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Trying to understand a circuit

  1. Jan 4, 2015 #1
    Hi everyone,

    I have been thinking, what would happen if I connect an AAA battery upside down?
    Did some searches online, and it seems like I will create a short a circuit and it will drain the battery (and some even say it is dangerous to this), but let's say if there is only a capacitor in the circuit, the resistance in the circuit shouldn't change, so it shouldn't create a short circuit or drain the battery even if I connect the battery upside down?

    Thanks!
     
  2. jcsd
  3. Jan 4, 2015 #2

    Bystander

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    What are the elements of the circuit you're trying to analyze?
     
  4. Jan 4, 2015 #3
    I am thinking about flipping the battery of my alarm clock upside down, but it can be anything, this is just a generic question, so I think perhaps making it only a single resistor will be the simplest, or is my question flawed in the sense that the resistor alone in the circuit will not drain the battery if I put it upside down?

    Thanks!
     
  5. Jan 4, 2015 #4

    davenn

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    OK upside-down is a really bad term. do you really mean you are just wanting to reverse the battery's polarity in the circuit ?

    if so, in general, this is a bad thing as it can damage the circuitry .... transistors, IC's etc, at 1.5V of your AA battery, you may well get away with it
    At a higher voltage source, the damage to the circuit is likely to be instant and damaging and may result in the "magic smoke" being released

    A resistor is a non-polarised component ---- the battery will drain regardless of which way around it's connected to just a resistor (or set of resistors)

    Dave
     
  6. Jan 4, 2015 #5
    Depending what the circuit has
    A motor will reverse
    A inductor would work as usual
    A LED won't light up and will take a very small current.
    A capacitor will charge as usual except electrolytics will would explode.
    A diode will conduct or not depending of its original polarization
    A resistor like Dave said would work as
    usual.
    A incandescent light will shine as usual

    A circuit with diodes and ICs like the alarm clock will usually burn even with low voltages ( I've ruined a cordless mouse by inserting its battery in the wrong way *facepalm*)
     
    Last edited: Jan 4, 2015
  7. Jan 4, 2015 #6
    Thanks for the replies!
    So actually the issue as a result of installing the batteries the wrong way are the electronic components will be damaged and the battery will just operate as normal.
    But any chance that the battery might "explode", I remember seeing from some google searches that incorrect installation will create a short circuit and might causes the battery to explode, but if I look at the explanations above, the components might explode, but there shouldn't be any damages on the batteries, or am I still missing something?

    Thanks!
     
  8. Jan 4, 2015 #7
    Depending on the circuit, it CAN behave as a shortcircuit and cause a battery explosion or leak if that doesn't have protection, when reversing polarity, only a diode in parallel to the circuit could cause it. And ICs could behave similarly.
    In any cause you shouldn't reverse polarity if the circuit has semiconductors, electrolytic caps... that can misbehave... I did that on class accidentally with a 555 and it smoked heavily and melted, what a mess lol.
     
  9. Jan 4, 2015 #8
    I thought I had a valid point to make regarding multiple cells, but now I'm not so sure I'm correct. One cell is straight-forward like everyone said it works reversed for resistance and electromotive force but semiconductors get "fried" or if they hold a trickle gets through. The catastrophe occurs when the battery is shorted which shouldn't occur in reversal of total polarity but what if say 1 out of 4 or 8 batteries are reversed. If the circuit is incomplete nothing occurs and batteries have a nifty nub-dent feature which prevents reversed alignment in a series of cells. This would lead to a greater potential "over-powering" the relatively smaller voltage of the reversed cell causing the voltage to exceed the cells physical limits. A fractionally higher voltage charges a battery, where in this case multiplied voltages cause destruction of the inverted cell. DO NOT ATTEMPT THIS!! Batteries could not only leak toxic and corrosive acids in liquid and gas forms but could ignite and possibly explode.
     
  10. Jan 4, 2015 #9
    shouldn't occur in reversal of total polarity.

    It can occurr, I saw it. Not only short circuit but less resistance and the battery could be damaged by overcurrent. A complex circuit with semiconductors can have unpredictable outcomes when reversed, from no current and no damage to overcurrent and destruction of the circuit components and the batteries/power supply.
     
  11. Jan 4, 2015 #10
    I'm not saying it can't happen I'm just saying it is usually anticipated and prevented by "accidental" means. In the case of 1 single battery it just dies and perhaps leaks. I've seen myself many battery failures several explosive usually they get hot and never function again, or they are over-charged until they boil and in liquid cells it explodes if they have enough energy density/potential. I don't know first hand about shorting lithium or other rechargeable batteries they have heat transfer issues proportional to density to begin with.
     
  12. Jan 4, 2015 #11
    Li ion batteries usually (yes there are bare cells available to make battery packs) have a protection circuit and fuse attached that avoid overcurrent so they won't explode in case of short circuiting, shortcircuiting a bare one would get hotter and hotter until thermal runaway and rupture, NiMH, NiCd and primaries usually don't (ones I saw) but they don't seem to explode (just heat)
     
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