Raoul-
Posts: 9
Joined: Thu Feb 21, 2019 8:25 am

Power problems RPI 3B+ with touchscreen

Wed May 22, 2019 12:28 pm

Hello,

I'm not sure if this topic fits in this subforum, but I couldn't find a better fit.
I'm using a Raspberry Pi 3 Model B+ with a LCD touch monitor, which is connected over HDMI. I'm powering the LCD touch monitor over the USB ports of the Raspberry Pi.

I've tried this setup in the following 3 situations:
1. Power the Raspberry Pi with a power adapter of 1.8A at 5V. I get a notification of undervoltage (yellow blitz) at the monitor.

2. Power the Raspberry Pi with a PoE switch and a PoE HAT. The monitor goes on and off all the time. The Raspberry Pi works fine with no undervoltage. I suspect the provided current is not enough, but at the specifications of this HAT is stated that it can deliver 2.5A (more than the power adapter).

3. Power the Raspberry Pi with a PoE switch and a Adafruit PoE splitter (https://www.adafruit.com/product/3785). The Raspberry and monitor work fine. There are no problems in this situation.

This raises a few questions:
1. Why do I get a undervoltage notification in situation 1, but not in situation 2 and 3?
2. Why does it look like the provided current is not enough in situation 2, but is enough in situation 1 and 3?

Could someone help me here?

Thanks in advance,
Raoul

gkaiseril
Posts: 576
Joined: Mon Aug 08, 2016 9:27 pm
Location: Chicago, IL

Re: Power problems RPI 3B+ with touchscreen

Wed May 22, 2019 5:32 pm

A double check of the power requirements of the Pi 3. The POE hat provides 5/2.5A DC power to the Pi. This the stated required power requirement for the Pi 3 series. Your power supply cannot provide enough amperage for the Pi3 needs.
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.

Raoul-
Posts: 9
Joined: Thu Feb 21, 2019 8:25 am

Re: Power problems RPI 3B+ with touchscreen

Wed May 22, 2019 8:08 pm

Yeah, but that has nothing to do with the voltage, right? The amperage/current is possibly not enough, but it says undervoltage.
Besides the undervoltage in situation 1 the pi and monitor both work fine.
The PoE HAT provides 2,5A at 5V and the splitter from adafruit provides 2,4A at 5V. As the Adafruit splitter provides slightly less power, I don't understand why it works with the Adafruit splitter, but not with the PoE HAT.

Kind regards,
Raoul

LTolledo
Posts: 1399
Joined: Sat Mar 17, 2018 7:29 am

Re: Power problems RPI 3B+ with touchscreen

Wed May 22, 2019 9:11 pm

1.8A is not enough!! your so-called "PSU" is struggling to power the RPi's required current.... resulting in undervolt alarm.

Did you researched the power requirement for the RPi3B+?
Can you paste the link where it specifically states you can power the RPi3B+ (with monitor, etc ) with just 1.8A?
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

gkaiseril
Posts: 576
Joined: Mon Aug 08, 2016 9:27 pm
Location: Chicago, IL

Re: Power problems RPI 3B+ with touchscreen

Wed May 22, 2019 10:18 pm

The warning is reported when the voltage drops below 4.63 volts DC. When the psu has to provide more amps than it can provide it lowers the output voltage to provide the higher amp output. Also it is easier to detect the voltage rather than the amps. If one reads the solution in the documentation it is clear this is a power issue or amps times voltage.
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.

Raoul-
Posts: 9
Joined: Thu Feb 21, 2019 8:25 am

Re: Power problems RPI 3B+ with touchscreen

Thu May 23, 2019 6:56 am

Thanks for your replies.
I know the recommended current for the Pi3B+ is 2.5A, but I also know less than 2.5A is often enough for the Pi to work at least. So I just tested it with my 1.8A power adapter. I know it's not ideal but it works.

What I don't understand is that the Pi PoE HAT, which provides 2.5A can't get the monitor on, but the 1.8A power adapter and the 2.4A Adafruit PoE splitter can. Am I missing something?

Kind regards,
Raoul

fruitoftheloom
Posts: 19504
Joined: Tue Mar 25, 2014 12:40 pm
Location: Delightful Dorset

Re: Power problems RPI 3B+ with touchscreen

Thu May 23, 2019 9:11 am

Raoul- wrote:
Thu May 23, 2019 6:56 am
Thanks for your replies.
I know the recommended current for the Pi3B+ is 2.5A, but I also know less than 2.5A is often enough for the Pi to work at least. So I just tested it with my 1.8A power adapter. I know it's not ideal but it works.

What I don't understand is that the Pi PoE HAT, which provides 2.5A can't get the monitor on, but the 1.8A power adapter and the 2.4A Adafruit PoE splitter can. Am I missing something?

Kind regards,
Raoul

Is your PoE Injector compatible to the same 802.3af standard as the PoE Hat ?
adieu

My other Computer is an Asus CS10 ChromeBit running Chrome Operating System.
HP Envy 4500 Wireless Printer supported by HPLIP software in Raspbian Buster.
Raspberry Pi Model 2B v1.1

Raoul-
Posts: 9
Joined: Thu Feb 21, 2019 8:25 am

Re: Power problems RPI 3B+ with touchscreen

Thu May 23, 2019 10:30 am

Yes, it is compatible with 802.3af PoE. It's the PoE splitter with microUSB from adafruit (https://adafruit.com/product/3785)

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