -
Why does impedance increase at high frequencies?
At high frequencies, the parasitic inductance (ESL) dominates the capacitive reactance, causing the capacitor to behave like an inductor and increasing its impedance. -
How can I minimize parasitic effects?
You can place capacitors in parallel. Combining different types (e.g., electrolytic for low freq, ceramic for high freq) helps cover a wider frequency range effectively. Parasitic effects vary with capactitor type, construction technology, and physical size. Combining different capacitors in parallel can help mitigate these effects. -
Which kind of capacitors exist?
There are several types of capacitors, each with its own characteristics and applications. Common types include ceramic capacitors, electrolytic capacitors, tantalum capacitors, film capacitors, and supercapacitors. Each type has different capacitance values, voltage ratings, and parasitic properties like ESR and ESL, making them suitable for various applications in electronic circuits. In general , ceramic capacitors are used for high-frequency applications since they have small parasitics, electrolytic capacitors for bulk energy storage at low frequencies, tantalum capacitors for stable capacitance in small sizes, film capacitors for precision applications, and supercapacitors for very high capacitance needs.