Drawing >500mA from USB

Discussion in 'Electronics & Electrics' started by imgod22222, Jul 5, 2012.

  1. imgod22222

    imgod22222 Member

    Joined:
    Jan 4, 2006
    Messages:
    729
    Location:
    Florida, America
    When building electronics that source power from a USB port, is there anything the device needs to do to get >500mA, or will the port happily supply whatever the device pulls?

    disambiguation, if needed:
    I know iPhones, etc check the D- D+ lines to be shorted before they draw >500mA on devices that support supplying that additional current.
    So on devices that support supplying that additional current, those devices will supply 5V to whatever is connected to +5V and GND (assuming it is kept within its operating range) correct?

    What's actually going on:
    I'm planning on using a usb car charger to take 12V from a SLA to 5V for my project. It draws about 1.5A, and the car charger is rated for 2A.
     
  2. Gecko

    Gecko Member

    Joined:
    Jul 3, 2004
    Messages:
    2,715
    Location:
    Sydney
    If you're connecting to a computer, a device can draw 100mA without any negotiation between the host and the device, and up to 500mA after negotiating (according to the USB spec, there are exceptions to this - I'm looking at you Apple).

    In your case though, 99% chance that your car charger will do 2A without any negotiation or other tickery.
     
  3. IzzehO

    IzzehO Member

    Joined:
    Mar 9, 2011
    Messages:
    936
    Location:
    4152
    This. Last time I opened one of those up it was a simple switch mode regulator outputting to the 5V rail directly without any digital logic or the like.
     
  4. NanoDuke

    NanoDuke Member

    Joined:
    Feb 4, 2007
    Messages:
    7,629
    Location:
    Sydney
    I did exactly the same thing.

    I took a Jaycar car 12V-to-5V USB device and used it to power.... um... *something*.
    The kicker was that I used 4*3.7V li-ion cells (which bugger me equals 14.8V)
    Ugh. Excuse my ramblings. I can't remember exactly how it was butchered up, but I do remember it was for a DIY night vision HUD.
     
  5. IzzehO

    IzzehO Member

    Joined:
    Mar 9, 2011
    Messages:
    936
    Location:
    4152
    Yeah that works fine. Although I'd ensure the conversion was switch mode. Linear from 14.8V to 5V would waste a lot of power in heat.
     
  6. dakiller

    dakiller (Oscillating & Impeding)

    Joined:
    Jun 27, 2001
    Messages:
    8,224
    Location:
    3844
    That is the spec, but I haven't seen any USB port that really follows that and you can take 500mA without any negotiation.
    The only issue is that you need the device to now be signaled to draw more than 500mA. Phones and stuff that can charge at 1A or more need to be signaled that they aren't plugged into a regular USB port and can now take more, otherwise they will be stuck at 500mA. Pullups on data pins or shorting the data pins are the common ways to do this, but there is a few 'standards', I know HTC differs from Apple.
     
  7. aXis

    aXis Member

    Joined:
    Jun 27, 2001
    Messages:
    5,548
    Location:
    Kalgoorlie, WA
    There is a spec for USB charging, and it doesnt require active connecitons. A few resistors will provide the right handshaking signals and ports will allow you to take at least up to 500mA, and if you check the spec there might even be higher current modes.

    That said a whole bunch of devices and chargers do non-standard or pseudo-standard behaviour
     
    Last edited: Jul 5, 2012
  8. MetalPhreak

    MetalPhreak Member

    Joined:
    Apr 8, 2007
    Messages:
    1,763
    Location:
    Perth
    It is the CHARGER that has the resistors across the data pins, to tell the DEVICE that it can draw more than 500mA.

    Most if not all USB chargers will happily supply up to their rated current without any negotiation or trickery. (except those dodgy super cheap ones that can't reach their rated current because it's a lie :lol:)
     

Share This Page

Advertisement: