A technician working on a rack of NIM instrumentation for a coincidence-counting…
PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Circuit foundations and passive networks · Semiconductor devices and analogue circuits
Inside the Instrument, Ch References, regulators and switching converters
Problem
A technician working on a rack of NIM instrumentation for a coincidence-counting experiment traces a 50Hz hum on an oscilloscope trace to a ground loop between two chassis fed from different mains sockets. Reasoning that "the protective earth is just another wire duplicating the signal return, so removing it breaks the loop", the technician unscrews and caps off the green/yellow earth conductor inside one instrument's IEC mains inlet. The hum disappears. Pleased with this, the technician also notices that a nearby low-dropout regulator module runs warm with its original 10μF tantalum output capacitor (RESR=0.3Ω), and replaces it with a 10μF ceramic capacitor (RESR=3mΩ), reasoning that "lower ESR always makes a supply better". Within a day the regulator is found oscillating at a few tens of kilohertz.
((a)) Identify the error in the mains-earth modification and state the hazard it introduces.
((b)) State how the hum should have been removed instead, referring to the return-current principle for ground loops.
((c)) Identify the error in the capacitor substitution and explain, in words, why a lower ESR made the regulator worse rather than better.