EEVblog Electronics Community Forum
Electronics => Beginners => Topic started by: Stefan on September 20, 2013, 01:10:38 pm
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Its very embarrasing, but apparently I cannot even read datasheets of switches.
Example:
Contact Rating: 0.4 VA max. @ 20V AC or DC max.
Contact Resistance: 50mOhm @ 2-4V DC, 100 mA.
Assuming I'm only interested in DC, what does this imply?
I don't even understand why they use VA. After all, if a switch is closed, the potential over the contacts is near zero, and when the switch is open the current is zero. So VA is always very low. Also many switches have different ratings for DC and AC (without even specifying the frequency) how does that make sense? A switch might only need a fraction of the period to operate, so it will essentially see DC anyway.
Ok, I'm trying to be a bit more specific:
If the switch is already closed, what is the maximum current I can put through it?
If I open the switch, what is the maximum permissible voltage spike?
If I close the switch what is the maximum permissible surge current?
thanks,
Stefan
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I don't even understand why they use VA. After all, if a switch is closed, the potential over the contacts is near zero, and when the switch is open the current is zero.
The critical thing happens at the transition from closed to open. How much current can the switch safely interrupt without arcing or welding the contacts together? This will vary with voltage. The rating of 0.4VA says to take the open circuit voltage times the closed circuit current and multiply them together.
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Yes, the VA matters because when the switch is being opened or closed, it will dissipate a large amount of power for a short period of time, as the voltage/current need to transition from the loaded to unloaded values which can't happen instantaneously. The switch bounces a bit and if the power is too high, the contacts will weld.
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Also with AC when the switch breaks the arc will stop at the zero crossing point vs dc which won't so current rating will be lower for dc.