June 2005, Issue 179

Accurate Capacitance Meter
Cypress PSoC High Integration Challenge 2004 Contest Winner

Whether you’re designing a power supply for a 750-W car radio woofer or a high-precision timing circuit for a model airplane launcher, you’ll encounter capacitors that are difficult to measure with conventional pocket multimeters. They’re either not up to the range (easily into the tens or even hundreds of thousands of microfarads) or lack the accuracy to meet the design requirements.

Traditional bridge circuit solutions measure capacitance by including the measured capacitor in a bridge where the balancing elements are known and accurate. Because alternating current is used, this approach is inapplicable to certain capacitor types (e.g., tantalum and aluminum electrolyte) in which a thin layer of metal oxide serves as the dielectric. These oxides exhibit certain semiconductor properties that naturally affect the accuracy when AC is flowing.

Although the measurement error introduced in bridge-circuit measurement varies from manufacturer to manufacturer, we’ve observed up to 35% relative errors in some cases. In addition, the range of a bridge circuit is limited to what you have to balance the bridge. To measure a large capacitance, you need bridge elements that will hardly fit in your pocket (or even your handbag).

Fortunately, there’s a way to work around both problems. The solution is to use direct current and the linear relationship between the capacitance and the time a capacitor takes to charge. Direct current isn’t affected by parasitic semiconductance. Only the current and the time it takes to fully charge the measured capacitor limit the range of the measurement. This method isn’t new, but it has been neglected when it comes to consumer products. There are inherent difficulties to overcome because it’s sensitive to current, voltage or frequency changes and drifts, hysteresis, parasitic resistances, and thermal effects. The classic analog or mixed-signal implementation involves precise, stable elements and circuit solutions to achieve good accuracy. This is usually beyond the budget of the average designer.

The DC method we’ll describe is free of the drawbacks associated with traditional bridge circuits. It was designed to accurately measure tantalum and aluminum electrolyte capacitors from 1 to 1,000,000 µF. It’s also applicable for measuring capacitors with ceramic, plastic, or tape dielectric with the same degree of accuracy.

The Cypress PSoC High Integration Challenge 2004 was the perfect opportunity to bring an old idea to life with contemporary embedded technology. Twenty years ago such a DC-based capacitor meter took up a double-sided board the size of a laptop full of TTL chips, but now it can fit in a single chip. The PSoC mixed-signal array looked like the perfect platform to build on, and the element of competition was the cherry on top of the pie.

We designed an accurate, CY8C27443-based capacitance meter specifically to measure the capacitance of electrolytic capacitors (see Photo 1 and Figure 1). As you’ll see, our meter is much more accurate than traditional bridge circuits. The problems caused by parasitic semiconductance are minimized by a stabilized source of direct current to charge the capacitor.