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View Full Version : 802.3af - PoE - How does it know what voltage?


Merijeek
02-08-2007, 09:43 AM
Okay, I've got me a fancy PoE switch here.

I plug in two IP cameras. One camera runs, if you power it from a transformer, 5VDC at 800mA. The other, if you power it from a transformer, runs 9VDC at 1000mA.

Now, I recognize that there's generally some forgiveness when powering some electronics - after all, if you splice that 5VDC transformer and put another hundred feet of wiring in there your camera is probably getting 4VDC if you're lucky, and right at the transformer you'd probably measure and find you're getting something like 6.5VDC.

But how does a PoE switch know how much voltage it's supposed to be sending?

-Joe

carnivorousplant
02-08-2007, 10:32 AM
The same way a light bulb knows if you plug it into 220v instead of 110. :)

There is probably a voltage regulator in there expects to see the correct voltage. Smoke will come out the back if you apply a much larger voltage.

FatBaldGuy
02-08-2007, 10:41 AM
It might help if we knew what a PoE switch is. Many/most transformers for cameras, computers, etc. can run on either 110 or 220 volts. The circuitry inside senses the input voltage and adjusts accordingly.

carnivorousplant
02-08-2007, 10:58 AM
It might help if we knew what a PoE switch is.

POE (http://joejava.com/poe.htm) is apparently a standard to deliver power over data cables.
If a device is using a block tramsformer, I wouldn't think it would comply with that standard. Does the OP mean that they are getting power without the transformers?

si_blakely
02-08-2007, 11:43 AM
It doesn't. Power over Ethernet (http://en.wikipedia.org/wiki/Power_over_Ethernet) just provides 48V DC (if it is 802.3). The device has to regulate that to an appropriate level.

If your PoE device can use a 48V supply, why don't the manufacturers just use a wallwart that outputs 48V. It would make for a nice easy standard PSU.

Si

AndrewL
02-08-2007, 01:06 PM
The circuit in the POE powered device is actually fairly complex. There's a negotiation process which goes on between the POE switch and the POE device in which the switch carefully puts small amounts of voltage on the port and looks for a characteristic impedance in response. This is to maintain compatablility with non-POE devices that might be damaged if unexpectedly powered. Once the POE handshaking has been performed the POE switch will be providing 48V on the port. However, the POE device circuit usually maintains electrical isolation between the port and the local device ground by chopping the 48V into AC and passing it through a transformer, and it's easy to change the 48V into whatever voltage the camera needs in the process.

Merijeek
02-09-2007, 06:08 AM
The circuit in the POE powered device is actually fairly complex. There's a negotiation process which goes on between the POE switch and the POE device in which the switch carefully puts small amounts of voltage on the port and looks for a characteristic impedance in response. This is to maintain compatablility with non-POE devices that might be damaged if unexpectedly powered. Once the POE handshaking has been performed the POE switch will be providing 48V on the port. However, the POE device circuit usually maintains electrical isolation between the port and the local device ground by chopping the 48V into AC and passing it through a transformer, and it's easy to change the 48V into whatever voltage the camera needs in the process.

Damnation, I had a feeling it was going to be something like that. I was basically hoping to find a way, probably by doing a splice through a jack, to siphon off power to power an external camera housing. They typically run at 12vdc or 24vac.

Looks like it may be something a bit beyond my kitbashing skills.

-Joe

Mangetout
02-09-2007, 06:49 AM
Strikes me that there's a market for generic POE receivers - perhaps with an isolated passthrough for the data and some kind of standard power output, perhaps with modular plug-ins for different output voltages...

yoyodyne
02-09-2007, 07:48 AM
Strikes me that there's a market for generic POE receivers - perhaps with an isolated passthrough for the data and some kind of standard power output, perhaps with modular plug-ins for different output voltages...They're called POE "active splitters" or "regulated taps".

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