pbrowne
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Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Thu Jun 21, 2018 3:10 am

Hi

I'm getting a Raspberry Pi 3 B+ and then the official POE hat when it's released (has it been released??). Now, I will be plugging in a RTL2832U RTL-SDR to a USB on the Pi. Connected to the RTL-SDR dongle is a Low Noise Amplifier (LNA) for the 21 cm hydrogen line at 1420MHz with the feed coming from 2.4 metre satellite dish antenna using a LNB designed for that frequency.

Question is, since I'm powering the Pi with POE, would it be possible to power the LNA which requires 8 - 12 V DC & 150 mA?

drgeoff
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Thu Jun 21, 2018 7:32 am

Probaby not. RPis run from 5 volts. There is no reason for the PoE hat to produce any other voltage. True PoE uses 48 volts at the injecting end.

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bensimmo
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Thu Jun 21, 2018 8:24 am

up volt from the 5V gpio line with a buck convertor?
or USB ports if that provides greater protection than gpio pins 2 or 4.

(just throwing the idea out there, I've not done it)

pbrowne
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Fri Jun 22, 2018 2:19 am

bensimmo wrote:
Thu Jun 21, 2018 8:24 am
up volt from the 5V gpio line with a buck convertor?
or USB ports if that provides greater protection than gpio pins 2 or 4.

(just throwing the idea out there, I've not done it)
Good idea, I was looking at this today.

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davidcoton
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Fri Jun 22, 2018 3:45 pm

pbrowne wrote:
Fri Jun 22, 2018 2:19 am
bensimmo wrote:
Thu Jun 21, 2018 8:24 am
up volt from the 5V gpio line with a buck convertor?
or USB ports if that provides greater protection than gpio pins 2 or 4.

(just throwing the idea out there, I've not done it)
Good idea, I was looking at this today.
Just watch the total load at 5V is still within the capacity of the POE -- I don't know what either side of that equation comes to, so I can't say whether it will work or not.
"Thanks for saving my life." See https://www.raspberrypi.org/forums/viewtopic.php?p=1327656#p1327656
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”

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bensimmo
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Fri Jun 22, 2018 4:31 pm

davidcoton wrote:
Fri Jun 22, 2018 3:45 pm
pbrowne wrote:
Fri Jun 22, 2018 2:19 am
bensimmo wrote:
Thu Jun 21, 2018 8:24 am
up volt from the 5V gpio line with a buck convertor?
or USB ports if that provides greater protection than gpio pins 2 or 4.

(just throwing the idea out there, I've not done it)
Good idea, I was looking at this today.
Just watch the total load at 5V is still within the capacity of the POE -- I don't know what either side of that equation comes to, so I can't say whether it will work or not.
I'm assuming the PoE is 2500mA capable, it's for a 3B+ after all.
Need here 150mA plus conversion loss. Which is probably less than adding a Pi camera? Certainly some LED HATs

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davidcoton
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Fri Jun 22, 2018 4:36 pm

bensimmo wrote:
Fri Jun 22, 2018 4:31 pm
I'm assuming the PoE is 2500mA capable, it's for a 3B+ after all.
Need here 150mA plus conversion loss. Which is probably less than adding a Pi camera? Certainly some LED HATs
Asumptions are dangerous, even when reasonable.
150mA at 12V is 1.8W, or 360mA at 5V. Still possible, but what else is also sharing the 5V supply?
"Thanks for saving my life." See https://www.raspberrypi.org/forums/viewtopic.php?p=1327656#p1327656
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”

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bensimmo
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 9:23 am

davidcoton wrote:
Fri Jun 22, 2018 4:36 pm
bensimmo wrote:
Fri Jun 22, 2018 4:31 pm
I'm assuming the PoE is 2500mA capable, it's for a 3B+ after all.
Need here 150mA plus conversion loss. Which is probably less than adding a Pi camera? Certainly some LED HATs
Asumptions are dangerous, even when reasonable.
150mA at 12V is 1.8W, or 360mA at 5V. Still possible, but what else is also sharing the 5V supply?
That's not for me to work out.
Try and see, worst is it doesn't work, not particularly dangerous.

360mA/400mA is still not a lot, I'd aim for 10V anyway :-).

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davidcoton
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 9:41 am

bensimmo wrote:
Sat Jun 23, 2018 9:23 am
Try and see, worst is it doesn't work, not particularly dangerous.
Worst is blown polyfuse and corrupted SDCard (possibly permanenmtly locked read-only). Not an area to experiment unless you can face that.

As for the voltage to the RTL-SDR, we don't know if:
  • It uses the voltage directly, in which case a lower voltage might result in lower current and power (good)
  • It uses a linear regulator, in which case the current will be roughly constant, so lower voltage saves less power
  • It uses a switching regulator, in which case the supply voltage makes little difference to the power consumption
If noise immunity is an issue (quite likely for a radio receiver), there may well be a linear regulator, but WJDK so far.
"Thanks for saving my life." See https://www.raspberrypi.org/forums/viewtopic.php?p=1327656#p1327656
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”

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bensimmo
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 12:59 pm

If the POE-HAT has a polyfuse? That's an assumption ;-) it probably does though.
, so what is your suggestion, just look at it and not bother ?
Write the idea off.

The question was can it provide the current (the PoE HAT) for the LNA.
The answer was maybe, if the HAT is specified like the PSU's.
I have no idea if the power into the Ethernet cable will be able to deliver enough to the HAT though.

I've never killed an SD card, with under Voltage/current, it gives you a warning or doesn't work and I drop both deliberately on a CC/CV 100w DC PSU.
It's a few quid though.

The voltage alterificators can have all their gubbins to protect the 5V gpio line from the LNA which it the thing needing 8 to 12V (and the LNB from that ?)

The RTL-SDR is sat on the USB, like any TV dongle does, it uses 5V from there, nowhere else.
I've not had one damage my Pi's or PCs.
:-)

Sometimes you just have to give it a go, see if it'll work or spend hours trying to find some special and then seeing if it works :-)
He sounds like he understands what may/may not work.

Unless I have the setup wrong in my head of course :-)

I guess it could/should be tested from a Pi 2.5A PSU via USB and then see the HAT spec when it in the shops as they cost more than the PSU.

stevend
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 1:30 pm

The Pi POE HAT is rated at 5V 2.5A - see the official document: https://static.raspberrypi.org/files/pr ... -Brief.pdf

klricks
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 2:15 pm

I think the question is whether the raw POE voltage (37-57VDC) is easily available and accessible from the POE hat?
If so it would not be hard to down convert to other voltages.
If not then you would have to resort to up converting from 5V which may or may not work.
Another option would be to parallel another POE extractor device.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Stretch w/ Desktop OS.

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davidcoton
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 3:03 pm

bensimmo wrote:
Sat Jun 23, 2018 12:59 pm
If the POE-HAT has a polyfuse? That's an assumption ;-) it probably does though.
, so what is your suggestion, just look at it and not bother ?
Actually my assumption (probably incorrect, on reflection) was that the POE power went via the Pi polyfuse. If it doesn't, and the HAT doesn't have a polyfuse or other current limit, the damage could be worse.
I'm advocating learning to use best scientific/engineering practice: research for the information, and use that to design the experiment to avoid unwanted damage to equipment.
bensimmo wrote:
Sat Jun 23, 2018 12:59 pm
I've never killed an SD card, with under Voltage/current, it gives you a warning or doesn't work and I drop both deliberately on a CC/CV 100w DC PSU.
Just because you haven't killed an SDCard doesn't mean it can't happen. There is plenty of anecdotal evidence that power problems at the wrong time CAN and DOES damage SDCards.
bensimmo wrote:
Sat Jun 23, 2018 12:59 pm
Sometimes you just have to give it a go, see if it'll work or spend hours trying to find some special and then seeing if it works :-)
He sounds like he understands what may/may not work.
See above. It depends on how you value your Pi equipment relative to your time. Not to mention the time to replace equipment when the experiment goes wrong.

I'm not concerned about your (bensimmo's) decisions, you've been here long enough to know the consequences and make your own decisions.
As to the OP and other readers, they need to know the possibilities and risks so they can reasonably assess what to try.
"Thanks for saving my life." See https://www.raspberrypi.org/forums/viewtopic.php?p=1327656#p1327656
“Raspberry Pi is a trademark of the Raspberry Pi Foundation”

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bensimmo
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Re: Raspberry Pi 3 B+ POE - Powering other devices 8 - 12 V DC & 150 mA

Sat Jun 23, 2018 3:59 pm

I believe the PoE actually powers via the GPIO pin's (2 & 4) from what somebody said, Given that's the only contact point as the 4 pins are from the Ethernet port. Which I had forgotten about, it might make it a tad more tricky unless the new onboard regulator can cope with the power fluctuation between the gpio pins and the power source. (as you tap off to get the 8 to 10V for the LNA). But would that actually be any different to it being powered via USB.


Might have a play, and see when the Pi3B+ droops with an increase 10V draw from the 5V line, I think i've got some cheap buck converters (step up) somewhere at work.



As a side and just as a note, the often used 3W IR LED (two of them) of night camera illumination e.g https://www.aliexpress.com/item/Free-Sh ... 64977.html make a PiZeroW and the camera module (as they attach to it and power through it on mine) become exceedingly hot, but the ZeroW doesn't falter. That' 1.8A extra through it?, or pretty much to the limit of the PSU attached.
Of course, I could test these with a step down from 5V and see how it goes :-s With a 3B+ with ethernet being used.. what do you think, no chance?

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