1. Resistors: Small Components, Big Impact
During a customer site visit, one of our experienced engineers received a report that “when a new scale is assembled with A&D load cells and indicators, the indicator value fluctuates by several tens of digits within an hour.”
The engineer conducted a comprehensive on-site investigation, carefully inspecting for potential issues such as loose screws, inadequately bonded terminals, contaminated PCBs, suboptimal load cell cable or terminal block materials, and improper shielding. Despite this diligent and systematic approach, the underlying cause could not be determined before leaving the site.
However, our diligent engineer considered that the only remaining possibility was the summing box the customer had assembled. He requested permission to examine the PCB inside the summing box.*
Upon returning, he presented the board for review. At first glance, the terminal block—fabricated from PBT resin renowned for its excellent insulation—and an impressive, glossy black resistor appeared problem-free. Nonetheless, as a precaution, I brought a magnet close to the resistor’s lead wire. The magnet was immediately drawn in and snapped onto the lead, strongly suggesting that it contained iron.
Iron, while cost-effective and durable, readily generates thermoelectric force when paired with copper, making it unsuitable for circuits managing microvolt signals. In this situation, the iron-containing resistor lead essentially functioned as a thermocouple*, transforming minor temperature fluctuations into voltage, which then caused drift in the indicator output.
Further investigation showed the resistor was engineered for heat and impact resistance, employing a copper wire with an iron core for the leads. We resolved the issue by recommending resistors with minimal thermoelectric effect. This experience underscored the critical importance of careful resistor selection in precision measurement systems.
*Summing box: A summing box combines the wiring from each load cell when constructing a scale with multiple load cells. Typically, buffering resistors are installed to ensure that the output of one load cell does not interfere with the others. These resistors are connected to the output terminals (SIG+, SIG-) of each load cell and must have a low temperature coefficient and minimal measurement error.
*Thermocouple: A thermocouple is a device that converts a temperature difference into a voltage, typically formed by joining two dissimilar metals. Various types are available to suit different temperature ranges and application needs.
2. Please, Just Ground It!
We began an investigation following a report from a customer in Southeast Asia who was experiencing frequent failures of weighing indicators, with some units malfunctioning after just two weeks of operation, despite multiple replacements.
Notably, the model in question is one of our long-standing, reliable products, making such rapid failures highly unusual. Location-specific factors are often the root cause in these scenarios. Given that the site was overseas and not readily accessible, we requested a wiring diagram from the customer.
What arrived was a challenging-to-decipher, worn-out scan—entirely in Japanese—dating back around 20 years to when the scale was originally assembled and exported by a Japanese manufacturer. Upon closer inspection, I noticed the notation “Power cord 2P,” indicating an AC connection using only two prongs, without a ground terminal. This quickly clarified the likely issue.
While a weighing indicator can technically operate without a ground pin, and many consumer electronics use two-prong power cords, weighing indicators differ. They are often installed with multiple, extended cables, sometimes running outdoors. If such a system lacks proper grounding, it becomes vulnerable to voltage surges from lightning strikes or static electricity, which can travel through the internal circuitry and cause significant damage.
Although two-prong outlets are common in Japanese homes, three-prong outlets with grounding are standard in offices and factories. Unfortunately, grounding is frequently overlooked because devices appear to function normally without it.
I strongly encourage anyone using precision equipment, including desktop computers, to ensure all devices are properly grounded for safe and reliable operation.