Departments

February 3, 1997

Signals & Noise

Design Idea suggestion

Richard Valentine's article, "Protection techniques ensure µC reliability in power-control circuits"(EDN, Oct 10, 1997, pg 171), mentions many aspects of, and gives good advice about, designing a µC system. Valentine has looked at nearly all possible faults. He has even implemented a relay to switch off the power line if the switching semiconductor fails.

But there is one possible failure that Valentine has forgotten, and it is a very relevant point. If the processor stops working and all regarding port lines are logical-high, then the motor will go out of control. It is very important to use a processor-generated ac signal, such as the watchdog pulse, to enable the motor-control signals respective to the relay-on stage. (See suggested circuitry in Figure 1's shaded box.)

Ing Hanns-Juergen Mostert
Moco Hardware Industries GmbH
Waldfeucht, West Germany

FIGURE 1

A simple solution for an annoying problem

EDN is and has for many years been the best electronics magazine. It is indispensable, and I love having it on the Web. I think the Web site is outstanding—the only quirk is the occasional tiny print. If I ran MS Explorer, it would be no problem; I just hit the size button to make it bigger. But Netscape requires many more keystrokes, so I have to grin and bear it. Take it easy on us older folks (the ones with tiny monitors and drugstore reading glasses).

I have a tip to pass on to EDN's Web users. If you notice funny things happening with the modem, such as slow browsing, clicking offline during a download, and even difficulty logging on, the problem may not be the modem. The tiny gold wires in the telephone connectors merely rest against each other; there is no clamping to make a solid connection.

Over time, dust and grime work their way in and make erratic connections. The equalization is extremely sensitive to noise and signal level, and the noise causes the system to repeat data transmissions, slowing the system. When it gets bad enough, the modem loses the carrier and clicks offline. The symptoms change from day to day, so it is hard to pin down the problem. Telephone connectors were never designed for data transfer. They may be OK for brief phone calls but not for the reliability a 10-Mbyte download demands.

The solution to this problem is simple. Dab a little white petroleum jelly on each connector (don't forget the ones in the basement!) and remove and insert the connector four or five times to clean off the dirt.

The jelly helps remove any surface oxide and dirt film, and it allows a true metal-to-metal contact. The contact resistance goes down by as much as a factor of 10 over an untreated contact and becomes very stable. This reduction is essential for the equalization to work. The jelly also forms a protective film against future contamination.

In 40 years, I have tried every commercial contact cleaner known to man. None have worked as well, or for as long a period of time, as plain white petroleum jelly. It works on any electrical connection, as long as the temperature is not so high that it burns the jelly.

Michael Monett
Anolog & Digital Design

ICs: older than EDN

I just read your "40th anniversary coverage"(EDN, Dec 19, 1996), and it was a lot of fun. I remembered most of that stuff (not bad, considering I'm only 40). However, there is one comment I wanted to pick on—I've been waiting for an opportunity to spring this one on people.

In his article (pg 85), Gary Legg states, "In 1956... even the most rudimentary, experimental IC was two years off." Actually, no. ICs had been around for several decades. What weren't around yet were solid-state ICs. On a shelf at home, I have some "hollow-state" (vacuum-tube) ICs, made by the German company Lowe, in the 1930s (I think). These ICs aren't the typical triode-pentode combination tubes. I call them "ICs" because they fit the definition: a complete circuit in a single package. Just plug it in, apply power, and you have a complete two-stage RF amp (Model 2HF) or three-stage AF amp (Model 3NF). The envelope contains both the active components and the passive ones (resistors and capacitors).

Of course, these ICs are something of a joke because they were dead-end technology. But they are interesting nonetheless.

Paul Koning
via the Internet