Sparks135
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Joined: Tue Feb 28, 2017 7:44 pm

Strange power situation needs input.

Fri Apr 28, 2017 9:25 pm

I have a Raspberry Pi 3 Model B and the following boards.

1. Auto Power shutdown ( Power from AC adapter plugged into this board)
2. Raspberry Pi 3 Model B ( Receives power from Auto Power Shutdown Board above item 1.)
3. Real Time Clock (Gets power from GPIO on Pi 3.)
4. 5 Port USB Hub receives power from separate AC Adapter)
5. Official Raspberry 7" touch screen (receives power from separate power Adapter)

Now the problem that needs a solution. The Power shutdown board can't be feed with the same supply that feeds either the 7" screen or the USB hub. If I use the power splitter that came with the 7" screen on a power adapter and plug one end into the Shutdown board and the other into the screen there is a low power problem. And if I take the splitter and try to feed the USB and the shutdown, or any combo the Pi fails to boot. Also the Power Shut Down only works with the power adapter plugged into the splitter. So now I have three power adapters, which is not optimum. Any ideas would be welcome.

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Burngate
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Re: Strange power situation needs input.

Sat Apr 29, 2017 5:45 pm

The Pi 3B requires quite a lot of current, and so does the screen (a real time clock won't want much)
It might help to know which Auto Power shutdown board you have, and also the power adapters you are trying to use.

Sparks135
Posts: 104
Joined: Tue Feb 28, 2017 7:44 pm

Re: Strange power situation needs input.

Sat Apr 29, 2017 7:44 pm

Burngate wrote:The Pi 3B requires quite a lot of current, and so does the screen (a real time clock won't want much)
It might help to know which Auto Power shutdown board you have, and also the power adapters you are trying to use.
Here is the auto power shutdown IR Board.https://www.amazon.com/gp/product/B06X9 ... UTF8&psc=1
Here is the main power adapter. Feedss Raspberry Pi-3, shutdown board and Real Time Clock
http://www.ebay.com/itm/361486440073?_t ... EBIDX%3AIT
Here is the other two AC Adapters. http://www.ebay.com/itm/131795179538?_t ... EBIDX%3AIT
Here is the USB Board used on self power mode, not feed from Pi 3. http://www.robotshop.com/en/7-port-usb- ... ry-pi.html

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Burngate
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Re: Strange power situation needs input.

Sun Apr 30, 2017 10:54 am

Well, this might not be much help.

The Pi requires at least 4.65v - below that the red LED goes out and the lightning bolt appears.
I haven't yet got a 7" screen - I'm wanting one but I'm too tight-fisted - so all I can go on is what I've read here.

The USB hub says it only takes a few mA, but will obviously take more depending on what's plugged into it.

If that main PSU is doing what it says it should, 3.5A should be plenty. It should be fully capable of driving both Pi and screen, though there seem to be differing experiences depending on whether the Pi feeds the screen or vice versa.
But having 6.5ft of lead makes me suspicious - it's rather long, given that the resistance of the lead can drop the volts quite drastically if it's thin or long.

What the Makerfocus auto shutdown board does is anyone's guess. Hopefully it doesn't drop the voltage to the Pi etc. too much.

Sparks135
Posts: 104
Joined: Tue Feb 28, 2017 7:44 pm

Re: Strange power situation needs input.

Sun Apr 30, 2017 1:12 pm

Burngate wrote:Well, this might not be much help.

The Pi requires at least 4.65v - below that the red LED goes out and the lightning bolt appears.
I haven't yet got a 7" screen - I'm wanting one but I'm too tight-fisted - so all I can go on is what I've read here.

The USB hub says it only takes a few mA, but will obviously take more depending on what's plugged into it.

If that main PSU is doing what it says it should, 3.5A should be plenty. It should be fully capable of driving both Pi and screen, though there seem to be differing experiences depending on whether the Pi feeds the screen or vice versa.
But having 6.5ft of lead makes me suspicious - it's rather long, given that the resistance of the lead can drop the volts quite drastically if it's thin or long.

What the Makerfocus auto shutdown board does is anyone's guess. Hopefully it doesn't drop the voltage to the Pi etc. too much.
Before I had the Markefocus board installed the screen and the Pi 3 and screen were powered with the Screen supplied Y splitter. Also what is strange is that the MarkerFocus won't work unless the splitter is installed. Even without it powering both the Pi and the screen. So I guess three power supplies is needed. I am at a loss.

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Burngate
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Re: Strange power situation needs input.

Mon May 01, 2017 8:45 am

You've mentioned twice the power splitter that came with the screen. I've looked at the Pimoroni, CPC and RS pages, but none mention a splitter, so I'm at a loss as to what you mean.
I'd assumed it was a short USB to two USB Y cord, so that nothing much could be affected by it, but I must be wrong.
Time, I suppose, to fork out sixty quid and see for myself.

Sparks135
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Joined: Tue Feb 28, 2017 7:44 pm

Re: Strange power situation needs input.

Mon May 01, 2017 2:26 pm

Burngate wrote:You've mentioned twice the power splitter that came with the screen. I've looked at the Pimoroni, CPC and RS pages, but none mention a splitter, so I'm at a loss as to what you mean.
I'd assumed it was a short USB to two USB Y cord, so that nothing much could be affected by it, but I must be wrong.
Time, I suppose, to fork out sixty quid and see for myself.
OK yes it is a USB micro cable splitter. Here is the link to the Official case for 7" and cable. https://www.amazon.com/gp/product/B01HK ... UTF8&psc=1
When I use the splitter plugged into both the power board and either the screen or USB the power up and down board does not respond to IR input. If I use the splitter with just the power up board it works fine, But if I remove the splitter the IR power board doesn't function.

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Burngate
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Re: Strange power situation needs input.

Tue May 02, 2017 3:53 pm

I said previously that I was suspicious about that power supply, and I've done some more thinking about it.

The advert describes it as a charger, not a supply. We all know there's a big difference - a supply has to maintain the voltage at the end of its cable at 5v ± 0.25v, whereas a charger merely has to supply enough voltage to charge a phone battery.

There's no information about the cable, apart from its length - 2m - but for a variety of wire gauges from 28 AWG to 16 AWG I've constructed a small table, with the last column being the expected volt drop when 2A is being drawn.

Code: Select all

AWG	Ω/km	Ω/m	Voltage drop
			            @ 2A and 4m
16	13.17	0.01317	 0.105
17	16.61	0.01661	 0.132
18	20.95	0.02095	 0.167
19	26.42	0.02642	 0.211
20	33.31	0.03331	 0.266
21	42.00	0.042  	 0.336
22	52.96	0.05296	 0.423
23	66.79	0.06679	 0.534
24	84.22	0.08422	 0.673
25	106.2	0.1062	  0.849
26	133.9	0.1339	  1.071
27	168.9	0.1689	  1.351
28	212.9	0.2129	  1.703
As you can see, with 5v going into one end, and with no less than 4.65v at the Pi - so a drop of 0.35v - the cable needs to be 21 AWG or thicker.
That's about 3/4 mm diameter copper in each wire, not counting the insulation.

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