Quick electrical question: (amps of AC adapter?)

Discussion in 'Et Cetera, Et Cetera' started by mindseye, Feb 29, 2008.

  1. mindseye

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    The AC adapter for my old laptop (the laptop is dead, but I still have the adapter) matches the specs of my new AC adapter -- same voltage, same polarity, but the old AC adapter is rated at 6.5 amps, and the new one is rated at 3.42 amps.

    I think it's safe to continue to use the old adapter. My understanding is that the amperage is the maximum amount of power the adapter can supply, and the new laptop will just draw less than the maximum.

    Can someone verify this? I'd prefer not to find out experimentally that my understanding is wrong. ;)
     
  2. SpeedoGuy

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    That matches my understand as well. But I'm not an electrical engineer so I can not offer any more surety beyond what you've reasoned for yourself.
     
  3. B_Think_Kink

    B_Think_Kink New Member

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    Google your adapter (if you know the brand and model #) it will tell you all the computers it can be used with. I was able to do that for my spare ac adapter.
     
  4. D_Goldsworthy Fingernuts

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    I am sure that i can find something in my readers digest that will help hold on!!! No noting about big cocks and ampage here? :)
     
  5. faceking

    faceking Active Member

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    THAT IS VERY BAD... discontinue now. ESPECIALLLLLLLLLLLLLY with a friggin' 3 ampere difference. That's big bad, no bueno. Maybe a handfull of milliamps, would be fine, as it would get buffered out, per se. But 3.... sweet jesus.

    Ohm's Law is something to not F with.

    Laptops are not heavy duty... and their motherboards run on extremely known currents... at a minimum it will screw up for battery memory, but over time... wouldn't be shocked if something fries for good on the board... as clock times and so forth need their currents expected.... and may tax a bit much in order to meet the demands and processing for their place in the "chain of command", per se.
     
  6. dong20

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    That's not entirely accurate, FK.

    Adapters will deliver the power they're requested to by the device - up to (and usually) a little beyond their rated capacity. They won't (or shouldn't) deliver full output unless it's demanded - otherwise a laptop (for example) would fry any time it didn't require anything other than the adapter's full rated output. Think about it, it's like a dimmer switch as opposed to a pole switch - it's not a binary state in terms of current flow.

    Also, consider universal laptop adapters. They are voltage adjustable, and thus current variable up to their rated capacity.

    That said, because the adapter is fixed voltage and capable of supplying enough power/current to damage the laptop I'd be careful in case the design of either isn't very well regulated in which case the laptop could try to draw enough power to damage itself.

    Be especially careful if the voltage output on the adapter is more than a volt or so off the stated input of the laptop. I'd agree with FK to use caution and obtain the correct adapter, though for slightly different reasons.
     
  7. mindseye

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    Thanks to everyone for the feedback. I tested the old adapter with the new laptop this morning (I backed up a couple of irreplaceable files first, though!). The laptop ran on the old "amped-up" adapter, but the battery monitor detected that something was awry and would not charge the battery off of the adapter.

    I'm taking this to mean that the adapter is usable as a short-term emergency replacement, but not much more than that.
     
  8. JustAsking

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    mindseye,
    You should be safe running the new adapter which has a higher amperage rating. The amperage rating is a figure of merit for the current capacity of the adapter. Given the same output voltage and the same load (the laptop itself) the adapter will deliver the same amperage, which will be below the capacity of the old adapter.

    I don't know why the battery won't charge, though. The voltage of the new adapter might be slightly lower than the old one.
     
  9. dong20

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    That would be my thought too. It could also be a higher voltage causing the battery charging circuits to reject the input if it's outside tolerance, Li-ion battery (I assume it is) charging circuits can be very fussy!
     
  10. Rugbypup

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    No i think adapters have fixed output otherwise its like saying you can connect your car battery to charge directly from the mains supply, with no charger, because a car battery dosent pull all 240v/?A.
     
  11. dong20

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    No, outputs can and do vary. For example, I have four laptops in front of me right now, each one has a different voltage output from its adapter, two are 15v, one is 16v and one 19v.
     
  12. JustAsking

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    This is not really a mystery. Ohms law has been known for over 100 years. It is

    I = E/R,

    where E is the voltage, R is the resistance.

    In this case, E is the same between the old and the new adapter, and R is the load the computer places on the power supply. Since E and R are the same with the old and the new adapter, I, the current, drawn by the compter from the new adapter will be the same as the old.

    There might be a slight difference in voltage or excess ripple on output of the new adapter, so the charging circuit doesn't like what it sees. Otherwise, the computer seems ok with it.
     
  13. Rikter8

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    The largest amperage draw on the laptop is during Post, or when you turn it on.
    Every device enters a self test, which draws the max current.
    The equipment CAN damage itself if it cannot draw enough current.
    Usually it will try to pull from other locations, OR generate MORE heat.
    If the battery is in the laptop, it will help you out temporaraly, but I would NOT use the AC adapter by itself, or for very long.
    Otherwise You might be replacing the motherboard - as the power section isn't a separate component.

    I would order another laptop adapter.
    I have a couple here for Dell's C series Inspiron that I can send you as well.
    (Check out Pricewatch | Find lower prices on computer hardware, laptops, computer parts and electronics or Ebay)

    Here's what happens to auto amplifiers when too Little current is available, and where the amplifier has to "Pull" for more. - see attachment.
    (One mosfet caught fire, the other few fractured from excessive heat)
     

    Attached Files:

  14. dong20

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    I've had that happen with low cost universals, either they were not putting out the current stated or they had poor rectifier circuitry or both. Some laptops are fussier than others, but it varies from model to model.
     
  15. dong20

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    That's not always the case, some models have separate power, main and charging boards, the Toshiba 6000/6100/9000/9100 series for example. I know because I've fixed so many. They have other (design) problems too. But other than that I agree, it's risky to use the incorrect adapter for long.
     
  16. pronatalist

    pronatalist Active Member

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    Generally, you should always use the power adapter that comes with the item, to be safest.

    Of course if there is a problem with that, often you can substitute a similar adapter, assuming one can even be found?, if the plug and polarity and voltage match, and the current-handling capacity is close enough. The item will use what it needs usually (assuming correct voltage—alert), not the whole amount of what the adapter can theoretically supply. But if you power up an item, that may be using a bit more power than the adapter was designed for, say like a small color tube TV on a black-and-white power adapter, you should check the power adapter a few times, after it's been on for a while, and make sure it only gets normally warm, not hot or overheating after a while. 20% "overload" may be okay, but 200% overload may be risky, if it even works at all. You really can't trust power ratings completely, because often they are the "maximum," not the typical usage.

    I've seen a music keyboard play just fine on an underpowered power adapter, but then it unexpectedly turned itself off, when cranking the volume on its internal speakers too loud. Seems it didn't much like the sudden voltage dip from a power adapter that couldn't quite keep up with its power demands.
     
  17. SpeedoGuy

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    Same experience here. And I'll never again try to charge my ipod on anything other than a top name brand adapter.
     
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