Interlab, L.L.C.,

	Sign In Sign-Up

Interlab, L.L.C.,

Q versus ESR

An Equivalent Series Resistance (ESR) is widely used, for diagnosis, characteristic of capacitors. What is ESR? Real capacitors never perform their major functions for 100%. There a lot of reason leading to that. Aging insulating film, drying acid in electrolytic capacitors, electric leakage, and deteriorating contacts - this is not complete list of reasons causing this. As a result capacitor irrevocably loose electric energy, it has to operate with. Where this energy goes? It converts into heat and scatters around. This is what calls dielectric loss, dissipation of energy. The intensity of this process depends on operating frequency, applied voltage, temperature and other factors. Resistor is a symbol of temperature-making device. This is why all energy-loss-related "sins" of capacitor are convenient to be attributed to some equivalent resistor - ESR. According to definition it is connected in-series with capacitor. The last one is supposed to be an ideal. One can say that ESR is a scape goat, liable for everything bad in capacitor being tested. Connecting in-series implies frequency dependence of phase shift between voltage across the capacitor and current through that. Their product, RC, is a time constant. A circuit with such a capacitor is sensitive to operating frequency. The more frequency, the more clumsy, in terms of electric polarization, the capacitor becomes. The phase shift between voltage and current gets less than 90 degrees. The tangent of missing angle, delta, is a measure of portion of scattered energy, tg = CR or tg = C(ESR). Its reverse value, Q = 1/tg , is a quality factor which shows saved to lost energy ratio. The more Q the better capacitor is.

When troubleshooting, haw we detect defective capacitors? By their quality. Mostly, technician is interested in final result - is it good or bad. As reader sees, ESR is not final criteria of capacitor in terms of suitability. Let say, the capacitor with 10 Ohms ESR is good or bad? One may note this is a senseless question. Why? Because the judgment depends else on the frequency as well as on the capacitance. Let's consider horizontal deflection unit in computer monitor, which operates at frequency 31.5 kHz. Decoupling electrolytic capacitor 100uF with ESR=5 Ohms, has tg = 99, that is = 89.4 degrees. This is a resistor rather than a capacitor! Now, consider 1uf capacitor at equal conditions. Its tg =0.99 that is =45degrees, which is still acceptable for electrolytic capacitors. Or, let say, same 100uf capacitor with ESR=5 Ohms will be good in the vertical scan with operation frequency 60 Hz. We have tg =0.19 and =11 degrees, which is pretty acceptable. We see that ESR itself is useless without knowing capacitance of capacitor and its operation frequency.

One can ask, why it's so widely used, especially in in-circuit diagnosis if it's so imperfect? In-circuit measurement of ESR is based on conducting high frequency current (50-100 kHz) through circuit with the capacitor being tested. The current mostly flows through the capacitor, because its low impedance at these frequencies. It's easily implemented in a measuring instrument. But, alas, it's still imperfect because requires knowing the capacitance and operation frequency and requires making decision for each capacitor basing on these data. Which is very inconvenient!

Unlike those methods and instrument, INTERLAB L.L.C. developed express-method and instrument which measure final and only characteristic, the troubleshooter needs - the quality Q. The meter does this with one touch! How? Look its page.