Welcome to the Thursday Toolkit series, about tools for building Physical Computing projects.
Equivalent Series Resistance, or ESR, is the resistance of a capacitor. Huh? Let me explain…
A perfect capacitor has a specific capacitance, no resistance, and no inductance. Think of a capacitor as a set of parallel plates, close to each other, but isolated. When you apply a voltage, electrons flow in on one side and electrons flow out on the other side until the voltage (potential difference) across the plates “pushes back” enough to prevent more electrons from flowing. Then the flow stops.
It’s a bit of a twisted analogy, but that’s basically what happens. A capacitor acts like a teeny weeny battery.
But no real capacitor is perfect, of course. One of the properties of a capacitor is that it has an inner resistance, which can be modeled as a resistor in series with a perfect capacitor. Hence the term “ESR”.
Resistance messes up things. For any current that flows, it eats up some of that energy, creating a voltage potential and more importantly: generating waste heat inside the capacitor.
ESR is something you don’t want in hefty power supplies, where big electrolytic capacitors are used to smooth out the ripple voltage coming from rectified AC, as provided by a transformer for example. With large power supplies, these currents going in and out of the capacitor lead to self-heating. This warms up the electrolyte in the caps, which in turn can dramatically reduce their lifetimes. Caps tend to age over time, and will occasionally break down. So to fix old electronic devices: check the big caps first!
Measuring ESR isn’t trivial. You have to charge and discharge the cap, and watch the effects of the inner resistance. And you have to cover a fairly large capacitance range.
This ESR70 instrument from Peak Instruments does just that, and also measures the capacitance value:
It’s protected against large voltages, in case the capacitor under test happens to still have a charge in it (a cap is a tiny battery, remember?). The clips are gold-plated to lower the contact resistance – and removable, nice touch!
In this example, I used a 47 µF 25V electrolytic capacitor, and it ended up being slightly less than 47 µF and having an ESR of 0.6 Ω as you can see.
It this cap were used in a 1A power supply to filter the ripple from a transformer, then its ESR could generate up to 0.6 W of heat – which would most likely destroy this little capacitor in no time.
Fortunately, big caps have a much lower ESR. It measured 0 (i.e. < 0.01 Ω) with a 6800 µF unit, for example.
As with last week’s unit, this is not an indispensable instrument. But very convenient for what it does.