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USB Output Voltage

does anyone know the standard output voltage for the USB port, and which wires inside the connector are the negative and positive, and maybe some links to some very basic usb guides, i was hoping to use the voltage for a small light for my keyboard, i like to do this simple diy stuff.
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isaacmc
Asked:
isaacmc
2 Solutions
 
Pete LongConsultantCommented:
Hi isaacmc,
USB 1=4.4Volts minimum

PeteL
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Pete LongConsultantCommented:
isaacmc,
The USB 1.0 Spec contains a cable length limit of 5 meters, a cable delay spec of 30ns and
a supply voltage minimum at any USB socket of 4.4 volts.
http://www.usb.org/developers/whitepapers/cablew~1.pdf
PL
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Pete LongConsultantCommented:
isaacmc,
ELECTRICAL SPECIFICATIONS
USB transfers signal and power over a four wire cable, shown in
below. The signaling occurs over two wires and point-to-point
segments. The signals on each segment are differentially driven into
a cable of 90W intrinsic impedance. The differential receiver features
input sensitivity of at least 200 mV and sufficient common mode
rejection.
There are two modes of signaling. The USB full speed signaling bit
rate is 12 Mbs. A limited capability low speed signaling mode is also
defined at 1.5 Mbs. The low speed method relies on less EMI
protection. Both modes can be simultaneously supported in the
same USB system by mode switching between transfers in a device
transparent manner. The low speed mode is defined to support a
limited number of low bandwidth devices such as mice, since more
general use would degrade the bus utilization.
The clock is transmitted encoded along with the differential data.
The clock encoding scheme is NRZI with bit stuffing to ensure
adequate transitions. A SYNC field precedes each packet to allow
the receiver(s) to synchronize their bit recovery clocks
The cable also carries VBus and GND wires on each segment to
deliver power to devices. VBus is nominally +5 V at the source. USB
allows cable segments of variable lengths up to several meters by
choosing the appropriate conductor gauge to match the specified IR
drop and other attributes such as device power budget and cable
flexibility. In order to provide guaranteed input voltage levels and
proper termination impedance, biased terminations are used at each
end of the cable. The terminations also permit the detection of
attach and detach at each port and differentiate between full speed
and low speed devices.
http://www.semiconductors.philips.com/acrobat/various/USBSTAND.pdf
PL
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lidskyCommented:

 PeteLong,
Just one crucial bit of information missing from your post. Max current available on a port.

Max current is 500ma. If you are using an unpowered USB hub then the total current available will be 500ma split across the ports.

 isaacmc,
I don't know what kind of light you are thinking of installing, but it will not be very bright.
500ma * 5v = 2.5 watts
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terageekCommented:
The physical connector has 4 terminals.  The 4 terminals are:
1 - VBus - Red Wire  (Nominal 5V, spec requires 4.4-5.25 Steady state.  Transient can dip to 4.07)
2 - D-     - White Wire
3 - D+    - Green Wire
4 - GND  - Black Wire (return current)
Shell - Shield - Drain Wire (earth ground)

In a series A receptacle (the wider one), the connectors are numbered 1-4 from left to right when looking into the receptacle where the connectors are hanging down.

In a series B receptacle (the narrower squarish one), the connectors are numbered 1-4 in a counter-clockwise direction starting in the upper right hand corner when looking into the receptacle with the narrow side up.

Actually, you may want to keep the current under 100mA.  In suspend mode, the long-term average current draw is supposed to be under 500uA, but you are allowed to draw up to 100mA for up to 1 second.

There are a number of ways to limit the current you draw.  You can put a resistor in series with the bulb, or you can build a current source using a low power op-amp which can be used to power a bright LED.
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Pete LongConsultantCommented:
ThanQ
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MidionCommented:
This solution has helped me out alot. I'm currently deployed in Iraq and was sent a christmas tree. The lights are powered by 2 C batteries. I used the information from this solution to help power my lights through a USB cable. I used the red cable for the Pos and the black cable for the Neg. Everything works great and the lights are actually bright instead of dim like when they had been powered by the batteries.
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Pete LongConsultantCommented:
Brilliant! Hope you have a quiet tour, and you, and your friends all come back safe.

Pete Long (formerly CPL P A Long Royal Engineers)
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