The Case of the Missing Signals

Norm Novotney, Shellitron Electronics

Several years ago I developed a sensitive signal processing front end for a rugged outdoor application. There were various amplifiers, conditioners, and a custom coupling toroidal transformer to provide ground loop isolation. The circuit was rigorously tested under severe environmental conditions including thermal cycling, vibration, ESD, etc.  It passed with flying colors.

In order to ensure supreme reliability, PCB board stiffeners were added to the design and
production transformers were additionally encapsulated in a plastic shell filled with a robust potting compound. Prototype field units were sent to Florida for further testing. All appeared well.

Just before I was to move on to another project, there were reports that the sensors would occasionally provide erratic results. It turned out that this only occurred on mornings when the temperature was near freezing. Having the units sent back, I was able to see for myself poor operation at low temperatures. Since extensive testing had been performed, this seemed quite mysterious.

A quick check clearly showed the sensor levels becoming extremely attenuated at the output of the transformers. What could be the cause? The original prototype transformers were installed and normal functionality resumed. It turns out that when very cold, the toroidal cores were being mechanically stressed by the glassy-hard potting compound–and this significantly degraded the transformer properties. In fact, a strong squeeze of the shell almost entirely eliminated the signal.  By simply switching from a rock-hard epoxy to a rubber-like RTV compound, the problem was “cured.”

Norm Novotney has spent over 25 years developing a wide variety of military, industrial, medical and consumer electronics products. He maintains an avid interest in minimizing product costs while maximizing efficiency and reliability. As a consultant for the past seven years, Norm has been counted on to solve the toughest problems with a solid dose of humor, candor, and hot soldering iron.