Using a power supply with a lower maximum current?

Discussion in 'General Electronics Chat' started by Robert Smith_1437948150, Sep 9, 2015.

  1. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    Hello, I've been told the following and I was just wondering if someone can confirm how accurate it is?

    If I purchased a phone that came with a 5V 2A power supply, it would be safe for me to use one rated at 5V 0.7A. I have been told that when the phone tries to draw more than 0.7A the voltage will drop, the phone will detect this and won't try to draw any more? (I know USB provides ways for power supplies to communicate what they can deliver, but I don't think many cheap, stand alone chargers do this, and just act as a regular power supply.)

    This isn't true though for laptops though as they aren't expected to use different chargers unlike phones. If I used a different power supply with a lower maximum current than the one provided with the laptop, either the power supply's voltage would drop and the laptop won't be able to draw the current it needs, or the power supply will maintain its voltage and try provide a higher current than it was designed to, potentially damaging the supply.

    On a fairly basic level, how accurate is the above?

    Thanks.

    Edit - This is all theory, I'm not actually wanting to do any of the above!
     
    Last edited: Sep 9, 2015
  2. #12

    Expert

    Nov 30, 2010
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    You seem to be suffering from bad advice. The current rating is a "capability" limit. All power supplies have a maximum rated current capability. The load decides what amount of current it needs. If you have enough capability, everything works. If you don't, the voltage gets lower and things start glitching.
     
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  3. R!f@@

    AAC Fanatic!

    Apr 2, 2009
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    Laptops can use different chargers. But some has a ID pin to prevent it from charging when using other oem chargers.
    To repair laptop I use my Bench PSU to power and charge laptops.
     
  4. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    Thanks, I understand that the rating is the 'capability' rating. ...but if a phone comes with a charger rated at 2A and then I used one rated at 0.7A, I have been told when the phone tries to draw more than the charger is capable of and the voltage starts to dip, the phone can sense this and won't draw any more.

    The cell may not charge but at least the charger won't melt.
     
  5. R!f@@

    AAC Fanatic!

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    If the phone detects a voltage dip the phone simply stops charging. That is if the phone is a quality one.
    Others may simply draw current and the voltage dips, the charger overloads and gives up. If the charger has short protection it will survive.
     
  6. #12

    Expert

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    It depends on which phone. Some are smart enough to shut off and some will overheat the power supply. If you don't know the internals of your exact phone and the manufacturer won't tell you (they won't), measure. The cheapest P.O.S meter from Harbor Freight can measure this range of voltage and current. Cut one wire on the output side of the charger, put the current meter in series with the wire, plug in the phone, and measure. If the current exceeds the label rating of the supply, the phone is not smart enough to refuse a power supply that is too small.

    Nobody can make a blanket statement that all phones, computers, etc. will protect a weak power supply, unless they are terribly misinformed.
     
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  7. cmartinez

    AAC Fanatic!

    Jan 17, 2007
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    That is very true for all the Samsung phones that I've had... currently I own a Galaxy A3 and it does detect when the charger is unable to deliver the required current for it to charge properly. The system then gives notice to the user and the charging process is stopped, even if the phone remains connected to the charger.
     
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  8. #12

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    That covers one brand. :D Only a few hundred brands left to check. ;)

    I have Vtech brand phones in my house, and they are as dumb as a sack of door knobs.
     
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  9. GopherT

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    Nov 23, 2012
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    Microsoft / Nokia phones have a "slow charge" warning if not plugged into a charger with 2A capability.

    iPhones just dim the screen and do an either/or decision. Either it lets you use the phone and not charge, or, it puts all available current to the battery once the screen turns off from inactivity.

    @#12 with Samsung, Apple and Nokia/Microsoft, we have nearly 45% of users covered. Now, anyone with an LG, Xiaomi, Huawei or Lenovo phone?
     
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  10. #12

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    If we assume this is only about cell phones, the whole crew here at AAC has a fair chance of covering most of the market.
     
  11. wayneh

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    Sep 9, 2010
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    Bottom line, in my opinion, is that you should never rely on using a power supply not rated to the job. Just because a few smart devices might enable you to get away with this, it's bad form and just not a habit worth falling into. Someday, you WILL pay a price for this bad habit, just like any other bad habit.
     
