How to select microswitch 1

[jyjy]

Hi all,

I have to select several microswitches of single pole double throw type for our control lever box. The switches I could get from my supplier has seperated inductive and resistive current maximum, the switch1 has 1A resistive and 0.5A inductive gold contact and switch2 has 7A res/4A ind gold overlayed silver contact.
Six switches will be used in the box and two of them will be switched to loads like solenoid and valve(inductive?)with 28VDC, and the other ones used as ground/open switch(discrete status to microprocessor).
So what I am a little confused is why are two kind of current given, from my understanding it´s no difference whether a resistive load or a inductive load flow through the switch? I only need to know the overall maximum current from the both, am I correct?
Another point is what is the difference between the two kind of switche contact? gold overlay sounds not so good under high voltage test e.g. 80V for 100ms.

Thanks!

 

[VEBill]

The problem with an inductive load is that when you open the contacts and try to interrupt the current there will be an arc.

I'm not an expert on the subject of contacts, but I read somewhere that the gold overlay layer can be destroyed if you test it using high current levels. It you plan to use the switch with high currents then it might be okay anyway. The gold layer makes it two switches in one, but once you use high current there is no going back. Await other's confirmation...

 

[ScottyUK]

Inductive circuits by their nature have a current which tries to keep flowing, generating a high potential across the opening switch contacts. On DC the arc will be persistent and relatively difficult to extinguish. The derating takes in to account the problem switching inductive loads.

Gold is a good choice for low level switching. There is a parallel discussion in another thread which is worth reading for some explanation.


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One day my ship will come in.
But with my luck, I'll be at the airport!

 

[Noway2]

Depending on your application, if you are switching an inductive load such as a solenoid or relay coil with DC, you may want to consider adding a flyback diode to provide a discharge path for when the coil de-energizes. The device(s) should be placed as close to the coil as possible. With a diode, it should be placed in reverse bias to the coil power supply. As ScottyUK said, the interruption of current can generate high potential voltages, which can damage electronic components. The diode acts as a voltage clamp and provides a bypass for the energy stored in the coil to disipate.

I worked on one circuit that was so sensitive to the inductive kickback from the annunciator relays that it was necessary to also place an RC snubber across the relay. I understand that this is fairly common in higher voltage applications, but this was on a little 12V relay.

I also place an RC snubber across AC driven coils to supress the arc, which varies according to what part of the cylce you managed to catch it in. Just make sure that the voltage of the capacitor is large enough and that that you use a sufficent wattage resistor.

 

[itsmoked]

I think that if you are switching amps.. at voltages over 12V that some silver alloys are "tougher" and have longer life and durability.

Little chart down the page sorta shows what I mean.

http://www.hsiswitches.com/compare2.htm

Each manufacturer has its own recommendations for what contacts should be used in each case. They must have this info because it is of primary interest to most users. They need a standard document to provide to their sales teams.

If you are going to use the classic Honeywell Micro switch ask them this question.. They know the answer.

 

[BrianR]

Generally switch ratings for inductive loads are 1/2 to 2/3 the resistive load rating. For low power switching as your application seems to be then gold over silver gives the best performance and life. Silver has the lowest electrical resistance but is sensitive to many contaminants so a flash of gold is used to give it protection.

For interest, when small signals are to be switched then gold plated contacts are needed and when mV signals are switched then mercury wetted contacts are best.

 

[Warpspeed]

All the above are good points.

Switches and relays can be used in many different types of circuits, and the modes of failure will be different under different operating conditions. Most circuits can be broken down into three types, ac, dc, and low level signal circuits. Different types of contact material can be more reliable in different circumstances.

Gold (or mercury) is always the preferred material for low level millivolt ac or dc signals. Heat damage from arcing or contact erosion is not an issue when switching for example, audio signals. But dirt buildup or contamination on the contact faces sure might be. Gold will not oxidise and it is a very soft malleable material. any microscopic particles will sink into the gold, the gold will flow. So gold contacts are ideal for high reliability in very low level circuits.

Using gold plated contacts to switch high voltages or currents is not a good idea, because a few arcs and sparks will vaporise the thin gold plating fairly quickly.

Whatever the base material under the gold will then be what does the actual switching. That is often perfectly o/k.

Try and contact the applications engineer of the company supplying the microswitches, and tell him exactly what you are doing. This is a very specialist area.

 

[Skogsgurra]

A side note: Mercury WAS a very good material for low-level applications. But is it still allowed?

Gunnar Englund

http://www.gke.org

 

[zeitghost]

I seem to recall that there was a preferred orientation for mercury switches.

 

[Skogsgurra]

Yes, those with an Hg pool needed to be mounted in a prescribed direction. There were some with "wetted" contacts (capillary action) and they worked in any direction.

Gunnar Englund

http://www.gke.org

 

[zeitghost]

The mercury switches in some semiconductor test equipment I used 20 odd years ago were never called on to switch appreciable currents but made contact while the power was off, following which the power supplies were energised.

This extended the life of the relay apparently, probably quite important when making many tests per second.