Switches
Figure 2. Contact resistance against current. Below the wetting current the resistance is high and erratic; above it, the contact self-cleans and settles to a low, stable value.
A gold plating is thin, often a fraction of a micron, and it wears. Under any real arcing it erodes fast, so a gold contact used above its element loses its plating and bares the base metal, after which it is worse than the silver it replaced. Gold also cold-welds more readily than silver at higher forces and currents, which can stick a switch closed. The rule that follows is easy to state and often got wrong: gold for dry and low-level circuits, silver or silver alloys for real loads, and a clear decision about which regime the switch is in rather than a hope that one contact will serve both. A switch that has to cross between the two, a small signal
most of the time and an occasional higher current, needs that duty spelled out to the supplier, because it falls in the hardest part of the range for any single material. Hard gold platings, alloyed for wear, buy some margin, but they do not change the rule.
The other help the design can give Contact material is the main lever, and not the only one. A higher contact force breaks through a surface film more readily, which is one reason a firmer switch can be more reliable on small signals than a very light one. A wiping action, where the contacts slide a little as they close, scrapes
the interface clean and is built into many switches for that purpose. A bifurcated contact, which touches in two independent places at once, gives the signal a second path, so that a speck of film on one point does not open the circuit; it is common in switches and relays meant for low-level work. Sealing helps from the other side, by keeping out the dust, moisture, and corrosive vapours that build the film, so a sealed switch holds a low resistance longer in a dirty environment than an open one. None of these replaces getting the material and the wetting current right, but together they often decide whether a marginal design is reliable.
Specifying for the signal Choosing a switch for a small signal starts from the signal, not the load. Establish the real voltage and current the contact will carry at its lowest, rather than the supply rating around it. If that current sits near or below the switch’s stated minimum, treat it as a dry circuit and specify a gold or gold- plated contact. Where the part must also handle an occasional real load, describe both duties and expect a compromise. Give the contact what mechanical help the design allows: adequate force, a wiping action, a bifurcated contact where the signal is very low, and sealing against the environment. And if a data sheet states no minimum current at all, ask for one, because its absence is itself the warning.
Figure 3. Choosing the contact by where the signal sits. Below the wetting current a dry circuit needs gold; a real load that arcs wants silver or a silver alloy; a part that does both has to be specified for both.
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The signal, not the switch A switch that fails intermittently in the field is usually behaving exactly as its physics dictates. It was asked to carry a signal too small to keep its own contacts clean, and it was tested at a current that hid the fault. The remedy is the right contact for the signal in particular: gold where the current is too low to keep silver clean, silver where the load is real, and a data sheet read from its minimum with the same care as its maximum. The dry circuit is not an exotic failure mode. It is an ordinary one, and it is designed out at the moment the switch is chosen, long before a unit is ever returned.
https://www.swiclick.com/ Components in Electronics October 2026 35
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