Electromechanical devices, including switches and elements such as indicators and alarms, have undergone major changes over the years to serve specialized HMI panel needs, report Joseph Torzillo, EAO vice president of sales, HMI components; and Lance Scott, EAO president, the authors of a new white paper from EAO Switch.
Further, switch designers have to contend with contact arcing, welding and bounce. "Arcing is a discharge of electricity — a spark — that can occur when contacts make or break," write the authors. "Welding occurs when contact material melts and fuses, causing contacts to stick. Arcing and welding degrades or burns contacts, reducing useful life, and are more severe in dc than ac applications. Solid-gold contacts are more easily melted and eroded by arcing, so they are limited to low-current switching where there is little or no arcing. As switched current increases, hotter arcs form and the potential for erosion and contact welding is greater. Arcing is more severe when handling high inrush currents, i.e., inductive and capacitive loads that require high initial current. Lamps may draw 10–12 times normal operating current when first activated. Relays, solenoids and motors may show high-inductive inrush currents when powering up. Minor periodic arcing with silver contacts is actually beneficial because it keeps contacts free from dirt and corrosion."
EAO says that another challenge in switch design is contact "bounce" or "chatter," a condition in which a contact rebounds for several milliseconds before it finally closes. "This is not a concern for power circuits, but causes problems in logic circuits that may interpret on-off bounces as data streams."
Read the full paper: "Key Components Enhance the Human Machine Interface."
