boing
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Display / Pi Power - Wiring

Mon Sep 14, 2015 7:47 pm

I've got a Pi 2 and the Pi display and I am experiencing power issues too. If I connect a 5V/2A power supply to the touchscreen controller board an then supply the Pi with a USB cable I've got a flashing power LED and the rainbow symbol in the upper right corner of the display. Same issue with the enclosed jumper wires powering the Pi from the touchscreen controller board.

On the other hand - if I connect the 5V/2A power supply directly to the micro USB port of the Pi and then power the touchscreen controller board with the enclosed jumper wires everything is fine, no flashing power LED and no rainbow symbol in die upper right corner of the display.

Is there any reason why one should not operate the touchscreen controller board/display this way?

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alexeames
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Re: Display / Pi Power - Wiring

Mon Sep 14, 2015 9:29 pm

Have you tried different USB cables?

Do they produce the same results? Some USB cables have very few strands of wire and are not really suitable for powering a Pi.
It could also be poor quality connectors in the cable.

For example noodle type USB cables usually have a much greater resistance than, say, the official one you may have got with your mobile phone.

Also. What happens if you use two separate PSUs or the GPIO leads? Does it work better then?
Alex Eames RasPi.TV, RasP.iO

boing
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Re: Display / Pi Power - Wiring

Mon Sep 14, 2015 9:55 pm

I have tried two different types of USB cables, quiet poor and good quality with no difference at all. I did not try an extra short USB cable because there is none in my gear. I tried with the enclosed jumper wires but got the same bad result. Using two separate power supplies works for sure.

Is there any problem in powering the touchscreen controller board through jumper wires from the GPIO pins of the Pi with the power supply connected to the Pi's micro USB port (this works fine with no power LED flashing and no rainbow square)?

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alexeames
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Re: Display / Pi Power - Wiring

Mon Sep 14, 2015 10:02 pm

boing wrote:I have tried two different types of USB cables, quiet poor and good quality with no difference at all. I did not try an extra short USB cable because there is none in my gear. I tried with the enclosed jumper wires but got the same bad result. Using two separate power supplies works for sure.

Is there any problem in powering the touchscreen controller board through jumper wires from the GPIO pins of the Pi with the power supply connected to the Pi's micro USB port (this works fine with no power LED flashing and no rainbow square)?
I first tried the right-angled HDMIPi USB to micro-USB cable that I had handy. I got the rainbow square and touch stopped working, also when attempting to video with camera module, it froze and crashed.

I tried a supposedly Samsung cable with similar results, but I can't be sure it's a genuine cable. So then I tried the GPIO jumper leads that came in the pack and that gives me perfect results. So I stuck with that for the time being.
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boing
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Re: Display / Pi Power - Wiring

Mon Sep 14, 2015 10:16 pm

alexeames wrote:So then I tried the GPIO jumper leads that came in the pack and that gives me perfect results. So I stuck with that for the time being.
I chose the GPIO jumper wires too, but where is your power supply connected to? To the touchscreen controller board or to the Pi? I use the display with the power supply connected to the Pi and everything is fine, the other way round (power supply connected to the touchscreen controller board) it isn't!

I would now like to find out if there is any danger with this way of powering the touchscreen controller board...
Last edited by boing on Tue Sep 15, 2015 7:17 am, edited 1 time in total.

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Re: Display / Pi Power - Wiring

Tue Sep 15, 2015 7:10 am

boing wrote:
alexeames wrote:So then I tried the GPIO jumper leads that came in the pack and that gives me perfect results. So I stuck with that for the time being.
I chose the GPIO jumper wires too, but where is your power supply connected to? To the touchscreen controller board or to the Pi? I use the display with the power supply connected to the Pi and everything is fine, the other way round (power supply connected to the thouchscreen controller board) it isn't!

I would now like to find out if there is any danger with this way of powering the touchscreen controller board...
I'm powering the driver board through its micro-USB power in and the Pi with the GPIO jumper leads.

The screen takes about 430 mA but I don't know the circuit. Hopefully one of the engineers will chip in and answer that one for us. :)
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Re: Display / Pi Power - Wiring

Tue Sep 15, 2015 5:23 pm

I tend to power the display directly and then use the jumpers to power the Pi. This works fine for me using the official Raspberry Pi power supply, note that not all power supplies are equal and it is possible that you've got a problem with it drooping slightly (this will not happen with the Raspberry Pi power supply)

If you power the Pi directly it'll be fine for the Pi but you'll end up with a significantly lower supply voltage on the display board (The Pi was not designed to pump power across the board whereas the display was!) but then again this may well be fine!

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 9:40 am

When powering the display with the power supply connected to the Pi and GPIO wires passing the power to the display it appears to be fine, I did not experience any low power issues with the display. My concern/question is if this wiring might damage the Pi seriously.

On the other hand, there are two major advantages with this type of wiring:

- The display shuts down if the Pi powers down. This is ideal for anybody using any kind of power switch attached to the Pi. You don't need to detatch the micro USB cable form the touchscreen controller board.

- In my setup, there is a Pi USV+ too which monitors the power supply of the Pi, switches to battery power if the main power gets lost and performs a clean shutdown of the Pi after a given time without mains. The Pi USV+ also enables the Pi to be supplied with anything between 5 and 25V and it has a power switch to startup / shutdown the Pi and anything attached to the Pi.

Powering the display first would mean that you can't use any type of USV/UPS board for the Pi any longer...
Last edited by boing on Wed Sep 16, 2015 10:23 am, edited 1 time in total.

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 10:02 am

No damage will occur if you drive the power from the Pi to the display, but you will find that the voltage on the display will be lower. I'm not sure at what voltage the display will drop out, I'd have to go through all the schematics and specs again but it's probably fine (since there are LDOs and SMPS's for everything)

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 10:29 am

gsh wrote:No damage will occur if you drive the power from the Pi to the display, but you will find that the voltage on the display will be lower.
Perfect! Thanks, Gordon :)

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 10:49 am

gsh wrote:...I'm not sure at what voltage the display will drop out, I'd have to go through all the schematics and specs again but it's probably fine (since there are LDOs and SMPS's for everything)

Gordon
Thank you for mentioning that. In the absence of any details, I hadn't tried a lower voltage, but I've just powered up the display from a 3.3 volts PSU, & it works fine.

Somewhere I have a 2.x supply, so if I can find it, I'll post back what happens...

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 12:05 pm

Display backlight started to flicker a bit at 2.7 volts, but still watchable. Cut out just under 2.6 volts.

Obviously that's just my sample, wire gauge, length, contact resistance, etc., but interesting anyway.

I don't have access to precision current measuring instrumentation these days, but I'll try later with my gear just to see what the display draws.

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Re: Display / Pi Power - Wiring

Wed Sep 16, 2015 4:31 pm

tvjon wrote:Display backlight started to flicker a bit at 2.7 volts, but still watchable. Cut out just under 2.6 volts.

Obviously that's just my sample, wire gauge, length, contact resistance, etc., but interesting anyway.

I don't have access to precision current measuring instrumentation these days, but I'll try later with my gear just to see what the display draws.
This sounds like a fun game. I have a bench PSU and micro-USB adaptor. Might have a play tomorrow. :lol:
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