Output of USB from Computer?

Discussion in 'General Electronics Chat' started by raziiq, Nov 12, 2009.

  1. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Hey There.

    I am very new to electronics. I wanna know a few things

    1) What is the total voltage we can take from a Computer USB (For example if i have to power up some circuit from USB cable that is attached to PC)?

    2) Amount of Current we can take from a Computer USB?

    3) Is it safe to use USB as a power supply to circuits?
     
  2. SgtWookie

    Expert

    Jul 17, 2007
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    The USB ports on desktop/tower computers are rated for 5v, not to exceed 500mA (0.5A) current draw. Laptop computers usually have less current capability; it varies between manufacturers / models.

    500mA/0.5A maximum. Laptops, generally less.

    I've had a Microchip PICkit 1 pugged into a USB port with a PIC12F675 uC flashing a dozen LEDs for the past year - no problems yet.

    The connectors weren't designed to pass high current. If you happened to short out a USB port, you may burn it up. If you want to be sure you won't burn yours up, use a 1/2A fuse between the USB port and your project.
     
  3. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    thanks for the reply, that was helpful.

    BTW if i need something to power up that needs more than 5V, then i should use external power supply instead of USB, right?

    Also the Parallel Printer ports have the same amount of voltage n Current? Ya??
     
  4. bill l

    Member

    Oct 11, 2009
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    I would not experiment especially if it's your primary desktop/laptop as you risk damaging the usb controller and I'd bet that would not be covered as a warranty repair.

    For 5 vdc, from 300ma up to 1 amp, phone chargers can be had for dirt cheap from thrift stores.
     
  5. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Ya thats why i am not planning to hook it up with my Computer first up. Wanna make sure i am not doing anything foolish before trying to connect circuits to my PC USB.
     
  6. SgtWookie

    Expert

    Jul 17, 2007
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    Yes.

    I suggest that you do not attempt to use the parallel printer port to power anything. They were not designed with powering devices in mind. If you attempt to do so, it is very likely that you will destroy not only the parallel port, but your computer's motherboard as well. It will be expensive to get repaired.

    If you feel that you must experiment with a parallel port, purchase a Multi I/O PCI add-on card and install it. You can find them for around $10 in various places. That way when you blow up the card, you are only out $10 instead of your entire computer.
     
  7. SgtWookie

    Expert

    Jul 17, 2007
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    If you are looking for inexpensive power supplies, this is the website to visit:
    http://www.mpja.com/

    Check out their "email specials" link first, at the top of the page. There is a 9v 1A wall wart in there for $0.99.
    If you are just looking for really cheap "wall-wart" type supplies, they are in this section:
    http://www.mpja.com/Category/Power_Supplies/Plug_Supplies/Plug_Supplies-Power_Supplies.asp
    "Wall-warts" are generally not regulated supplies. They just output low DC voltage with a good bit of ripple on it.
     
  8. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    The outer two connections are the + and -, they are a bit longer so that power is connected before serial.

    Ports are rated at 500mA max, but generally 300mA is expected. Some external hard drives have a Y cable to connect to the system, one for power and data, the other for more current/power only.

    Going over the limit can damage the motherboard, so you may want to test with a wall outlet - USB jack power adapter, or 12V - USB jack power adapter.
     
  9. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Oh thanks , i ll look at them for sure.
     
  10. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    What does it mean by going over limits? Is it that if i use a 12V bulb to light it up from my USB, it will damage my USB n motherboard?
     
  11. thatoneguy

    AAC Fanatic!

    Feb 19, 2009
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    Trying to draw over 500mA. It is usually well controlled and limited by the motherboard. Others, especially some cheap PCI adapters will meltdown the bus if the USB supply is shorted.
     
  12. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Alright, got it, so as SgtWookie said, i should use a fuse in between the USB n my circuit to avoid any kind of damage right?
     
  13. SgtWookie

    Expert

    Jul 17, 2007
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    Do not try to power your 12v headlamp from your computer power supply; you will burn it up.

    If you want to experiment, try it with a salvaged computer supply; not one in a working computer.

    Your headlamp will require at least 12v, 5 Amperes.
     
  14. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Ya got that, i wont try it with my Computer. I ll try it with 12V Battery or something, but i think i should go for LEDs instead of these bulbs, as i have just seen a lots of LEDs for different purposes, they are quite bright too
     
  15. SgtWookie

    Expert

    Jul 17, 2007
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    Yes, LEDs can be quite bright nowadays.

    Be careful to not stare at these new super-bright LEDs; you will permanently damage your vision if you look directly at them for more than a few moments.
     
  16. raziiq

    Thread Starter Active Member

    Dec 15, 2008
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    Now thats really scary :eek: , are there any precautionary measures i need to take in order to perform experiments with these LEDs?
     
  17. SgtWookie

    Expert

    Jul 17, 2007
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    Just don't stare at them. The light from a bare super-bright LED's lens is generally focused in a very narrow beam. If you stare right at them at close range, your retinas will get "burned".

    If you want to really "play it safe", just use a sheet of ordinary typewriter paper or white printer paper between your eyes and the LEDs; several inches from the LEDs. The white paper will diffuse the light so you don't get such a concentrated dose.
     
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