dekaman
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another pi4 power supply query

Fri Jun 28, 2019 2:54 pm

I was wondering a few things, I have some canakit 3b+ power supplies and Im guessing they are ok, not looking to buy a Pi4 for a while its just for future reference, I have some Anker multi chargers that I wonder would be powerful enough one has usb c to usb c connectors, and if its powerful enough it may be enough to other things connected (not powered on at same time) just as I have limited amount of sockets.

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Re: another pi4 power supply query

Fri Jun 28, 2019 3:10 pm

dekaman wrote:
Fri Jun 28, 2019 2:54 pm
I was wondering a few things, I have some canakit 3b+ power supplies and Im guessing they are ok, not looking to buy a Pi4 for a while its just for future reference, I have some Anker multi chargers that I wonder would be powerful enough one has usb c to usb c connectors, and if its powerful enough it may be enough to other things connected (not powered on at same time) just as I have limited amount of sockets.

Phone Chargers and Power Supplies are different, just but the Official RPi 4B Power Supply when you buy the Pi....
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dekaman
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Re: another pi4 power supply query

Fri Jun 28, 2019 9:20 pm

Well one of my multi chargers can charge a gpd win 2 which is a powerful machine so I assume this is fine. I am not sure the spec of the other multi chargers I have but they aren't normal usb spec. they are power blocks that you put a cable into

LTolledo
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Re: another pi4 power supply query

Fri Jun 28, 2019 9:33 pm

dekaman wrote:
Fri Jun 28, 2019 9:20 pm
...one of my multi chargers can charge a gpd win 2 ...
the gpd win 2 has internal battery and internal circuity to handle charging and drawing power from the internal battery ..... the RPi4B (or other RPi models, for that matter) has none....

but then again the best way is for you to try it yourself and report to us the results...
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PhatFil
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Re: another pi4 power supply query

Fri Jun 28, 2019 10:41 pm

One thing i have learnt hanging around in here is that not all usb chargers are created equal.

But for everyone running a pi happily from a charger device there is at least another suffering from underpower issues.

Your charger MAY be sufficient, but then again it may struggle to supply a sufficient voltage when your pi attempts to draw more power than it can supply at 5V+.

relying on your charger will be a gamble, do you feel Lucky??

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rurwin
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Re: another pi4 power supply query

Fri Jun 28, 2019 11:50 pm

The thing about USB chargers is that they are not required to provide 5V. At zero current they will provide the rated voltage but as the current goes up, the voltage will go down. The lower end of the voltage range specified for USB (1 & 2) chargers is below the minimum voltage at which a Pi will work (around 4.6V in practice, although the spec is 4.75v) So they need to be over-specified or, to put it another way, don't expect to be able to draw as much current as they say they will provide.

USB=C may be different, and the official Pi4B power supply provides more than 4.75V over its full current range.

The other thing about USB power supplies, is that they are required to provide no more than 5.25V at their output terminals and most provide exactly 5V. If they use a separate wire, like many off-the-shelf chargers do, then there will be a voltage drop on the wire. You can be well under 5V by the time the power gets to the Pi. Using a wired power supply is a very good idea.

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HawaiianPi
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Re: another pi4 power supply query

Sat Jun 29, 2019 2:27 am

What he said ^

I used to have a 5V, 2.4A charger that dropped well below 5V at only a 1A load. Devices like phones and tablets do not require precise voltage control to charge their batteries (phones only need a little over 4.2V to fully charge). Because of this, many phone chargers have poor regulation and will not maintain a stable 5V under load.

It may not be true for all phone chargers, but this is why they are not recommended as power supplies. Unless you have a constant current load tester to verify the charger, you can't be sure it will be stable enough. It might seem to be working fine for weeks or months, and then you try something that really pushes your Pi hard and it crashes, reboots and corrupts your SD card because the charger couldn't deliver the extra load.

If the charger uses a separate USB cable, that only adds to the potential problems (some cables have too much resistance). Although this should be less of a problem than it was with micro USB.
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rurwin
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Re: another pi4 power supply query

Sat Jun 29, 2019 11:00 am

HawaiianPi wrote:
Sat Jun 29, 2019 2:27 am
(some cables have too much resistance)
I would argue that all USB cables have too much resistance.
It's difficult to get datasheets for USB cables. One I've just checked uses 28AWG for signals and 24AWG for power.

24AWG copper wires have a resistance of 0.08 ohms/metre.

With a 1m cable, that would be 0.16 ohms. (1m for the power, 1m for the ground).

V = I R
If the current is 2A, then the wire will drop 2 x 0.16 = 0.32V

5V - 0.32V = 4.68V

That is out of spec before considering that the power supply itself will not supply 5V at that current. And that's a pretty good USB cable; it uses thicker wires for power, many of them don't.

The Official (Pi 1, 2 & 3) power supply uses 18AWG cable and supplies 5.1V
That's only 0.02 ohms/metre but it's 1.5m long so 0.06 ohms for the cable
With 2A of current it will drop 2 x 0.06 = 0.12V

5.1V = 0.12V = 4.98V

I too hope that USB-C will be better than the dead-reckoning that micro-USB depends on.

jahboater
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Re: another pi4 power supply query

Sat Jun 29, 2019 11:17 am

rurwin wrote:
Sat Jun 29, 2019 11:00 am
The Official (Pi 1, 2 & 3) power supply uses 18AWG cable and supplies 5.1V
That's only 0.02 ohms/metre but it's 1.5m long so 0.06 ohms for the cable
With 2A of current it will drop 2 x 0.06 = 0.12V

5.1V = 0.12V = 4.98V

I too hope that USB-C will be better than the dead-reckoning that micro-USB depends on.
The Official Pi4 power supply also uses 18AWG cable.

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