A sinusoidal oscillator to supply an AC current to the capacitor under test.The block diagram in Figure 1 shows that the ESR meter is made up of four fundamental sections: Accordingly, I have incorporated a sine wave source into the design to avoid this possibility. In some ESR meters, a square wave or pulsed source is used to test the capacitor, and the resulting inductive spikes can cause an abnormally high ESR reading. A glance at the equivalent circuit model shown in the sidebar should make this clear.Īll capacitors have an inductive component which can possibly interfere with the ESR measurement. This is the basis of the ESR meter I describe in this article. Measure the resulting AC voltage developed across the capacitor’s terminals (Vcap) and you can find the ESR by dragging out Ohm’s Law: These losses can be lumped together as ESR, which looks like a small resistance in series with an ideal (lossless) capacitor.Ī simple technique for measuring ESR is to supply the capacitor with a known AC current (Icap) at some frequency where the reactance of the capacitor is very low so that the ESR dominates. Measuring ESRĪs detailed in the sidebar (“What a Capacitor Really Looks Like”), a number of factors contribute to power loss in a capacitor. There’s no telling what abuse and degradation they may have suffered, and I definitely don’t want to use a component in my next project that’s going to let me down, no matter how pristine its appearance. I have a bunch of capacitors I have accumulated over many years - some of which have been salvaged from old equipment or cycled through several projects. I find this device to be a useful gadget to have around my work bench. In concert with this “retro” approach, the measurement result is displayed on a conventional moving coil panel meter. Moreover, this design is easy to build and set up, using only common through-hole parts (no surface-mount devices!) and no microprocessors. With the instrument I describe in this article, you can test your store of capacitors or those in some vintage equipment you may be restoring, so you can weed out the ones that may not be up to par. This can happen if the capacitor has been subjected to electrical stress or elevated temperature, or even while it's just sitting around in storage, not connected to anything. The internal resistance, called “Equivalent Series Resistance” (or ESR) can increase, causing power loss and heating. Why mention capacitors? Because high capacitance types like aluminum electrolytics and tantalums can slowly deteriorate over time. That includes me, you, and, surprisingly, most of those capacitors you've been hoarding in your junk box for years, just waiting for a project to put them to use. Time marches on, and eventually everything goes downhill.
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