# Help needed with some homework.



## Mstrkurt (Jul 2, 2009)

Hey guys, i'm in need of some help as i've encountered some problems. I have researched the question but i've been unable to find a suitable answer.

Q: Why is the a.c current rating for a switch, higher than the d.c rating?

Can someone explain how, or why it is?


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## octaneman (May 13, 2009)

Hi Mstrkurt


The conversion of AC to DC is calculated by finding its DC equivalent for example:
To calculate the DC from AC you use the following equation: Sqrt (2) * RMS voltage.

You will have to calculate the peak voltage by multiplying 1.4 (square root of 2) , so for example 110V RMS will have a peak voltage of 155v. To get it's DC equivalent you divide by 1.4.


You can find the Vdc by using this equation : Vdc= AC voltage / pi . So to calculate 110 Vac to Dc its : 110 / 3.1416 = 35Vdc. 

Other calculations for current and power are done using Ohms law. Keep in mind that in the real world some of the DC will be lost because of filtering and rectification. 

Manufacturers also take into account the materials needed to make the switch. Calculating the amount of surface contact, heat, insulation, arcing between points and most important part of it all the cost to build. All these factors play heavily into the design of the switch needed to do a particular job. 

Hope this helps.


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## Done_Fishin (Oct 10, 2006)

ac voltage passes from a positive or negative peak through zero. DC doesn't. its stable (within certain fluctuations).

when a switch, that is under voltage, opens the tendency is for the circuit to try to maintain a circuit, so a spark occurs as the switch starts to open and will feed current through the air to maintain a feed to the load. This continues until the distance between the contacts is so great that the circuit cannot be fed any more. In order to feed that spark the voltage actually increases. 
In an AC circuit this is not so easy to happen because of the zero crossing which limits the time and available voltage to arc across the opening contact.

If the contacts get too hot due to the arcing effect, they will burn out. So manufacturers work out the safe voltage that the contacts can withstand when opening before damage occurs.
Don't forget also that when making contact there is also a "bounce" effect which again causes arcing.

I suggest you check out spark suppression in switches .. using capacitors to quench the spark.

EDIT 

RA EMC Awareness - Mitigation - suppressing arcs & sparks


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## Mstrkurt (Jul 2, 2009)

Thank you both very much, that has helped out a lot


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