Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Idiot with a simple question

  1. Mar 12, 2010 #1
    I need to replace a switch that has printed on it 3A 250VAC or 6A 125VAC. Is it alright to replace it with a switch rated 3A 125VAC 1.5A 250VAC ? Thank you.
     
  2. jcsd
  3. Mar 12, 2010 #2
    Yes, no, maybe?

    Those numbers are telling you how much power it can handle. The new switch can only handle half the power of the old switch. My prediction: it will work but the new switch will not last very long as it will heat up too much and wear out. It all depends on how much current the load will draw. Since the old switch is kaput, it's safe to guess that the old switch, which was twice as rugged, was overdrawn too.
     
  4. Mar 12, 2010 #3

    berkeman

    User Avatar

    Staff: Mentor

    Where is the switch used? Why are you replacing it?
     
  5. Mar 12, 2010 #4
    It is for a FM modulator to hook up an ipod to my car. I attatched a photo of the package that illustrates whats going on here. The switch labeled switch is what I want to replace(the switch that came with it isn't round like pictured). The reason I am replacing the switch is because it is a rectangle shaped one and I wanted to get a round one because it would be easier to mount it into where I am putting it (just drilling a round hole instead of making a rectangle hole). I had this switch laying and was wondering if it could be used here. So I guess I need to know what ratings to look for to replace this switch.
     

    Attached Files:

    Last edited: Mar 12, 2010
  6. Mar 12, 2010 #5

    berkeman

    User Avatar

    Staff: Mentor

    Ah, that helps a lot. Since it's a low-voltage application, just be sure the switch is rated for the current that it will pass.
     
  7. Mar 12, 2010 #6
    Strangely enough mains switches don't last very long in cars.

    You should really use a DC rated switch - you will find these at a proper auto store.

    This is because the requirements for contacts etc are totally different for DC switches, life is actually tougher for them.
     
  8. Mar 12, 2010 #7

    berkeman

    User Avatar

    Staff: Mentor

    No kidding -- I didn't know that. Is there some place I can read more about the differences? Thanks.
     
  9. Mar 12, 2010 #8

    vk6kro

    User Avatar
    Science Advisor

    This device will only use less than 50 mA, so neither switch would be used to its maximum ratings.
    The new switch will work perfectly.
     
  10. Mar 12, 2010 #9
    Seeing your strapline I would suggest looking in one of the weighty tomes produced by the ARRL on components.
    I think the RSGB also does something similar, I will try to look it up.

    Google sometimes comes up with all sales stuff rather than technical articles and I fear this is a case in point.
     
  11. Mar 12, 2010 #10
    perfect. Thanks alot!
     
  12. Mar 12, 2010 #11

    berkeman

    User Avatar

    Staff: Mentor

    Thanks, I'll do that when I get home tonight. :smile:
     
  13. Mar 12, 2010 #12

    vk6kro

    User Avatar
    Science Advisor

    Because an AC arc stops when the polarity reverses and a DC arc cannot do this, the separation of the contacts in a DC switch has to be greater than for an AC switch at the same operating voltage.

    However, this is a 250 volt switch being used at 12 volts and it is being used at less than 5% of its rated current, so there is no problem with using it.
     
  14. Mar 15, 2010 #13
    In a prior life, I was in the HVAC designing business, and we had two issues with relay contacts.

    One was the "forming" voltage. Essentially, you need a minimum amount of current and voltage to keep the contacts clean. The silver alloy contacts used in power relays will otherwise build up a layer of oxide and fail.

    The other issue was related to polarity. Our microcontrollers were programmed to close / open the relays independent of line phase. Otherwise, continual operation at one point of the line phase can prematurely erode the contacts.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook