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sihubert
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mUPS with two different DC power inputs?

Wed Feb 13, 2019 1:47 pm

Hi everyone,

I wonder what are the differences between PoE-in and DC-in in the mUPS?

Could I use both in parallel, possibly with slightly different DC voltages? If yes, what would be the "priority" between those two inputs?
Do both inputs feed power to PoE-out? If yes, I don't understand why MikroTik would ship an accessory PoE injector...

For a bit of context, I would like to use two redundant DC power inputs, PSU1 and PSU2. That is:
- use PSU1 to feed PoE-out and charge battery
- if PSU1 is down, use PSU2 to feed PoE-out and charge battery
- if both PSU1 and PSU2 are down, use battery to feed PoE-out.

Is this scenario possible using the two power inputs of the mUPS (PoE-in and DC-in)? Or do I need external hardware?

Thanks in advance!
 
mkx
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Re: mUPS with two different DC power inputs?

Wed Feb 13, 2019 5:04 pm

I can't say how things work on mUPS in particular. I have read an explanation by @Normis that when device (a RB device with PoE in) has connected both PoE in and barrel-plug, it'll use which ever power source has higher voltage by some margin (makes sense as it only involves a pair of diodes to make this happen). If one intends to use both power supply options, it is best to have some systematic difference in voltages so that power source used won't change too much.

The scenario of changing power source is the following: let's say that PoE in has 0.5V lower than barrel. Then device will start using barrel power source. As power draw increases, the power adapter will drop voltage slightly and it might decrease to 0.5V below the PoE in. So device will switch over to using PoE in. This will drop voltage on PoE in and allow barrel to increase again, causing barrel voltage to rise significantly above PoE in voltage. And RB will change power source over again.
This changing doesn't harm much the device itself as voltage fluctuations won't be critically high. However it might upset both power adapters due to constant switching between full power draw and idle states.

One of intended installation places of a mUPS is also directly on tower / antenna pole, where AC power might not be available. Hence possibility to power device over ethernet cable using provided PoE injector. If, however, AC power is available near the mUPS, it is always advisable to use barrel-type power input due to lower power drop.

Your scenario is quite feasible if mUPS behaves the same way as other RB devices. What I'd do is the following:
  • feed barrel-type power input using provided 24V power adapter
  • feed PoE-in using provided PoE injector and non-standard 18V, 2A adapter (this gives same wattage as provided power adapter).
    Possibly use short ethernet cable between PoE injector and mUPS so that voltage drop (and power loss) is kept at minimum. This might not be feasible if you need long ethernet cable to connect to some switch ...

Accroding to mUPS brochure, mUPS regulates input voltage to 24V if input voltage is lower than that. Hence mUPS-powered devices won't notice the switch-over from barrel power source to PoE in.
BR,
Metod
 
sihubert
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Re: mUPS with two different DC power inputs?

Thu Feb 14, 2019 1:27 pm

Thanks a lot mkx for your detailed answer! Very useful information indeed.
I'll test your suggested procedure and come back to share the results. My only worry is if power inputs from PoE-in and barrel are simply shorted together, but I don't expect such dangerous design from MikroTik...

In the meantime, anyone having more accurate information is more than welcome to share :wink: .

One of intended installation places of a mUPS is also directly on tower / antenna pole, where AC power might not be available. Hence possibility to power device over ethernet cable using provided PoE injector. If, however, AC power is available near the mUPS, it is always advisable to use barrel-type power input due to lower power drop.
I perfectly understand the usefulness of having both RJ45 PoE-in and barrel-type connectors on mUPS. However, since barrel DC-in is there, I don't understand the need for an accessory PoE injector included in the mUPS package. That is, except if two power inputs are tolerated - or better, if such redundacy is considered by design.
 
mkx
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Re: mUPS with two different DC power inputs?

Thu Feb 14, 2019 5:07 pm

I perfectly understand the usefulness of having both RJ45 PoE-in and barrel-type connectors on mUPS. However, since barrel DC-in is there, I don't understand the need for an accessory PoE injector included in the mUPS package.

Why does this surprise you? If installation demands PoE in as power source (which is quite likely, the same reasoning says voltage up-regulation performed by mUPS is a good thing), then one would need to purchase PoE injector. So why not include it? Price of the injector is quite low, shipping costs can easily surpass it. As almost all (if not all without exception) RB devices support passive PoE in, this PoE injector can come handy anywhere so no harm done is it's not needed for particular mUPS installation.
I'm using one (had to purchase it separately) at home where RB is placed near power receptacle ... but I want to use PoE to connect RB to UPS (which happens to be on the other end of ethernet cable). And would use another one if I had one lying around. Hint: if you'll purchase mUPS and won't use PoE injector, please kindly send it to me ;-)
BR,
Metod
 
sihubert
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Re: mUPS with two different DC power inputs?

Thu Feb 14, 2019 6:30 pm

Okay, I understand now your comment about mounting it directly on a tower. Thanks for clearing that up!
Anyway it was definitely just a minor remark, and a bit off-topic :roll: I'll come back with news about the tests.

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