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nevolex
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One power supply for 2 devices

Fri May 01, 2020 9:48 am

Hi guys,

I am planning to buy a RB4011iGS+RM
it has a Max power consumption without attachments of 18 W

and an Audience
it has a Max power consumption without attachments of 28 W

I would like to use one power brick for both devices, connecting it to the RB4011iGS and using POE out for the Audience. Total power consumption is 45 W, I assume there will be some loss on poe line, though.

However, if I would get a MT48-480095-11DG power supply (45.6‬ W max total / 48 V, 0.95 A)

Will that work?

Thank you
Last edited by nevolex on Fri May 01, 2020 11:45 am, edited 1 time in total.
 
svmk
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Re: One power supply for 2 devices

Fri May 01, 2020 11:14 am

Also, I have a similar question. I would like to know if I can connect several (3-5-10) Mikrotik devices via PoE to one backup power supply of the required power. Will such a connection cause unstable operation of devices.
 
pe1chl
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Re: One power supply for 2 devices

Fri May 01, 2020 11:38 am

The specified max power out for PoE output from the RB4011iGS+RM is too low to power the Audience... it is about 22W max.
I would say you cannot power the Audience that way.
When you want to avoid having a power connection near the Audience I would say you can use a PoE inserter (RBGPOE) and the normal power supply included with the Audience both placed near the RB4011iGS+RM and then power it over the ethernet cable.
 
pe1chl
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Re: One power supply for 2 devices  [SOLVED]

Fri May 01, 2020 11:40 am

Also, I have a similar question. I would like to know if I can connect several (3-5-10) Mikrotik devices via PoE to one backup power supply of the required power. Will such a connection cause unstable operation of devices.
Always check the specification of the used power of the devices, the max power per port of the supplying device (router or switch), the max total power it can deliver, and the max power of the powersupply.
See above.
 
svmk
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Re: One power supply for 2 devices

Fri May 01, 2020 12:14 pm

As for ensuring sufficient power supply, this is a necessary condition. I'm more interested in the stability of devices connected to the same power source. With this connection, the “minus” DC of all devices is connected to one point, there is no galvanic isolation.
 
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mkx
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Re: One power supply for 2 devices

Fri May 01, 2020 12:54 pm

If devices are inter-connected with conductive cables, then they are not truly galvanicly isolated. And conductive cables include UTP cables. If you want to galvanically isolate devices, then you need to use a stretch of optical cable on interconnect ... and certainly use separate power adapters, possibly conencted to mains via separate fuses.

However, OP was (implicitly) asking about powering multiple devices via PoE, which by definition doesn't provide galvanic isolation. With this concept there are only a few considerations and in all-indoor installation only one really matters: the consideration mentioned by @pe1chl which is PoE PSE capacity (both total and per port). Surely there are other considerations when doing outdoor installations and there galvanic isolation becomes a factor ... however, overvoltage protection is likely bigger problem.
 
pe1chl
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Re: One power supply for 2 devices

Fri May 01, 2020 1:54 pm

With galvanic isolation, you need to differentiate between "large overvoltage" like lightning strike, and the normal voltage differentials found in a large building.
For large voltages, fiber is the only safe way. And of course there is no Power-over-Fiber so you will need to arrange local powering.
But for normal voltage differential, in fact 802.3af/at PoE *is* isolated, or at least it should be. A device powered from 802.3af/at PoE should have a small switchmode converter between the ethernet line and the device ground. And so does ethernet itself.
It is not correct that UTP cable does not provide isolation! An ethernet link has an isolation transformer.
Typical isolation voltages to expect from ethernet and PoE are 1500 V. Of course this is not sufficient for lightning.

This is not the case for "passive PoE", that provides no isolation guarantee. I don't know if typical MikroTik devices provide isolation for passive PoE, probably not.
(there is a switchmode converter but it probably operates in buck-mode rather than transformer mode)
 
svmk
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Re: One power supply for 2 devices

Fri May 01, 2020 4:12 pm

This is not about a "large overvoltage" - all the devices are in the same room, connected to the same LAN and are powered from the same source. All external links are fiber optic. Will several Mikrotik devices work stably if connect them, in addition to the main power supply, to a single backup power source (for example, 24 V batteries) through passive PoE.
 
pe1chl
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Re: One power supply for 2 devices

Fri May 01, 2020 4:47 pm

Will several Mikrotik devices work stably if connect them, in addition to the main power supply, to a single backup power source (for example, 24 V batteries) through passive PoE.
Please re-read answer number 4.
 
sindy
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Re: One power supply for 2 devices

Fri May 01, 2020 6:31 pm

I guess this new question is actually different from the topic title and asks whether simultaneous connection to two power sources (to one via PoE and to the other one via the regular DC connector) won't cause any trouble. If so, the answer is "yes, this is safe, the source with higher voltage will be used, and the 'failover' between them will happen unnoticeably".

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