PoE Midspan With Switch Tested

By: John Scanlan, Published on Mar 23, 2016

Both PoE switches and midspans generally provide power to network devices.

One question that is regularly raised in IPVM classes is whether combining the two will cause problems, e.g., the camera to be fried.

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PoE *** ************ **********PoE is commonly used to power IP cameras, eliminating a separate cable and supply for power. In most cases, a PoE switch combines both switching and power supply into a single unit. However, for older non-PoE switches or where PoE demand outstrips supply, a separate unit called a midspan is often used. More: ***: ****** **. ************* ***** *** ** Video ************.

Test *********

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Measuring *****

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Midspans ****** **** ******

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Camera ******* ** ******

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Explaining *******: ** ******* *****

** ** ***** *** results:

  • *** ****** ******** ********* to *** ****** ** the **** ****** ********* power ** ******.
  • **** *** ******* ******, the ****** ** *** providing ***** ** *** camera *** ** *** injector ******* ***** ********* to ****.
  • **** *** ****** ** receiving *** ***** ** this ********.

*** ****** **** ** the **** ** *** to *** *** *********** process ******* ****** *** source. ** *** *****, the ****** ********** ****** as ********** *** **** the ******* ********, ***** sends ** *****. *******, the ******* ******** **** not ******* *** **** the ******, ** ** power ** ****. ** this ***, ****** ** preserved *** ** ***** is ********* ** *** devices ****** ************ *********.

PoE ***** ******* ****

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** *** **** ** our ******* *** ******, no ******* ** **** nor ******** ** *** midspan, ** *** ****** sends ** *********** ******** power.

Comments (2)

Nice work, John!

On a more basic level I would mention that there is really no danger of frying a camera because of double POE for the simple reason that even if both the midspan and the endspan were to both (mistskenly) provide power, this wouldn't do anything to the camera as it is just power in parallel.

So the same voltage, though double the potential max amps. But since same resistance it just draws the same.

Here's a worst case scenario to show what I mean: Here I have taken two passive POE sources and put one on the 1236 and the other on the 4578, by using to injector cables and a custom keystone.

So this means that they are both always on. I was hoping that the byte brothers would show this as 12345678 POE, but instead it just alternates between 1236 and 4578 every seconc or so.

In any event the camera works fine since there is no increase in volts. I have the closeups if you need want to see anymore.

Looks goods! I was also hopeful that power panel would test / display 12345678 at the same time.

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