On Oct 24, 2008, at 8:32 AM, Steve R wrote:
> > At 11:50 PM -0400 10/23/08, Charles Davis posted: >> On Oct 23, 2008, at 8:23 PM, Steve R wrote: >> >>> I've googled galore and not found an answer. Maybe the simplest way >>> to ask is, if I am using a USB hub for only one device, does the >>> USB >>> hub act as a pass-through cable extender so that it need not be >>> powered? Is there enough power coming from the internal USB port to >>> continue through the hub to the USB device, If you only have one device plugged into the hub, then, yes, it acts basically as an extender cable. There is a miniscule amount of power that the circuitry of the hub uses, but with almost any single attached device it is not a meaningful amount. The only time the power the hub draws may be meaningful is for larger or power hungry devices that draw more power than the usb spec allows a device to draw. I do not have any experience with this, but maybe an external HD or scanner that draws all its power from the usb port COULD have issues in not getting enough power. But if the device is a keyboard, mouse, digital camera running on batteries, a printer, scanner or external HD with a wall plug, there should be no issues. >> possibly, but what is supposed to tell anything else plugged into >> the >> hub that it's NOT the one wanted, and to not draw any power? See above, he is only talking about 1 device plugged in. > > Is a USB hub with unpowered devices like lamps plugged into a power > strip (USB hub) that is plugged into a wall receptacle (USB port)? > The cords (cables) are always powered regardless of whether the lamp > (device) is turned on? A basic electrical primer, vastly simplified: Electricity flows in a loop, or circuit. Whether it is DC (direct current) always flowing in one direction or AC (alternating current) which changes direction 60 times a second (or 50 depending where you live) it needs a complete closed loop to flow. Any break in the loop and no current will flow and no power is used. That break can be a lamp switch, cut wire or a burned out bulb. As soon as you complete the circuit by fixing the break (turning on the lamp switch, repairing the cut wire or replacing the bulb) current flows again. Therefore, a lamp plugged into an outlet with the switch off does not complete the circuit and draws no power and there is no current flowing through the wires. When you switch the lamp on or your dog bites through the lamp cord, the circuit is completed and power and current flow through the wires. For simplicity, I'll divide usb devices into two categories, but there are going to be many exceptions. USB devices without an external power source like mice and keyboards will always draw some power to run their internal circuitry polling the keys or the optical sensor to detect input. They will draw more power when doing something than when at idle, but even at idle they draw some. USB devices with a wall plug and/or a power switch like a printer or external HD will draw basically no USB power when they are not turned on. When you turn them on, they will draw some USB power, but not a lot since most are using usb strictly as a data conduit and not power to run an electrical circuit like a keyboard.. So a USB device that is on, whether it is a printer with the power switch turned on or a mouse waiting to be moved, will draw power. A USB device that is not turned on like a printer or scanner with the power switch turned off or power brick unplugged will not draw any usb power. HTH, Len --~--~---------~--~----~------------~-------~--~----~ You received this message because you are subscribed Low End Mac's G3-5 List, a group for those using G3, G4, and G5 desktop Macs - with a particular focus on Power Macs. The list FAQ is at http://lowendmac.com/lists/g-list.shtml and our netiquette guide is at http://www.lowendmac.com/lists/netiquette.shtml To post to this group, send email to [email protected] To unsubscribe from this group, send email to [EMAIL PROTECTED] For more options, visit this group at http://groups.google.com/group/g3-5-list?hl=en Low End Mac RSS feed at feed://lowendmac.com/feed.xml -~----------~----~----~----~------~----~------~--~---
