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How much current and voltage can USB support?

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ehsanelahimirza

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USB current

hi

i want to know that how much a USB port can support CURRENT and VOLTAGE
actually i want to use portable HDD. it should be powered by extra power supply.
if power can be gained by using 2 or 3 ports please tell me how to do that
 

IanP

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Re: USB current

The USB will operate guaranteeably up to 500mA (2.5W) ..
If the HDD can work of slightly lower voltage, you can try to connect several Schottky diodes (with Uf<0.2V) to several USB ports and increase the output current that way ..

Regards,
IanP
 

M!k

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Re: USB current

I don't think it's just so easy. A current of up to 500mA can only be drawn after enumeration. Before that (or by using just diodes and no USB-uC for enumeration process) the max. allowed current is 100mA (= one unit load) per port.
From the USB specs:
7.2.1.4 High-power Bus-powered Functions
A function is defined as being high-power if, when fully powered, it draws over one but no more than five unit
loads from the USB cable. A high-power function requires staged switching of power. It must first come up in
a reduced power state of less than one unit load. At bus enumeration time, its total power requirements are
obtained and compared against the available power budget. If sufficient power exists, the remainder of the
function may be powered on. A typical high-power function is shown in Figure 7-45. The function’s electronics
have been partitioned into two sections. The function controller contains the minimum amount of circuitry
necessary to permit enumeration and power budgeting. The remainder of the function resides in the function
block. High-power functions must be capable of operating in their low-power (one unit load) mode with an
input voltage as low as 4.40 V, so that it may be detected and enumerated even when plugged into a buspowered
hub. They must also be capable of operating at full power (up to five unit loads) with a VBUS voltage
of 4.75 V, measured at the upstream plug end of the cable.

Concerning the voltages here's an excerpt from the USB specs:
7.2.2 Voltage Drop Budget
The voltage drop budget is determined from the following:
• The voltage supplied by high-powered hub ports is 4.75 V to 5.25 V.
• The voltage supplied by low-powered hub ports is 4.4 V to 5.25 V.
• Bus-powered hubs can have a maximum drop of 350 mV from their cable plug (where they attach to a
source of power) to their output port connectors (where they supply power).
• The maximum voltage drop (for detachable cables) between the A-series plug and B-series plug on VBUS is
125 mV (VBUSD).
• The maximum voltage drop for all cables between upstream and downstream on GND is 125 mV (VGNDD).
• All hubs and functions must be able to provide configuration information with as little as 4.40 V at the
connector end of their upstream cables. Only low-power functions need to be operational with this
minimum voltage.
• Functions drawing more than one unit load must operate with a 4.75 V minimum input voltage at the
connector end of their upstream cables.

Btw.:
low-power: <100mA (one unit load)
high-power: 100mA - 500mA (1 to 5 unit loads)


Mik
 

ehsanelahimirza

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Re: USB current

when i started to work on using power by USB ports i came to know that it can dekever only 5 volts
but my requirement was 12v and 5v both
a normal HDD operates at

12V--->800mA
5V---->750mA

a combination of USB ports cant provide it even
plz tell me how to obtain this power. is it would be possible through serial or parrallel ports
 

IanP

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Re: USB current

There should be planty of power available inside of your computer (laptops excluded) ..
Try, if you can, to use +5V and +12V from the computer's SMPS ..
Other option is to buy/build a 12V/2A regulated power supply and add 5V/1A switching step-down converter ..

Regards,
IanP
 

ehsanelahimirza

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Re: USB current

i have one external supply to use but i want to make it portable in a sence that
no power is to be given by extra supply or from inside of pc bcz everywhere we r not allowed to open the PC(public ) and the other case with me that in the whole computer lab of my university there is no extra connection for electricity. i have to contact lab administrator each time. so if it is possible to use the power of serial/parallel/USB ports plz tell me
 

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Re: USB current

No computer port can provide you with sufficient power ..
COM(s) and LPT(s) carry no power at all - if you are lucky you can draw some mA from selected serial port pins, but that is all ..

You will have to think of something else ..

Regards,
IanP
 

asit

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Re: USB current

Joystic port can be a solution It can supply lot of current.

WARNING: There is lots of current (many ampreres) available on the PC joystick port without any built-in current limiting. Short circuit on the circuit connected to PC joystick port can cause wires heating, burn thin circuit board traces, crash your computer, damage your PC power supply and even catch fire. I have burnt one flatcable going from PC multi-IO-card to the joystick connector because of short in circuit connected to joystick port! So if you make mistakes your multi-IO-card or soundcard can be in the line to suffer sudden joystick port death. Usually you should be quite safe with if you do not short circuit +5V output to anything and do not apply any external signals to PC joystick port.

check out : https://www.epanorama.net/documents/joystick/pc_circuits.html
 

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