parasitic power

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JohnRoberts

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While I was putting a few of my computers onto a X4 KVM switch  I noticed without any of the USB jacks connected with only one computer powered up the monitor video did not pass...

As soon as I connected one USB jack the video worked, apparently the switch electronics get their power from the USB jacks.

JR
 
It's not a mechanical switch. Whether relays or switch-chips or buffers, there's got to be juice.

I thot you threw a wire out the window and found free power. Parasitic from a 100KV power line or 50KW AM transmitter. When I lived near the old WCAU I could pull many Volts off a yard of wire.
 
Many years ago there was a story of a guy in the UK who lived close to a new multi KW TV transmitter. He wound a coil of some number of turns around his attic space and powered his house lights from it. He got nicked when people near him complained they could not receive the new transmitter despite being very close to it.

Cheers

Ian
 
The USB standard anticipates supplying modest amounts of 5V power.

Another interesting technique for an electronic device to steal operating power is my heater thermostat, that connects in series with the heater element load, so even in the summer, sends it's modest operating current through the heater element.

JR
 
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ruffrecords said:
Many years ago there was a story of a guy in the UK who lived close to a new multi KW TV transmitter. He wound a coil of some number of turns around his attic space and powered his house lights from it. He got nicked when people near him complained they could not receive the new transmitter despite being very close to it.
Cheers
Ian
Are any of us supposed to believe that?

A TV transmitting antenna is at the top of a 100 to 300 meter tall tower.  Most of it's power is aimed at the horizon.

No he is not going to get more than a few watts. We tried it near the base of an extremely powerful FM transmitter.  If you held a old 48 inch florescent tube at the right place it would light dimly.
 
> Are any of us supposed to believe that? 

More to not-believe:
http://forums.theregister.co.uk/forum/1/2013/08/14/boffins_hawk_powerless_radio/

While reading 1930s Wireless Worlds I did run across the question, and the legal opinion that it amounted to "stealing", but no technical evidence any significant Watts had been sucked. A bigger and more practical issue was that domestic electric was supplied for Lighting and Appliances at two different rates. Radios are not lights, but were universally plugged to light outlets.
 
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