Relay Driver Circuit Design A relay driver is a small circuit with a large job, because the coil and the contacts have very different requirements. The coil needs a defined current and a path for the energy it stores, and the contacts have to switch a load whose nature may…

PCB Cleaning: Aqueous And Solvent Processes Cleaning a printed circuit assembly is a process decision rather than a cosmetic one, and it is made for three reasons: to remove a flux residue that would otherwise be chemically active, to remove ionic contamination that would otherwise promote leakage and corrosion, and…

Inrush Current Limiting Circuit Design When a power supply is connected to the mains, the bulk capacitor is empty and it looks like a short circuit for the first few milliseconds. The resulting inrush current is limited only by the wiring, and it can be tens of times the operating…

Coulomb Counting: Design Rules and Process Limits A coulomb counting gauge measures the current into and out of a battery and integrates it over time. The principle is simple and the practical difficulty is that every small error accumulates without limit, so the offset of the measurement is the parameter…

ENIG Black Pad: Causes, Detection and Prevention An electroless nickel immersion gold finish protects the copper and it presents a flat, solderable surface. It also introduces a corrosion cell at the nickel surface, and the failure it produces is hard to see. How the Finish Works The nickel is deposited…

Optical Encoder Signal Conditioning Design An optical encoder produces two square waves in quadrature and often a third index pulse once per revolution. The signal conditioning circuit turns those into counts, and its quality decides the resolution, the immunity to noise and the maximum speed of the axis. The Outputs…

Reflow Profile Setup And Thermocouple Measurement The numbers displayed on a reflow oven are not the profile. They are the temperatures of the heating zones and the speed of the conveyor, and the profile is what the board actually experiences as it passes through them. The difference between the two…

Process Qualification and Validation for a New Assembly Line A new line, a new machine or a new process is qualified before it is used for production. Qualification is the evidence that the process produces what the drawing specifies, and it is often reduced to a run of boards that…

RF Shield Can Design And Attachment A shield can is a metal cover placed over a circuit that would otherwise radiate or receive energy. It is also a mechanical part: it has to be attached to the board, to survive the assembly process, and to allow the components underneath to…

Bridge Sensor Excitation Circuit Design A bridge sensor turns a physical quantity into a small differential voltage, and that voltage is proportional to the excitation applied to the bridge. The excitation circuit therefore sets both the sensitivity of the measurement and the error caused by self heating. Why the Bridge…