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Help with resistors?

Discussion in 'General Electronics Discussion' started by sethmaster, Feb 15, 2010.

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  1. sethmaster

    sethmaster

    6
    0
    Feb 10, 2010
    Hey i need some help. What resistors do i need to lower the voltage from a 9volt battery down to 5 volts?
     
  2. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    It's called a 78L05, provided you are intending to power a small digital circuit with it...
    Maybe you could be so kind as to tell us what you're trying to accomplish...? :rolleyes:
     
  3. sethmaster

    sethmaster

    6
    0
    Feb 10, 2010
    resistors

    Thanks. I am trying to power a usb device using a nine volt. I figured that would be handy for dead cellphones and anything else in an emergency.
     
  4. Resqueline

    Resqueline

    2,848
    2
    Jul 31, 2009
    Ok, just as I thought. Resistors are completely unsuitable for that task. The mentioned voltage regulators are cheap & easily available.
    The 78L05 can only handle up to 100mA, the 78M05 can do up to 500mA, and the 7805 is a 1A device.
    The 78L05 is in a small TO-92 transistor package, and the other two are in a bigger TO-220 power transistor package.
    They all draw a few mA by themselves so they can't be left permanently connected to the battery.
     
  5. 55pilot

    55pilot

    434
    3
    Feb 23, 2010
    Cell phones are not you typical USB devices. If they detect that they are connected to a USB hub, they behave like a USB device. They negotiate with the hub for their allowed current and live within the budget they are assigned. The maximum allowed current per the USB spec is 500mA @ 5V or 2.5W.

    However, if a cell phone detects power on its USB port but does not detect a host, it assumes that it is connected to a charger and draws as much current as it needs. That can be as much as 1A or 5W.

    If you take a typical 9V Alkaline (Energizer type) battery, hook it up to a 7805 and connect it to a cell phone, one of two things will happen.

    1) Your 7805 could drop 3W to 4W and if it is not properly heat sinked, it will heat up and die in under 10 minutes.

    2) If your 7805 has a really good heat sink and does not heat up to the point of failure, your typical Alkaline battery will be exhausted in about 15 minutes, giving you under 25% charge to your typical 4Wh cell phone battery.

    If you really want to charge a cell phone from a 9V battery, you will need a step down DC-DC converter that does not waste 40% of the power in heat.

    ---55p
     
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