Better to Switch Ground or Power?

Discussion in 'General Electronics Chat' started by suprageezer, May 26, 2007.

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  1. suprageezer

    Thread Starter New Member

    May 26, 2007
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    Hello,
    I am going to wire a light bulb using a toggle/rocker switch in a 12VDC Circut. Is it better to switch the fused power wire or switch the ground. I have been told if I switch the ground the switch will last longer. Why is this?

    Thank You
    Rick
     
  2. cumesoftware

    Senior Member

    Apr 27, 2007
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    It makes no difference. But if you want to follow convention, switch the power line.

    Also, the switch will last the same, since the current crossing it will be the same (and even thought, the current is not that significant for the endurance of the switch, as long as the ratings are respected).
     
  3. subtech

    Senior Member

    Nov 21, 2006
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    It is a good idea to switch the + side any time you can whether working with
    AC or DC. For some, turning off a switch leads them to believe that everything downstream of the switch is de-energized. This would not be the
    case if you choose to switch the neutral or negative.
     
  4. recca02

    Senior Member

    Apr 2, 2007
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    putting switches on live trminal is good from safety point of view for
    small kids(or mentally a small kid) poking their hands/fingers in supply .
     
  5. suprageezer

    Thread Starter New Member

    May 26, 2007
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    This circut is for a set of fog lights, the reason I am asking is, I had been told in the past that once the current has passed thru the light, by switching ground the spark on the switch contacts is much smaller. Something very similar to this is when you work on a car battery, its safer to remove the negative terminal first since the spark is much smaller. Are there any engineers that could shed some light onot exactly why this is?, Aslo yes I know I should switch the lights with a relay, it's ground vs. power switching reasons I am hoping to learn about.

    Thank You
    Rick
     
  6. recca02

    Senior Member

    Apr 2, 2007
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    sorry didnt get the situation correct.
    the position of switch shudnt really matter as far as life is concerned.
     
  7. thingmaker3

    Retired Moderator

    May 16, 2005
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    Old wive's tale. Myth. The current is the same at any point in a series circuit.
    Another old wive's tale. It is safer to connect negative last and remove first when using the car frame instead of the battery terminal.

    Kirchoff's Laws are Laws, not suggestions. :) Voltage dropped across the load is not relavant. Voltage drop across the switch will increase from zero to supply whenever the switch is opened. There's no difference if one side of the switch goes from +12 to GND or if the other side of the switch goes from GND to +12.

    If you want a smaller spark, use silver plated contacts. If you want the switch to last longer, buy a more robust switch.
     
  8. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
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    You can suppress the transients across the switch with a .01uf 2000kv capacitor. It will help a small amount. Lamps are inductive and resistive loads you could put a diode to ground just after the switch this would dump the energy in the bulbs coils to ground when the switch is opened.

    Nether of these is really needed.
     
  9. Dennis Atwood

    New Member

    May 26, 2007
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    have you had many automotive switches fail on you? because i never have.

    just a little off topic, but speaking of car batteries...when jumpstarting, yes, it's supposedly safer to connect the ground wire to chassis last and remove first because the spark is further away from the battery, and on the rare occasion you might get some hydrogen gas from the battery, it won't light off... though, I can tell you because I've actually lit hydrogen a few times as training exercises at work, (i'm a fireman) you'd need alot. alot more than a car battery makes when charging. it dissipates VERY quickly.

    also, i always cut the negative cable on the battery at car wrecks because if a piece of grounded metal should contact the (-)terminal, it doesn't necessarily spark, whereas if i left the battery grounded to the chassis, all i'd have to do is touch the positive terminal with a tool to get some pretty decent sparks. and gasoline will ignite :D
     
  10. recca02

    Senior Member

    Apr 2, 2007
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    in view of above question then what about a fuse?
    i think they are always placed before the appliances,
    they can also be connected after the appliance near ground in that case cant they?
     
  11. Dennis Atwood

    New Member

    May 26, 2007
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    if you place the fuse after the appliance at the ground, and the appliance shorts, then the entire chassis of the appliance would be hot. this is a bad, bad idea.
     
  12. recca02

    Senior Member

    Apr 2, 2007
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    oh yeah didnt think about that,
    jeez i dont know whats wrong with me these days.
     
  13. cumesoftware

    Senior Member

    Apr 27, 2007
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    I think this is not the case, since here the circuit works at 12V DC and the power supply is not grounded. As I said, it doesn't matter where the switch is, but it is a good idea to have it to switch the positive terminal, just to follow the convention.
     
  14. thingmaker3

    Retired Moderator

    May 16, 2005
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    Per the original post, the power supply in question is indeed grounded.;)
     
  15. Dennis Atwood

    New Member

    May 26, 2007
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    usually fog lights only use one wire anyway, and the ground comes from it's metal enclosure, which is bolted on to the car. or in the case of a plastic case, a short ground wire is screwed to a nearby piece of the car's metal, thus grounding it.

    i thought we were speaking generally about appliances and fuses.
     
  16. cumesoftware

    Senior Member

    Apr 27, 2007
    1,330
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    I didn't saw that was for a car, but you won't have that danger. Also, most car headlights and taillights have the ground terminal directly attached to the "chassis".
     
  17. mrmeval

    Distinguished Member

    Jun 30, 2006
    833
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    Same reason you put the switch on the positive or hot side. When the fuse blows or a switch is off people will assume that the circuit is dead after that point. Otherwise you'd go to pull power at the breaker and still have voltage on the line. That would be fun when installing a garbage disposal or working on a grounded appliance.
     
  18. suprageezer

    Thread Starter New Member

    May 26, 2007
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    ThingMaker, Thank you for that explaination, that's what I was hoping to learn, the actual reason why it doesn't make a difference. Would you mind if I used your response on another forum so that others might learn as I have? The reason I ask is I had mistakenly learned it was better to break ground and posted that and was soon rebuked by others but they gave no explaintion. I felt I needed to learn the truth so I came here and you gave me that excellent answer that I would like to share with others.
    Thank You
    Rick
     
  19. subtech

    Senior Member

    Nov 21, 2006
    123
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    Recca

    I think you should be much more careful whilst "slinging" your opinions around.
    In almost all situations, a fuse, circuit breaker, or switch is best situated as close to the source as possible and almost never in the neutral or negative side of anything. A simple sketch and a little thought should bear that out.
    Also, placing the switch, fuse, or breaker (take your pick) in the + or energised conductor is not just a "convention", its done for safety.
     
  20. recca02

    Senior Member

    Apr 2, 2007
    1,211
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    well in no way did i mention abt the fuse as an opinion that was a question that just popped up ,i m already embarrassed abt it now,
    did you mean abt the switch?or some other opinion in some other post?
    btw i also realise that connecting switch after the appliance can cause shock hazard since the appliance remains connected to the live terminal even after switch is turned off.
     
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