flipped switch, still has power?? why?

Discussion in 'The Projects Forum' started by electrfried, Apr 14, 2015.

  1. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    Put switch in-line on the negative wires of a 4 pin molex computer cable, coming off à laptop style brick transformer. 2 pin rocker switch soldered the 2 negs to one pole, and terminated the two wires from the other one to the connector. When switch flipped off still get minor voltage movement? Running two computer fans in a laptop stand that I want to be able to shut off. Any thoughts?
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    sounds like you have wired the switch wrong, can you show a picture of what you have done?
     
  3. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    Simply put a 2 pole switch between the two grounds from the supply and the two going to the fans.
     
  4. WBahn

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    Mar 31, 2012
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    That picture doesn't even begin to show how you actually wired it and your verbal description isn't cutting it, either.

    Please just make a schematic (a hand sketch or Paint drawing will do fine) of how you wired it up.

    My guess -- and that's all it is at this point -- is that you are allowing a sneak path between the two positive pins.
     
  5. gerty

    AAC Fanatic!

    Aug 30, 2007
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    What do you mean by minor voltage?
     
  6. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    P (+)5v -------------------------------(+)5v F
    L (-)-------pole1_switch_pole2-----(-) A
    U (-)-------pole1_switch_pole2-----(-) N
    G (+)12v----------------------------(+)12v S

    Granted they only pull from the 12v leads.
    Like said super simple.

    Guess I need to just test the adapter leads and the switch for continuity.

    Assume 3-4v, the fans make a subtle eerie noise.
     
    Last edited: Apr 15, 2015
  7. alfacliff

    Well-Known Member

    Dec 13, 2013
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    why switch the grounds? the normal way is to switch the supplies, not the grounds. that way, anything connected to both the 5 and 12 volt lines will not have any power on it.
     
  8. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    Was told it wouldn't matter and simply cutting in the grounds was much easier.
     
  9. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Switching grounds is easier and o_O dumb u know. :D
    You should always switch positive., not the ground
     
  10. WBahn

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    Mar 31, 2012
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    Tell that to McDonnel Douglas, who switched the grounds on most systems on the F-15 (and they aren't alone, it's a quite common practice).

    There is nothing magical about switching the power versus switching the ground -- after all, which is "power" and which is "ground" is a matter of arbitrary definition to begin with.

    Having said that, as soon as you have two supplies involved that share a common, then you should not switch the common as now one supply has a means of talking to the other via the other lead. You also have to consider sneak paths to chassis grounds and other potential return paths.

    Better to switch the two power leads as then the remaining connection between the two circuitries is the same lead, hence no sneak path. There is still the potential for sneak paths between the common and other sources/sinks, such as the chassis, but the way most systems are designed makes that much less likely.
     
    Last edited: Apr 15, 2015
  11. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    I forgot that a F-15 guy was here. :oops:
     
  12. alfacliff

    Well-Known Member

    Dec 13, 2013
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    switching "grounds" might work until there is a little moisture to provide a ground path, or as in this casse if more than one supply is connected (+5 and+12). there can be current flow between the circuits connected to different supplies. how do they switch the ground on the f-15's when the entire airplane has everything bonded together to ground? the case of each radio is grounded, the antenna shields are grounded, and such.
     
  13. WBahn

    Moderator

    Mar 31, 2012
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    You put a switch between the load and the airframe instead of between the power lead and the load.

    Where it pays off is in systems where one part is physically isolated from the rest. If I have, for instance, a solenoid valve that has to be activated by a switch some distance away, then to switch power I have to route power to the switch and then back to the valve. Or, I can have power go to the valve and then a single wire that goes to the switch and goes from there to the airframe. Fewer wires has lots of advantages in a combat vehicle.
     
  14. alfacliff

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    Dec 13, 2013
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    and then you have a lot of stuff that has power on it because it is disconnected from ground. disconnecting the power is the best way, a dropped screwdriver wont arc as much when it gets between two radios that way. and most leakage is from the case of equipment, how do you keep leakage current from energizing things? aircraft environment is usually cold with a lot of condensation. if you have two supplies connected to sometnhing on your plane do you still sw2itch grounds? how do you keep current from flowing from one supply to theh lower voltage supply?
     
  15. WBahn

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    Mar 31, 2012
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    They why has it been a very common practice two switch the grounds in many aircraft designs? The answer to your last question is that you design the system taking that into account. Also note that there is nothing that says that EVERYTHING must be switched the same way.

    If you go to a truck/tractor parts store you will find solenoids (particularly continuous duty solenoids) that bring out both coil connections specifically so that the user has the option of controlling the power or controlling the ground. There are also solenoids in which only one terminal is brought out with the other grounded internally and solenoids in which only on terminal is brought out with the other tied to the input power pole internally.
     
  16. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    I think it just clicked. In some quirky way the positive leads are then able to use each other as grounds when you make the negatives common?
     
  17. WBahn

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    Mar 31, 2012
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    Basically.

    Consider the following:
    DualV.png
     
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  18. electrfried

    Thread Starter New Member

    Apr 14, 2015
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    I will do some experiments tomorrow, as it still keeps making me go cross-eyed knowing that the power supply has all of the wires independently routed. I will pull the leads from one side and see if the remaining side shows signs of the ghost voltage. It's odd that the 5v, led thermostat shuts down no problem but the high power Corsair fans manage to keep pulling even with the inline rheostat turned all the way down. If need be I can just install 2 switches, but I want to look into the specs of all the electronics if they can all run on 12v then I would prefer to just remove the 5v altogether. Many thanks, it has been a long time since I fiddled with DC projects, that are not in an automobile.
     
  19. alfacliff

    Well-Known Member

    Dec 13, 2013
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    didnt you see that the two batteries share a common ground? the only reason for switching grounds is the same as disconnecting the ground wire in a car when removing the battery, less chance of shorting the positive terminal.
     
  20. alfacliff

    Well-Known Member

    Dec 13, 2013
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    most starter solenoids with two terminals have one for pulling in the solenoid and the other for shorting the ballast resistor in the ignition.
     
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