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DC and AC rating current for micro-switch

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mawad

New Member
Hello,
I have a question regarding to the rated current for a AC and DC current for micro-switch. I found in some micro switches that the rated current when apply DC voltage is less than the AC voltage. Such as this example: **broken link removed**
It is shown that the 1A 125VAC (UL); DC5V 30mA. I don't get why? I was thinking that the current should be the same for both AC and DC because of the voltage will apply on the same terminals. Could you help me to understand why there is a difference?
Best wishes,
Mohammed
 

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
It's not. In a very simple example. Take a slow moving switch. An arc can develop between the parts before they even mate eroding the contacts especially with DC. The AC voltage constantly changes and even goes through zero 60 x/second. Slide swiches in AC applications don't really cut it. They are used in ceiling fans to set the direction, but the direction needs to be changed when the fan is stopped.

An AC light switch is snap-acting and so are contactors.

Another reason is that 120 V AC goes from -155 to 155 volts which increases the peak current.

At the low end of the scale, contacts have a typical "wetting current" of about 10 mA unless other materials are used or the contacts can renew themselves, so a slide contact might be more preferrable. Mercury wetted and RF applications are types of relays with special purposes. So are relays that can handle high voltage such as 40 kV.

HP (or horsepower) rated contacts are yet another wrinkle. Current carrying capacity and switching capacity is yet another. Voltage breakdown is yet another.
 

BrownOut

Banned
When the switch connect AC, the current is zero 120 times per second ( for 60Hz US systems ) or 100 times/sec ( for 50Hz ). When the switch is turned to "off", the current continues to arc across the contacts. But as the AC voltage is zero those many tmes a second, the arc is extinguished very quickly. However, DC voltage is never zero, so any arc across the contacts would remain forever, or until the switch burns up. Thus, the DC rating must be much lower to prevent perpetual arcing.
 
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