The main technical indicators of capacitors are capacitance, withstand voltage and temperature. In addition to these three main indicators, the more important of other indicators is equivalent series resistance (ESR). Some capacitors have a golden strip line with a big hollow letter “I” printed on it, which indicates that the capacitor is a LOW ESR low-loss capacitor. Some capacitors will also mark the ESR value (equivalent series resistance), the lower the ESR, the smaller the loss, the greater the output current, and the higher the quality of the capacitor.

ESR is the abbreviation of Equivalent Series Resistance, that is, “Equivalent Series Resistance”. The ideal capacitor itself will not have any energy loss, but in fact, because the material of the capacitor has resistance, the insulating medium of the capacitor has loss. This loss appears externally as a resistor and a capacitor in series, so it is called “equivalent series resistance”. Another concept similar to ESR is ESL, which is equivalent series inductance. Early coiled inductors often have high ESL. The larger the capacitance, the larger the ESL. ESL often becomes a part of ESR, and ESL can cause phenomena such as series resonance. However, relative to the capacitance, the proportion of ESL is very small, and the probability of problems is small. Later, due to the improvement of capacitor manufacturing technology, ESL has been gradually ignored, and ESR is used as in addition to capacity, voltage resistance, and temperature resistance. The main reference factor for the selection of capacitors.

The unit of series equivalent resistance ESR is milliohm (mΩ). Generally, the ESR of tantalum capacitors is usually below 100 milliohms, while aluminum electrolytic capacitors are higher than this value. The ESR of some types of capacitors can even be as high as several ohms. The level of ESR is related to the capacity, voltage, frequency and temperature of the capacitor. When the rated voltage is fixed, the larger the capacity, the lower the ESR. Similarly, when the capacity is fixed, the selection of a high-rated voltage variety can also reduce the ESR; therefore, there are many advantages to choosing a capacitor with a high withstand voltage; the ESR is high at low frequencies, and the ESR is low at high frequencies; high temperatures will also increase ESR.