Re: DC in Cat5

[Richard Malcolm-Smith <rich@xxxxxxx>]

happy_hammer66 wrote:

> instead of using multiple AC/DC converters for every little toy i
get
> and to improve presently low levels of WAF, i was thinking of
having
> two or three central DC supplies at node 0 (set for various levels
ie.
> 5V 9V 12V) and running the DC on pins 7 and 8 of the RJ45.
> 
> Is this a no-no?
> 
> The only problem i forsee is whether the amount of current you can
> supply via cat5 cable is too low, but some of the toy's should
draw
> quite small amounts of power.

If the devices will connect to each other by any non-isolated means,
(Serial, open collector output etc) then the answer is hell no!

Many things have the ground of the DC not connected to the ground of
the
signal wires. My own remote LED to display weather station info has the
18v plugpack with the + as the ground of the circuit, a -5v regulator
for the Vdd of the logic, and the - from the PSU gives the -18v that
the
mammoth LED displays require.

I have an opto on the serial input to this so it can run via cat5 and
still be ballanced. I still have protection on the opto like a 4.7v
zener across the opto and its resistor, and 100ohm resisitors inline
with both cables. I got game and plugged it into a phone line to see
what would happen (worst you can do in a patching situation) and the
100ohms get slightly warm.

I just drive it from an RS232 port directly. I figure the zener and
resistors will take care of anything. I had probs with it inverting the
bitstream untill I figured that I was using a phone cable not a data
cable (DOH!)

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