Carbon composition resistors
Some really old history tells an instructive tale.
When I worked at Bertan High Voltage through the 1980s we used 100 K, ½ W carbon composition resistors as part of the RC high voltage filters in various modules delivering up through 7500 V (Figure 1). Then one day in the mid-1980s, those high voltage filters started to fail.
Figure 1 The 100K carbon composition resistors used as part of RC high voltage filters in modules delivering up to 7500 V.
Examination revealed that we had gone from using resistors once obtained from Allen-Bradley to other resistors then being made by IRC. It turned out that the Allen-Bradley parts were of true carbon composition structure with their resistive elements made as cylindrical cores of material while the IRC parts were actually film resistors that were acceptable per the published carbon resistor specification sheets, but troublesome for our own purposes.
Both vendor’s parts were sold as being compliant to the military specifications for RC07 resistors and indeed they were both compliant to that. However, the RC07 requirement only called for a voltage withstanding capability to 250 V and our application required those resistors to withstand impulses up into the multi-kilovolt range.
The Allen-Bradley parts could do it and there was even an article on the subject which Allen Bradley had once published, but the IRC film resistors could not do that.
Unfortunately, as we next found out, Allen-Bradley had discontinued their product line. Our purchasing department had therefore gone ahead to IRC in the belief that the two vendors’ parts were equivalent. Unfortunately, for our purposes, they weren’t.
I asked a sales rep why this had happened. His explanation was that Allen-Bradley could no longer obtain sufficient quantities of raw materials to make the cylindrical cores. The core materials had in large part been composed of slag left over from steel mill production and the steel industry was then in a severe economic slump. They weren’t making enough steel and generating enough slag to satisfy Allen Bradley’s resistor production requirements.
Was that a folk tale? I never confirmed the story independently, but that’s what I was told.
Also, it turned out that Allen-Bradley had been at the time the only resistor manufacturer still using the cylindrical core configuration and there was nobody else to turn to. We had to change from using carbon composition resistors to specially designed resistors, then made by Airco-Speer if my memory serves, which were actually rated for high voltage service.
Leaving the 1980s behind, this article in Figure 2 came along from Stackpole in EDN in 2004.
Figure 2 “Carbon-comp resistor takes you forward into the past” article by author Bill Schweber published back in 2004.
By then, I was working at a different company, so I never re-examined the high voltage issue, but just the other day, strictly on a whim, I took another look at Stackpole and found article in Figure 3.
Figure 3 Recent viewing of Stackpole showing the carbon composition resistor series as discontinued.
I guess the era really has ended.
John Dunn is an electronics consultant, and a graduate of The Polytechnic Institute of Brooklyn (BSEE) and of New York University (MSEE).
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