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

[Nicholas Twist]

Hello again Adrian and everyone else.

My understanding of plug packs is the following (please correct me if
I am wrong). The rated voltage at the end of the plug is rated usually
higher than the voltage listed on the pack itself. When this is the
case it usually means the plug pack is not regulated but providing
enough voltage to power the electronics (I've gotten 12-16v dc off
packs rated at 9-12v dc respectively). Usually (notice the vague qualifier), this means there is some sort of regulation inside the unit themselves. The Linksys wrt54g we all seem to be enamored with is one such device where the electronics inside run off 3.3v..

However, I think those plug packs for those are designed to give the proper voltage for the circuit board straight and like you are suggesting we need to be careful not give them too much voltage (well it couldn't be much lower so it must be powering the device directly ;-). The only other working voltage for a circuit board like that is 3.3v and there isn't much headroom from 5v to 3.3v for it to be stepped down easily...

I have a few wall warts I've collected over the years and can make them available to the whanganui GNU/Linux slash wireless crew if I can dump them on someone to hand out if needed (I am trying to clear out stuff). I also have a proper 12v regulated power supply if that is needed too. One thing you have to be careful with in regards to mating good quality electronics to various mish-mash plug packs is that some plug packs are terrible when it comes to filtering out the AC and can have all sorts of junk coming out of them.

Best,
Nicholas

On 11/10/2008, Adrian Soundy <liberation_software at paradise.net.nz> wrote:
> I just found one of the multimeters(old one) I was using wasn't correct so
> some measured voltages were wrong
> The PSU which comes with routers is 5.2v
>
> With 27m cable
> Boot 5.2v - 1.1v = 4.1v
> Radio on 5.2v - 1.9v = 3.3v ( too low to run)
>
> With 20m cable
> Radio on 5.2v - 1.4v = 3.8v ( must be just enough as it worked)
>
> The regulator in the router regulates the input to 3.3v
>
> Changes length / PSU voltage to:
> 0   -> 15m	standard (5v)
> 15m -> 50m  7.5v
> 50m -> 70m  9v
> 70m -> 100m 12v
>
> This should be right now
>
> Adrian
>
>
>
> -----Original Message-----
> From: wha-net-discuss-bounces at one.server1.org
> [mailto:wha-net-discuss-bounces at one.server1.org] On Behalf Of Adrian Soundy
> Sent: Saturday, 11 October 2008 3:31 p.m.
> To: 'discssion list for wha.net.nz'
> Subject: Re: [wha-net-discuss] wireless net test 2 poe
>
> 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.
>
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