Neon lamp blunder
There was this test system that comprised a huge row of equipment racks into which various items of test equipment would be mounted. Those items were a digital multimeter, an oscilloscope, several signal generators and so forth. Each section of the rack assembly had a neon lamp mounted at its base which was supposed to indicate that 400 Hz AC line voltage was turned on or turned off for the equipment mounted in that rack section.
Planned essentially as follows in Figure 1, the idea did not work.
Figure 1 Neon lamp indicator plan where line voltage was always present and applied to the equipment installed within each section via a power relay where singular SPST contact set operated that section’s neon lamp.
Line voltage was always present but would be applied to installed equipment within each section via a power relay of which one SPST contact set was to operate that section’s neon lamp. The problem was that each section’s neon lamp would always stay lit, no matter the state of the relay and the state of equipment power application.
No neon lamp would ever go dark.
There was much ado about this with all kinds of accusations and posturing, finger pointing, scoldings, searching for a fall guy and so forth but the problem itself was never solved. What had been overlooked is shown as follows in Figure 2.
Figure 2 The culprit was the stray capacitance from the wiring harness that each SPST contact was wired through that kept each neon lamp visibly lit.
Each SPST contact was wired through a harness which imposed a stray capacitance across the contacts of the intended switch. When the SPST was set to be open, that stray capacitance provided a low enough impedance for AC current to flow anyway and that current level was sufficient to keep the neon lamp visibly lit.
Brilliant, huh?
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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