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  12. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    Thanks everyone, some really interesting info.

    Focusing on phones for the time being, it seems each have their own way of handling things if connected to an underpowered supply (at least those of a higher quality).

    Does anyone know if some/any phones prevent damaging the charger by not drawing any more current when it senses the voltage begin to drop?

    I think I'm right in thinking when connecting a phone to a USB port on a PC, the USB port is able to communicate that it is limited to 500mA, but cheap, regular phone chargers don't do this and therefore may rely on sensing the voltage dropping.

    Regards,
    Robert
     
    Last edited: Sep 10, 2015
  13. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    I think you know that answer from all of the above comments. Yes. That is how those phones work. More to protect the phone than the charger but the net result is the same no matter the motivation.
     
  14. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    Thanks GopherT,

    I'm just a bit confused as to what is protecting the power supply? Is it the phone or the power supply itself?

    Say I have a 'dumb' phone that just tries to draw a fixed amount higher than supply is capable of, the power supply could limit the current, constantly decreasing the voltage so the supply is providing the current it was designed to do.

    In the above case it doesn't matter if the phone can detect the voltage dropping as it wouldn't make a difference. If the phone can detect the voltage dropping then it's the power supply that is protecting itself anyway and the phone doesn't need to do anything.

    So when would it be beneficial for the phone to detect the voltage drop? Is it as you say to protect the phone, why does it do that?

    Regards.
     
    Last edited: Sep 10, 2015
  15. wayneh

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    I don't think there's one answer. Some chargers have thermal cut-outs that reset once it cools off. (My Macbook charger will do this.) Modern AC adapters likely have current limiters built in. I see this function on some of the ICs used in these adapters, so I assume some designers take advantage of that.

    The phone needs to decide whether to charge and at what rate. The designers have decided what conditions are required for however many charging modes are available. If the supply cannot fit into one of those modes, the phone won't start the charging process.
     
  16. cmartinez

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    I'd say the simplest answer would be: "sometimes the chargers are protected against over-sourcing current, sometimes they're not, and sometimes the phones are protected against over-drawing current, sometimes they're not"
    But normally... the phone is the one that's protected, if at all
     
  17. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    Thanks.

    In a scenario where just the phone is protected and using an underpowered supply. When the phone tries to draw more than the supply is capable of, what exactly happens?

    Does the power supply try to provide the current the phone is drawing, at its designed output voltage. Without any protection this may damage/overheat the power supply.

    So when does the phone's protection kick in, when is that needed?

    Can someone give me a quick step by step of what would happen in the above scenario, and also in a scenario where both the charger and phone have protection?

    Regards.
     
    Last edited: Sep 11, 2015
  18. #12

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    Nov 30, 2010
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    You have been told several times that each design is different and nobody can (wisely) make a blanket statement about all of them. Now you want someone to speculate about all the different ways and combinations and what each designer planned for and how they implemented their opinion of how to make things work. So...no. It isn't likely that anybody here has all day to speculate on a dozen or a hundred designs, then explain how they guess each one would work.
     
  19. Robert Smith_1437948150

    Thread Starter Member

    Jul 26, 2015
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    I never asked or expected anyone to do that at all.

    I just thought all chargers and phones would work pretty much the same way in principle (i.e. phones detecting the voltage dropping and chargers reducing the voltage to keep a fixed current), and then manufactures can choose how the device handles those situations (i.e Nokia's slow charge or an iPhone dimming the screen).

    I was asking about the first part, which I thought was universal across all phones/chargers, apparently not.
     
  20. dl324

    Distinguished Member

    Mar 30, 2015
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    You're giving phone manufacturers more credit than they deserve. They expect you to use the charger they provide and/or specify. If you choose to use something else, you're on your own.
    If you think back 20 years ago when cellphones were just starting to become affordable for most people, you'll appreciate the fact that most non-Apple phones have chosen to use USB connectors for power and communication. Back in the wild west of cellphones, manufacturers could hold you hostage with their proprietary stuff. I stayed with iPhone until they changed their connector to be incompatible with their older models. If I needed to buy new accessories Android based phones became a more attractive option.
     
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