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Re: [Discuss-gnuradio] discussion on USRP-->Wall Socket for Power Line C

From: John Ackermann N8UR
Subject: Re: [Discuss-gnuradio] discussion on USRP-->Wall Socket for Power Line Comms
Date: Tue, 21 Feb 2012 16:13:22 -0500
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Well, I'd put it that large resistors result in high impedance. You still have the full line voltage across the output of the two resistors, but any load resistance will form a voltage divider and drop the voltage very quickly.

A simple next step would be adding an appropriate load resistor and using an op-amp as a follower to provide a low impedance output to drive the BasicRX.

I'm not sure why you'd need to unground anything -- the output is an AC signal, and the resistors limit any current to a miniscule and safe level. Folks have been plugging this design into all sorts of grounded gear with no issues at all.

On 2/21/2012 3:57 PM, Evan Merewether wrote:
Two issues:
1) Large resistors results in large attenuation.  This circuit attenuates
120VAC to ~5V for the microcontroller.
2) You would need to un-ground all of the components (USRP + PC + you) and
float everything to somewhere around 60V

-----Original Message-----
From: address@hidden
[mailto:address@hidden On Behalf Of
John Ackermann N8UR
Sent: Tuesday, February 21, 2012 1:45 PM
To: George Nychis
Cc: discuss-gnuradio
Subject: Re: [Discuss-gnuradio] discussion on USRP-->Wall Socket for Power
Line Comms

On 2/21/2012 3:00 PM, George Nychis wrote:
Okay!  So apparently there is some interest in power line communication
for GSoC.  But, what we would want to do is already have a safe way of
connecting the USRP in to the wall socket for the student(s), and for
the future of GNU Radio and USRP power line communications development.

So, as a goal of this thread I'd like to get some feedback on how we can
make this possible.  Ideally, something off-the-shelf would be great,
providing the highest amount of safety for those experimenting with it.

Here's a very simple approach:


There was a bunch of discussion about this on the time-nuts mailing
list, and some folks suggested changes for increased safety (including
putting two resistors in series so that if one fails short -- which is a
very unusual occurrence -- there's an extra layer of protection.  But in
general, with large value resistors the current is limited to a very
safe level -- and it's current that kills, not voltage.


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