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a question about electrical switch ratings

Discussion in 'The Garage' started by KentMoney, Aug 27, 2011.

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  1. KentMoney

    KentMoney XS650 Addict

    Hey guys,

    I've tried searching for an answer but I am not finding it.

    I recently purchased a microswitch that I would like to use for my blinkers. The packaging states that it is good for 5A at 28V. What does this mean if I am using a 12V system like most motorcycles? Does this mean I can get about 10A at 12V?

    Basically, can I run the blinker circuit through this switch or do I need to use some sort of relay?

    Thanks!
    Kent
     
  2. DogBunny

    DogBunny Motorcychologist Top Contributor

    Bigger amperage is always better when it comes to motorcycles and switches.
    I have a lot of experience with microswitches on motorcycles. 5 amp, 28V DC, which is the same rating as 5 amp 125 V AC, is generally as high-rated a microswitch as you will find, and should be okay for your blinkers.
    I have found NKK brand switches which are made in Japan, not Taiwan, which are rated at 6 amps. I would try to find them. It's not that they are rated for an extra amp, what's more important is that they are a quality Japan-made switch.

    Use good soldering technique. Avoid over-heating the switch. Have an extra switch on hand in case you screw up.
     
  3. jammerj

    jammerj XS650 Member

    If you decrease the voltage, you increase the amperage, and vice/versa, if you were pushing 5amps @12volts, you'de be pushing 10 amps @ 6 volts, or 2.5@ 24volts.
     
  4. KentMoney

    KentMoney XS650 Addict

    right. so if it was rated at 5A at 28V, then I could easily get 10A into it at 12V. right?
     
  5. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    The formula is I=V/R
    I= current (the amperage)
    V = voltage
    R = resistance

    So you can see if V goes up, so does I, if there's no change in R.


    BUT- switches are a different kind of thing. The voltage rating is how well it suppresses arcing inside when the contacts open and close. The amps rating is how beefy the contacts are I suppose, like thin contacts would melt in a high current circuit that needed big wires.

    If the switch voltage rating is for AC, then you can use the the amp rating stated on the switch for up to 30VDC as a rule of thumb. Batteries are of course VDC (Volts, DC).

    If it's stated in DC on the switch, you can't exceed the voltage without arcing and you can't exceed the amps without a meltdown.
     
  6. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    In the switch rating, 5 amps is 5 amps you can't double the amperage because you halve the voltage you are using. like XJWMX sez
     
  7. DogBunny

    DogBunny Motorcychologist Top Contributor

    In my own words, what XJWMX said is that a switch's amperage rating is dependent on how well it is able to handle arcing at a given voltage.

    The arc created by opening or closing a contact that is carrying a 5 amp load at 12 volts DC will be less than the arc created by opening or closing a contact that is carrying a 5 amp load at 24 volts DC.

    Will a switch rated at 5 amps, 24 volts DC, be able to carry 10 amps at 12 volts DC? Maybe no, maybe yes, it depends on how the switch suppresses arcing, but a switch rated at 5 amps, 24 volts DC will certainly be able to carry more than 5 amps at 12 VDC.

    Back to the OP's original question:
    I have a lot of experience with microswitches on motorcycles. 5 amp, 28V DC, which is the same rating as 5 amp 125 V AC, is generally as high-rated a microswitch as you will find, and should be okay for your blinkers.
    I have been running headlight switches for a long time on more than one bike through 5 amp, 28 VDC microswitches, and a headlight draws more current than blinker lights.
     
    Last edited: Aug 28, 2011
  8. KentMoney

    KentMoney XS650 Addict

    OK, that's what I wanted to hear. I don't want to have to change these switches often if I use them for my blinkers.

    Thanks guys
     
  9. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    That's not what I said.

    When the mfgr makes a switch he puts a rating on it. If it's an AC rating it'll be good with DC at the AC rated amperage as long as you don;t exceed 30 V.

    If it's a DC rating and you use DC you can't cheat by moving one up and the other down. If it says 5A you can't exceed that, no matter what your voltage. Same with voltage. Could you fudge? Maybe some, but show me the formula for it. I wouldn't feel comfortable just guessing

    But the right switches are so common it doesn't have to be a question.
     
  10. DogBunny

    DogBunny Motorcychologist Top Contributor

    I'm just trying to understand. You said:
    If it's an AC rating it'll be good with DC at the AC rated amperage as long as you don;t exceed 30 V.

    So, if it's an AC rating, the rated voltage is irrelevant and the switch can handle 30 VDC at the rated amperage, no matter what the AC rated voltage?

