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Current and voltage rating

  1. Jan 17, 2012 #1
    so i went to buy a laptop chrger with rating 19.2v and 3.2A but he gave me a charger with rating 19.2v and 4.2A and said the current rating is not important why is that?
     
  2. jcsd
  3. Jan 17, 2012 #2
    You better go back and ask charger with correct ratings. If he says something else ask him why? ask him to explain that.
     
  4. Jan 17, 2012 #3

    russ_watters

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    Staff: Mentor

    A laptop power supply is a voltage source, not a current source. It provides a constant voltage and the laptop draws whatever amperage it needs, so the power supply needs to be able to handle that need. If the capacity is too low, the power supply burns up. There is no "too high".

    So the current rating is important only insofar as it shouldn't be too low. The salesman gave good advice.
     
  5. Jan 17, 2012 #4

    sophiecentaur

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    Power supplies with lower current rating may be a bit cheaper but - a bird in the hand, as they say. It's only the supply voltage that is relevant, as long as the PSU can supply what you want.
     
  6. Jan 17, 2012 #5
    Well it's not that the current rating is not important. It is important. But the current rating is a maximum rating. So the higher the better. Having a higher rating gives you a kind of safety factor. It means the charger is able to generate more heat without failing.
     
  7. Jan 17, 2012 #6
    thanks that explained
     
  8. Jan 17, 2012 #7

    sophiecentaur

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    Not sure I'd agree that 'the higher the better'. Any appliance will need current protection (which is why table lamps shouldn't have 13A fuses). There is no fuse / protection between PSU and laptop so the lead (at least) needs some sort of cap on the amount of current it is 'allowed' to carry. 30% more than specified is absolutely fine but I wouldn't feel good about the possibility of 10A going through a thin supply lead, if it ended up with a table leg squashing it and shorting the conductors. People tend to ignore this aspect of safety - it's all about shocks but fires are just as deadly and can involve more victims.
     
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