Looking for Cross Reference Charts

Discussion in 'Electronics Resources' started by Charlie C, Apr 26, 2006.

  1. Charlie C

    Thread Starter New Member

    Apr 26, 2006
    3
    0
    Hi, I am looking for some cross reference charts for current capacity

    example

    2.5A @ 125v AC / 1.25A @ 250v AC / 350mA @ 125v DC / ???@ 12v DC


    I am trying to assess some AC design switches for 12v automotive use.

    If you have a magic formula to do the calculations, that will do just as well.

    Thanks, Charlie
     
  2. Papabravo

    Expert

    Feb 24, 2006
    10,152
    1,794
    I don't think it is as simple as that. Your calculation suggests that current times voltage equals a constant. In the AC example the constant is 312.5, and in the DC example it appears to be 43.75 which would make the unknown quantity 3.6459 Amps. Over a small range of values this may appear to be a linear relationship but it is not. It is hyperbolic.

    Switches are rated according to their contact material and their insulation properties. Using your logic the AC switch could be applied at 1 kV and 312 mA, or 10 kV and 31.2 mA, or 100 kV and 3.12 mA. This would be madness since spectacular arcing might result as the switch is opened and closed.

    I don't think I would use a swtich at a higer voltage than it was rated for. I might use an AC rated switch in a DC application, but that's about it.
     
  3. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    A better way to start would be to know what you want to switch and how much the maximum current will be. Then you can look for a switch that will do it. Your AC switches may be perfect but you haven't given us enough info.
     
  4. Grant

    Member

    Mar 5, 2006
    17
    0
    Hi Charlie,
    Switches made for both AC and DC are de-rated for use on DC because of the arcing that can occur. DC switchgear is in general much larger physically than AC switchgear for the same electrical capacities (think of the old large knife switches). I would be very wary of using an AC switch in a DC application.
    Cheers,
    Grant
     
  5. Charlie C

    Thread Starter New Member

    Apr 26, 2006
    3
    0
    I am looking at using these switches in a 12v DC setting, near as I can work out the amperage draw I will be working with is around 2 to 2.5 amps at startup settling back to about 1.8 amps when running.

    When reading the spec sheets on several switches, a few have noted the workable amp ratings at under 30v DC, which are usually about the same amp rating as at 125v Ac.

    Most are showing ratings for 125vDC but not any ratings for a lower voltage and I was trying to find some kind of a relationship between the higer and lower voltage situations
     
  6. windoze killa

    AAC Fanatic!

    Feb 23, 2006
    605
    24
    With such a low current I would think any switch that is rated at 2.5A would be fine. It doesn't really matter about the voltage part of the rating to much because you are only using 12V it will be well within the rating of the switch. The voltage rating of a switch is mainly there as an indication of the safe working voltage to prevent arching. The current rating of the switch is the current carrying capability of the contacts. Having said that there will be some interaction between the current and the voltage but at the levels you are wanting this would be minimal.
     
  7. Charlie C

    Thread Starter New Member

    Apr 26, 2006
    3
    0
    Thanks to all for the feedback.
     
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