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Electrical switches principles and switching problems

Switches are important part in electronic systems. It is one of most frequently used human interaction with electronic devices methods. But switches are mechanical component that are vital part of any equipment.

Electrical switches

Electrical switches are as old as electricity. The function is always the same – it makes or breaks current in a circuit. In early 19th switches were used for DC circuits, while later for AC and then they serves for switching wide spectra signals starting from audio and ending with digital.

Well switches have changed comparing to those before 100 year, but principal is the same old as electric itself. When switch connects circuit path it has a resistance of mΩ, when current path is broken, then resistance is high MΩ and higher. This resistance and maximum voltage that can be applied to insulation is often major important and vital feature that leads to switch stability.

Contact resistance

Switch resistance is determined by switch contacts – the moving metal parts which touch when switch is ON. The amount of contact resistance depends on the area of contact, contact material, the amount of force that presses the contacts together and the way that force have been applied. For instance when contacts are wiped together then there is much lower resistance than when contacts are simply pushed together. Large area contacts are used for high current circuits while small contact area switches for low current – usually used in electronic devices. And plus the real true contact area is smaller than physical area of contacts, because it is impossible to have precisely flat contacts(you can check this in any ignition circuits where contact areas are burned.).

Contact resistance increase during time because of oxidation and contamination. The only way to deal with this is to wipe contacts. But disadvantage of wiping action is that this action may abrade contacts what can remove contact platings like nickel-alloy or gold.

Electro plating allows constructing contacts of bulk material which is mechanically suitable. Plating also reduces contact price comparing if used as a bulk material for whole switch. This way we can get contacts with wanted electrical and chemical properties. Choosing contact materials is never easy as those contacts that are low resistance are vulnerable to chemical attack by atmosphere, but some of them may provide contact sticking and may weld. Other problem is burning and oxidation. The spark current may melt contact and combine with oxygen in the atmosphere. Oxidation is rather more problem than melting, because oxides are non conductors and this cause to rise of resistance and degradation of switch. Lets see several contact materials used in switches:

  • Silver – It has low resistance but it corrodes. It may be used in High current and high pressure switches;

  • Palladium-Silver – Less contaminated but has higher resistance. Suitable for general use;

  • Silver-Nickel – resistive to burning and sticking but has higher resistance. Suitable for general use;

  • Tungsten – Hard metal and has high melting point but oxidizes easily. Used for high power switching;

  • Platinum – Stable metal and resists chemical attack, but can be used for high voltage and low current. Used for special purposes;

  • Gold – Resistive to corrosion and can be easily plated in other metals but can be used for low current switches and is widely used in electronics.

So we can see that silver has the lowest resistivity of all metals, but silver is badly corroded by the atmosphere, so silver contacts have a short life. If there is a way to exclude atmosphere contamination, then in high current circuits silver contacts or silver coating are desirable. Usually silver contacts are placed in closed coating with inert gases.

In other hand Golden plating is very common solution because of its resistivity to corrosion. But golden contacts can only be used in relatively low current circuits and moderate pressure can be applied because of metal softness. Of course gold contacts aren’t immune ro corrosion. For instance sea-water may produce chlorine when current passes the water. Golden contacts can be corroded with chlorine. So at sea golden switches are not the best choice unless circuits are isolated from sea water.

Of course there are other metals used for contacts like molybdenum, but they are used for special purposes where special properties are needed like high resistance to burning where contact arcing occur.

One Comment:

  1. Need some graphics!

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