[wha-net-discuss] wireless net test 2 poe

[Adrian Soundy]

I run some test to look at the POE requirements for the mini routers to see
why we were having problems.

I set up a router with 2 multimeters, one for voltage and other to measure
current
The PSU which come with the routers supplies 6.4v although marked as 5v

I measured current at various times:

Booting  ~= 440ma
Once radio starts up current varies between 570ma & 750ma
So max is about 750ma

I hooked up a long piece of cat5 cable (27m) and tried again
Routers boots but radio never comes on
Measured voltage at router was ~5v down from 6.4v

I thought I would calculate what the voltage drop would be at 750ma so I
searched the web for cat5 resistance and found following page which talked
about resistance and voltage drops with POE :
http://freenet-antennas.com/PHP-Nuke/modules.php?name=News&file=article&sid=
4

Resistance for cat5 as used by POE is 0.094 ohms metre.
So I calculated what my voltage drop for 27m was using  voltage = current *
resistance

Voltage = .75 * (27 * 0.094) 
= 1.9v

This means when radio switches on the voltage would drop to  (6.4 - 1.9 ) =
4.5v which wasn't enough to run it
When booting it draws only 440ma = 1.1 volts drop = 5.3v which was enough
So as soon as radio switched on the voltage dropped below minimum.
I calculated that 20m was about the maximum cat5 length on standard PSU so I
cut cable to 20m tried it again and it all worked.

There is a couple of ways to fix problem

Use plug pack with more voltage using following table depending on length :
Lengths for standard plug pack voltages (7.5v, 9v, 12v )
Up to 15m standard psu 
15m -> 35m = 7.5v
35m -> 55m = 9v
55m -> 70m = Not standard voltage
70m -> 100m = 12v

You really need a regulated PSU to hold voltage at correct voltage while
current changes. If the voltage get too high it causes the regulator in the
router to overheat and to cut out. One PSU I tried was marked at 9v/2a but
when measured was more like 12v. so it was probably only 9v when drawing
2amps. 

Another way is to have a separate regulator at the router end so that it
always supplies 5v to router similar to what is built in to some other
routers. Then any PSU over the required voltage would do. 
Really you need a switch mode regulator (DC to DC converter). The cheap
linear mode regulators would work but generate heat and also waste power.
You don't really want to generate any more heat in a waterproof box on the
roof then necessary.

A commercial product is the Dlink dwl-p200($100)
http://www.dlink.co.nz/Products.aspx?Sec=1&Sub1=12&Sub2=20&PID=119


So if anybody has a source for switch mode regulated plug packs of 7.5v or
9v then lets us know.