RE: Re: Power over ethernet and ePODs

["Keith Doxey" <lists.diyha@xxxxxxx>]

Hi All,

 

2A @ 10V = 20W

 

The "proper" PowerOverEthernet (POE) specs can supply somewhere around that amount of power .....

 

BUT......

 

the way it works is to use a higher voltage somewhere between 40 - 55volts and then use a switchmode power supply at the distant end to providfe the correct regulated voltage. The benefits of this are twofold

1. The current for a given power is less at a higher voltage

2. Lower current means lower volt drop and with the higher starting voltage the percentage of power lost is much less.

 

There are TWO serious flaws with your idea.

 

1. You need 2 AMPS - CAT5 is only rated somewhere between 250-350 mA. Admitedly you can use 2 wires to double the current rating but you have to consider what happens if one wire breaks or a plug has a bad connection, the other wire is then overloaded.

 

2. Not only do you get voltage drop - but if the power supply is grounded on its 0V rail you also get ground lift at the distant end.

 

Assuming a 12V power supply and 2V drop the distant end will receive 10V but this will be across the pair.

 

The true voltages with respect to the power supply ground will be

Epod 0V will actually be +1V

Epod 10V will actually be +11V

 

Thats fine as long as no other connections exist to the Epod. But as you are using ethernet there will be a 1V offset sitting on the RS422 connection to the hub. This probably wont have any harmful effects.

 

If you then connect something else to the Epod that has a grounded connection eg an Amplifier etc, this will cause the Epod 0V rail (currently at +1V) to be dragged down to ground possibly giving a nice hum to the sound. Because the voltage drop occurs due to the resistance of the cable and you have effectively reduced the resistance by providing an alternative path back to the PSU there will be less voltage drop. This means there will be more voltage delivered to the Epod. 

 

Dont forget the safety aspect as well, to power 5 Epods you need a power supply capable of 10A. Connecting a cable with a 250mA rating to a supply capable of 40X that is ill advised without careful fusing arrangements. It is effecctively the same as connecting an electric kettle directly to the incoming mains feed with lighting flex!

Keith

www.diyha.co.uk
www.kat5.tv

-----Original Message-----
From: Kevin Hawkins [mailto:lists@xxxxxxx]
Sent: 29 July 2002 12:42
To: ukha_d@xxxxxxxSubject: RE: [ukha_d] Re: Power over ethernet and ePODs

    Hi Graham

 

     - it's probably not a very good idea to try and anticipate a voltage drop down a cable to reverse engineer the correct voltage at the other end. A few reasons spring to mind...

 

    1) The ePods and other devices take a varying current and it is the current when passing through some resistance that causes a voltage drop (yep ohms law) and so the voltage drop down the CAT5 will vary with what the ePod is doing, the backlight contributing significantly.When the unit was in power save mode the voltage may rise causing risk.

 

    2) Possibly the biggest resistance in the line would be caused by any connectors that were in circuit but their actual resistance may be a little variable depending on physical issues. (I haven't looked at the POE design to see if there are any connectors in line.) Where there is resistance, heat is generated.

 

    3) Not all runs to epods would have the same voltage drop so you would need different source voltages anyway.

 

   The ePod appears to require 10V DC (an unusual voltage) at up to 2A. Presumably the 2A would be with the machine running full tilt, all slots occupied and charging the battery as well. In typical use this may be much less and you might not have a problem with the CAT5 anyway. What would be interesting to know is if there is further on board regulation in the ePod in which case it may well compensate for any small voltage losses anyway. Not sure how well regulated the ePod psu's are.

 

    The technical solution would be to have a spare wire coming back from the ePod end that carried no current and was used to sense the voltage at the ePod end and automatically raise or lower the source supply to compensate but again this facility is only in the more expensive PSU's and would not work for multiple loads. e.g. several ePods.

 

    I would be tempted to try it and see,  feeding the existing supply through a longish length of Cat5 and seeing if the ePod powered correctly, it is unlikely to do any real harm to the ePod (I guess there could be some data corruption potential).What sort of length of run of Cat5 are we talking about ?? Avoid connectors if poss.

 

    The cable will have a spec somewhere on resistance per metre although there may not be a guideline to current carrying potential as it is intended for data signals only. I know people use them for speakers and 2A into 8 ohms would be 32Watts or 16W into 4 ohm speakers. 1A would be only 8 watts. The voltage drop would sort of be ignored though just producing a less accurate sound. All I am saying is that 32W of power is more than people are using them for so this is pushing it in comparison. How many conductors are available for the power ? Pairing them if poss could help - I haven't looked at the article.

 

    I am sure Keith would have a view on a current handling capability. 2A is quite hefty though.

 

    Kevin

 

 

 -----Original Message-----
From: graham_howe [mailto:graham@xxxxxxx]

So the only remaining question is about the power supply itself.
Obviously I don't really want to have 5 power supplies at node zero
for the 5 ePODs and anyway I doubt very much that the ones supplied
with the ePODs would be sufficient over a 50m run. So what I really
need is a big power supply that can support 5 ePODs concurrently
(hence bigger current draw) and can supply adequate voltage over 50m
of cable (hence higher voltage to allow for drop).

Does anyone have an idea where I might get a suitable PSU and also
could anyone with the elecronics knowledge tell me what spec I should
be looking for (I assume it all comes down to Ohms Law, but I
wouldn't trust myself to do the calculations).

Regards

Graham





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