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-   -   Power supply from USB (http://www.physicsforums.com/showthread.php?t=51021)

kdkdkd Nov2-04 06:16 AM

Power supply from USB
 
hi,

I want to power a digital pen using a USB connected to the PC. What is the exact procedure for this...as in do i require any external circuitry?
I also heard about this DTR pin which when permenantly enabled, acts as a power supply.
can anyone enlighten me about these things?
thanks.

imabug Nov2-04 07:38 AM

Perhaps something in the USB FAQ might help answer your question

chroot Nov2-04 02:05 PM

Take a look at the USB connector pinouts:

http://www.networktechinc.com/technote.html

The USB connectors have four pins. Pin 1 is +5V, pin 4 is ground; these pins provide +5V power. The USB specification allows a peripheral to draw up to 500 mA from the power pins. Pins 3 and 2 are the differential + and - signalling pins, respectively.

The DTR pin is one of the control pins on the PC's serial port, NOT the USB port. You could potentially use any of the computer's serial port pins for minimal power supply, but it's not a good idea, since they aren't designed for that purpose. There's no specification on how much current it may or may not be able to provide.

If you're designing a USB peripheral, you can use up to 500 mA of current from the USB's power pins. If you're designing a serial-port peripheral, you should really externally power the device.

- Warren

kdkdkd Nov2-04 10:40 PM

Thanks Warren and imabug.

Guess I'll go for a USB rather than a serial then.


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