Introduction: I Was Surprised
A few years ago, after more than 25 years of working with these things, I learned something new about ceramic capacitors. I was working on an LED light-bulb driver and the time constant of an RC circuit in my project simply did not seem to be right.
I immediately assumed that there was an incorrect component value installed on the board, so I measured the two resistors making up a voltage-divider. They were just fine. I desoldered the capacitor from the board and measured it. It, too, was fine. Just to be sure I got new resistors and capacitor, then measured and installed them. I fired up the circuit, checked that the basic operation was proper, and went to see if my RC time-constant problem was resolved. It was not.
I was testing the circuit in its natural environment: in its housing, which itself was in an enclosure to mimic a “can” for ceiling lighting. The component temperatures in some instances reached well over +100ºC. Even in the short time that it took me to get around to retesting the RC behavior, things could get quite hot. My next conclusion, of course, was that the temperature variation of the capacitor was the issue.
I was skeptical about this conclusion as I was using X7R capacitors which, as I had known for many years, only varied ±15% up to +125ºC. To be sure and to confirm my memory, I reviewed the data sheet for the capacitor that I was using. That is when my ceramic capacitor reeducation began.
Background on Some Basic Ceramic Capacitors Types
For those who don’t have this stuff memorized (like virtually everyone), Table 1 shows the letters and numbers used for ceramic capacitor types and what each means. This table describes Class II and Class III ceramics. Without getting too deep into details, Class I capacitors include the common COG (NPO) type.
These are not as volumetrically efficient as the ones in our table, but they are far more stable with environmental conditions and they do not exhibit piezo effects. The ones in the table below, however, can have widely varying characteristics; they will expand and contract with applied voltage, sometimes causing audible buzzing or ringing, piezo effects.
Table 1. Types of Ceramic Capacitors
Of the many capacitor types above, the most common in my experience are X5R, X7R, and Y5V. I never use the Y5Vs because of their extremely large capacitance variation over environmental conditions.
When capacitor companies develop products, they choose materials with characteristics that will enable the capacitors to operate within the specified variation (3rd Character) over the specified temperature range (1st and 2nd character). The X7R capacitors that I was using should not vary more than ±15% over a temperature range of -55ºC to +125ºC. OK, so either I had a bad batch of capacitors or something else was happening in my circuit.