How to choose swiching current?

[JimboJones21]

I have to select a mechanical switch to switch 4 rs485 lines. I see that the short circuit current for a rs485 transceiver is 250mA, so I will need a switch for 1A continuous worst case.
But how do you figure what the switching current for the switch should be?

 

[IRstuff]

Huh? What exactly are you asking?

0.25A should be more than good enough, unless you've got some odd circuit problems. The Ios shouldn't be occurring on a regular basis, so there should no need to design for that with that much margin.



TTFN

 

[JimboJones21]

With a mechnical switch there is the continuous current rating and switching current rating. I can see how to find the continuous current by just basically looking at your load, but how do you figure the switching current rating. So if you had 250mA flowing through the switch and you turn the knob how do you what the rating should be to handle 250mA being disconnected?

 

[itsmoked]

The continuous rating refers to the heating the switch will get based on the contact resistance. The switching value refers to the current the switch can tolerate switching 100,000 times or whatever the switch life has. If you increase the switch load the switch will still interrupt (within reason) but will have a shorter life.

Now taking about your case.... I doubt there are any switches that aren't rated for at least half an amp on the planet!

More importantly, you need a switch with gold contacts for your application.

 

[bill318]

I agree with itsmoked, you need to be more concerned with the minimum current the switch can keep its contacts clean with and gold would be the way to go with such a small signal current.

 

[JimboJones21]

Thanks for the responses. I have a Lorlin sample and Grayhill sample. They're both PCB mount. The Lorlin one is flimsy compared to the Grayhill switch. But the Lorlin has a contact rating of 500mA @ 250V and the Grayhill 250mA @ 115V. The Lorlin switch is $12 and the Grayhill is $45? I'm surprised because I thought that Grayhill would have a higher rating. I guess the extra cost is for military spec?

http://www.lorlin.co.uk/m.htm

 

[itsmoked]

Man those prices are about 6X of what I'd expect to pay to switch "485"

 

[IRstuff]

That's because he's buying relays that can switch 115 Vac.

TTFN