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TEST&MEASUREMENT Capacitor ESR Tester the good, the bad and the leaky... Design by Flemming Jensen How about an in-circuit capacitor tester to take the strain out of tracking down faulty capacitors? No need to solder out any capacitor, simply check it in-circuit, from thousands of microFarads down to a hundred nanoFarads. In most cases, parallel coils or low value resistors are no problem. Even shorted caps may be revealed in-circuit and polarity is irrelevant for the tester. High ESR? Replace! The most significant property of a capacitor is its capacitance but besides that there's another important factor, namely the so called ESR, or Equivalent Series Resistance. An ideal capacitor is a purely reactive component with a 90-degree phase angle between current and voltage. In the real world, however, a capacitor needs to be modelled as an ideal capacitor in series with a resistor representing the losses introduced by the component. The equivalent circuit is shown in Figure 1. Sure, capacitance can be measured by means of a capacitance meter, which is pretty common nowadays, but unfortunately this test won't tell anything about the capacitor's quality -- we need to know the ESR as well. Over time, electrolytics tend to dry out, which will raise their ESR and inevitably the voltage drop inside the capacitor. Evidently, the pure reactance Xc can not produce heat, due to the 90-degree phase shift between voltage and current, but the ESR can, and in switching circuits the resultant heat will cause a further degradation of the capacitor's quality i.e., a further rise in ESR. It's fairly common to find electrolytics that on the face of it have only lost just a few 14 Elektor Electronics 9/2002 TEST&MEASUREMENT How does ESR influence circuit behaviour? In (fast) switching circuits, a low ESR may be crucial for proper circuit behaviour. For example, in a TV set, high capacitor-ESR may lead to inability to quit stand-by mode, incorrect picture height or width, synchronisation problems, interference or hum bars. In Switch Mode Power (SMPSUs) supplies, high ESR caps may lead to blown semiconductors, blown fuses or no start up. In power circuits, a rising ESR will make the capacitor warm up, leading to even higher ESR and eventually circuit breakdown. The usual method to troubleshoot these problems involves soldering out the capacitors, measure the capacitance and solder the good ones back in. A tedious task, but what's even worse, ailing capacitors often don't show a low capacitance, get soldered back in again and then the troubleshooting gets really time consuming. ideal capacitor ESR reactive part X C = 1 2 f . C 012022 - 11 Figure 1. The most important property of a capacitor is its capacitance but beside that there's another important factor, namely the socalled ESR, or Equivalent Series Resistance. percent of their rated capacity although their ESR is in the hundred Ohms range. Obviously, such a component acts as a load just running hot and wast