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May 1967 Electronics World[Table of Contents] People old and young enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World was published from May 1959 through December 1971. All copyrights are hereby acknowledged. See all Electronics World articles.
Capacitor stability at bargain prices
Any capacitor changes its microfarad value when temperature varies. And some capacitors change more than others. In some circuits, capacitance drift with temperature can cause real problems.
Look at circuits where you have fractional microfarad values of paper, film, ceramic or mica capacitors. During warm-up from room temperature to 65° C ambient, a capacitor with a temperature coefficient of, for example, 500 parts per million per degree C will increase capacitance value by 2%. This change is enough to cause troublesome drift in tuned circuits, where inductance also increases with temperature. It can knock the accuracy of a timing circuit off, or mess up the performance of a differentiator network. For these applications, we have a new kind of capacitor that beats anything we've seen in the stability race. It's the new Mallory Polystyrene Capacitor. They're made of stretched polystyrene film and high purity aluminum foil. The assembly is fused into one piece, with the polystyrene forming a solid case of clear plastic that you can look through and see the foil. Their temperature coefficient is less than 150 parts per million per degree C, which is about half that of polyester film capacitors. And the coefficient is negative; capacitance goes down when temperature goes up, compensating for the upward drift of inductance elements in tuned circuits.