    So, a switch rated at 5 amps, 1 VAC can handle 5 amps 30 VDC?
     
  11. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    The rule of thumb takes into account that there's no such thing :)

    I was curious about the op question myself, because I didn't know the answer and I didnt like that, so I just went to a switch mfgrs site and read their notes.
     
  12. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    Amps = 2 x (watt of turn signal bulb / 12)

    Look up the watts for the turn signal bulb and plug it in to the above. If Amps is 5 or less you can use your 5 A switch without worrying. Note the 2x because I assume the switch is runnning front and rear at the same time.


    I should add that there's some cushion built into electrical ratings, unless it says Absolute Maximum rating. Exceed a nominal rating a little with turn signals on your own bike, maybe okay. Don't try it on medical equipent....
     
    Last edited: Aug 28, 2011
  13. DogBunny

    DogBunny Motorcychologist Top Contributor

    The rule of thumb takes into account that there's no such thing

    Aw, c'mon, that's a cop-out. I am not into emoticons, but imagine a smiley face.

    You can't just say that an AC rated switch at a given amperage is good for the same amperage at 30 VDC without giving the voltage of the AC switch, especially by citing as your authority a "rule of thumb."

    I have before me a micro switch that is rated at 5 amp 28 VDC, 2 amp 250 VAC. PM me, and I would be happy to take a picture of the switch in it's package, with those ratings printed on the package, and to email the pic to you.
     
  14. Components are de-rated for DC operation, as there is no 0V crossing every 1/60th of a second like there is with AC. When the switch actuates, it is always at full voltage and amp draw, consequently, the arc is heavier and the contacts are far more stressed than they are in AC service.
     
  15. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    I don't want to be pm'ed picture of switches unless naked women are modeling them.

    Hey, it's not a cop out. The rule of thumb for dc is simply that you can use the ac rated amperage up to 30 volts dc. It takes into account that ac switches are line voltages, not something like 1vac.

    Also the rule of thumb for dc applies only if there's no separate dc rating on the switch, which there is on your example. In your case go by what it says on the switch. Hey, I only know what I read. I'm not a scientist.
     
  16. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    And the voltage spec just has to do with contact life. The relationship between the dc life and the ac life would be some sine function. My head hurts already,
     
  17. DogBunny

    DogBunny Motorcychologist Top Contributor

    I don't want to be pm'ed picture of switches unless naked women are modeling them.

    That could be arranged.

    Okay, I'm going to leave this be. But I'm not really happy with anyone's answers, nor with any of the research that I did on the 'net, or with the ratings that are on the various switches that are before me, or even with my own answers. I will say once again, that I use these switches as kill, start, horn and headlight switches, and that I have done so for a long time on two bikes, lots of other people do too, and they are adequate if you don't screw them up when you solder your connections.
     
  18. xjwmx

    xjwmx It's just the unknown. Top Contributor

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    Yes. But the key to it all, how to figure out what you need, how you might get along with less, and etc., is the following. I hope the op sees it.

    "Amps = 2 x (watt of turn signal bulb / 12)

    Look up the watts for the turn signal bulb and plug it in to the above. If Amps is 5 or less you can use your 5 A switch without worrying. Note the 2x because I assume the switch is runnning front and rear at the same time.

    I should add that there's some cushion built into electrical ratings, unless it says Absolute Maximum rating. Exceed a nominal rating a little with turn signals on your own bike, maybe okay. Don't try it on medical equipent.... ?"
     
  19. sgallaty

    sgallaty Antihero

    Also, if you marginally exceed a switches rated specs it will not fail right away - depending on how you exceed it.

    What generally happens is that the contacts arc, and the switch starts to introduce voltage drop via resistance and poor contact.

    This may not even be outwardly obvious, but your lamps will be dimmer.
     
  20. gggGary

    gggGary Stop that! XS650.com Supporter Top Contributor

    Is this horse dead yet? I don't care, I'm going to keep beating it.
    Yes the that switch will be fine for turn signal duty IF it's built by a good company. IE it meets it's own specs.
    Per the manual; turn signals = 27 watts X 2 plus the indicator bulb 3.4 watts or 58 watts / 12 volts = 5 amps IIRC there are some inrush currents on incandescent bulbs but for the very limited number of cycles that switch will see you should be fine. Your best guide is what Db said, he and others have used them, they work reliably. Moisture and then dirt are your enemies keep it clean and dry not wet or damp and it should serve fine for the life of an XS650.
     

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