Switches


Fine on the bench, intermittent in the product: dry circuits, wetting current, and when a switch really needs gold


A switch that passes every test on the bench can turn intermittent once it is carrying a real signal. The reason is the dry circuit, and it decides when a contact needs gold rather than silver. Liu Junxiong, R&D engineer at Swiclick, takes a closer look


A


switch is one of the few components a designer trusts on sight. It closes a circuit, the data sheet promises a hundred thousand operations,


and attention moves on. Then a unit comes back from the field with a fault that will not repeat on the bench: a control that works at the desk and fails in the product, or one that reads open for no reason, then reads closed again when prodded. The switch measures fine every time it is checked. The fault is not in the switch. It is in the size of the signal it carries, and it has a name worth knowing before it reaches a customer. It is the dry circuit.


Why a good contact goes open A switch contact is not the clean metal-to- metal junction the schematic implies. Left in air, a silver or silver-alloy contact grows a thin film across its face: an oxide, a sulphide, an adsorbed layer of moisture and airborne organics, a skin of dust. That film is a poor conductor and often an insulator. In ordinary switching it is never noticed, because the film is dealt with by the very things a designer is usually taught to suppress. A few volts across the closing contact can break the film down electrically, a mechanism known as fritting, and where the voltage and current are high enough, a small arc and the heat that comes with it burn the interface clean on every operation. A clean contact then reads a few tens of milliohms. The switch looks perfectly reliable because it keeps cleaning itself.


Take that energy away and the picture changes. The self-cleaning arc, for a silver contact, needs on the order of twelve volts to strike, so a logic rail at three or five volts never produces one, and at very low voltages even fritting may not break the film. When


34 October 2026


Figure 1. In ordinary switching a few volts and a real current break through the surface film, by fritting and a small cleaning arc, and the contact stays low. Below that level the film is not broken and the contact reads high or open.


the same switch carries only a couple of volts and a few milliamps or less, nothing removes it, and the contact can close mechanically yet still read as an unstable resistance of kilohms, or as an open circuit, because the signal must pass through a surface layer that nothing is clearing. This is a dry circuit, taken loosely as one working below roughly ten milliamps and a fraction of a volt, beneath the level at which a contact keeps itself clean. A switch that spends its life reading logic signals, sensor outputs, or a microcontroller input lives there, and it is exactly the kind of case that a bench check at a comfortable test current will pass.


Wetting current and the minimum the contact needs


The current that keeps a contact clean has a name: the wetting current. It is the minimum


Components in Electronics


a contact needs to break through its surface film and hold a low, stable resistance, and below it the contact is unreliable. The figure depends on the contact material and plating and on the environment; a silver contact commonly wants at least a few milliamps, and more in a dirty atmosphere. A switch meant for signal use will state a minimum current alongside its maximum rating, and that minimum is the number that matters for a dry circuit. It is also the one most easily missed, because a data sheet leads with the headline that a part switches several amps, not with the quiet line that it also needs a milliamp or two to be trusted.


The mistake this sets up is common. An engineer picks a heavy-duty switch rated for a power load, uses it to read a logic signal at a fraction of a milliamp, and cannot see why a part rated so far above the duty behaves


so badly. A contact built to interrupt a real current, with a hard, arc-resistant surface, is often the worst choice for a dry circuit, because that surface does nothing to help a tiny signal through its film. The switch has to match the signal, and the minimum current is where that match is decided.


When silver is not enough, and what gold costs


Where the signal is too small to keep silver clean, the answer is a contact that does not need cleaning. Gold is close to inert. It grows no oxide or sulphide film in normal air, so a gold or gold-plated contact stays conductive down to the smallest signals, which is why it is the standard choice for dry-circuit and low-level switching. The case is strong, but gold is not a default, and it brings its own limits.


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