For the math heads, I was looking at the voltage solution and the current
solution and noticed that if I set the radical to zero, that is the point
where the quadratic blow up. I knew that was the minimum working voltage.
Anything above that and we are not a complex solution.
So I set the radical (perhaps the discriminant) equal to zero and solved.
-----Original Message-----
From: [email protected]
Sent: Saturday, March 11, 2017 11:35 AM
To: [email protected]
Subject: [AFMUG] So anti-climatic
This is what I was looking for when I started this quest.
The formula is so simplistic, it almost seems like for the amount of work I
put into it it should be more complex.
If you cannot see the image, there is the bottom line.
If you have a device that uses a switch mode regulator, which is many things
these days because the are efficient, cheap and small, you have a device
that is essentially a constant power load.
If you have to power that constant power load, (like an SM or a VDSL
ethernet line extender) over wires that have a significant amount of
resistance, like 1000 feet of 24 gauge twisted pair, there is a lower
voltage beyond which they will not operate. (I would say to not exceed
their input voltage either without zener protection).
That voltage is:
V(supply min) = 2 sqrt(PR)
Where P is the power of the load.
R is the power wire resistance.
In my case, I need 48.99 volts or more.
