julyjim
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How much external power from Zero?

Sat Apr 21, 2018 2:03 pm

My project calls for several I/O devices - 2.8 TFT , LCD (via I2C expander), DDS chip and possibly more whose power will be supplied via Zero .
Since this is going to be portable application I like to know HOW much power I can get from Zero header power pins +5V ( pins 2 / 4 ) and 3.3V (pin 1).

For clarity - I know , thanks to this forum , how much power I can get from GPIO pins. No problem there.

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Gavinmc42
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Re: How much external power from Zero?

Sat Apr 21, 2018 2:14 pm

Enough from the 5V on the header, maybe only 100ma more on the 3V3.

Have you got the specs on those parts?
How many milliamp's do they need?
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julyjim
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Re: How much external power from Zero?

Sat Apr 21, 2018 4:16 pm

No, I do not have real data.
Just starting from source - Zero.

The load will vary.
For example small 2.8 TFT takes whopping 80mA for backlight!
But on my 4x20 LCD I can turn the backlight off in software.

Here is AD9851 DDS chip "power" straight from datasheet

Low Power: 555 mW @ 180 MHz
Power-Down Function, 4 mW @ 2.7 V
Ultrasmall 28-Lead SSOP Packaging

I think the biggest "problem" will be that some of these devices have "power on LED" which is strictly for "show" and not necessary contributing to operation. But too small to cut off.

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Gavinmc42
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Re: How much external power from Zero?

Sun Apr 22, 2018 12:23 am

AD9851 uses max 70ma at 3V3 125MHz, that should be fine.
You might want a separate power supply if you need 5V 180MHz

Run the LCD backlight off the 5V, the logic off 3V3?
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges

julyjim
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Re: How much external power from Zero?

Sun Apr 22, 2018 12:40 am

I do not want to start another debate about the subject , however
the unofficial schematic for RPi 3b clearly identifies distribution of 3,3 V from 5 V - via 3.3 regulator.
The question or speculation remains - how does Zero gets 3.3V output?

Yes, I am planning to use 3.3v whenever possible.

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rpdom
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Re: How much external power from Zero?

Sun Apr 22, 2018 5:28 am

julyjim wrote:
Sun Apr 22, 2018 12:40 am
I do not want to start another debate about the subject , however
the unofficial schematic for RPi 3b clearly identifies distribution of 3,3 V from 5 V - via 3.3 regulator.
The question or speculation remains - how does Zero gets 3.3V output?

Yes, I am planning to use 3.3v whenever possible.
The simple way to tell is to have a look at the board. The Zero uses a PAM2306 regulator for 3.3V and 1.8V supplies. Theoretical maximum output current according to the datasheet is 1A per output.

julyjim
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Re: How much external power from Zero?

Sun Apr 22, 2018 2:15 pm

All I see next to the power USB is "C2 = 1F 54S" chip

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rpdom
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Re: How much external power from Zero?

Sun Apr 22, 2018 3:00 pm

julyjim wrote:
Sun Apr 22, 2018 2:15 pm
All I see next to the power USB is "C2 = 1F 54S" chip
Which model of Raspberry Pi Zero do you have? On a Zero 1.3 (Camera, no WiFi) I can see U3 which is the voltage regulator chip. On a Zero W it isn't labelled, but is about the same place on the board.

julyjim
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Re: How much external power from Zero?

Sun Apr 22, 2018 6:22 pm

Zero W V1.1

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mahjongg
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Re: How much external power from Zero?

Sun Apr 22, 2018 7:41 pm

The schematic (partial) of the RPI Zero W is here: https://www.raspberrypi.org/documentati ... educed.pdf

the zero w uses a PAM2306AYPKE, (https://www.google.com/url?sa=t&rct=j&q ... 7dXPmiVil3) but practically speaking the 3v3 current it can deliver is mostly decided by the inductors used.

My wet finger estimate would be 200mA extra beyond what the zero itself uses.

the 5V current is practically unlimited, (as in several amperes) as 5V in is directly connected from the microUSB input to microUSB output, only the trace to the 5V GPIO pins will limit the current in practice. obviously the "garbage in garbage out" rule applies, if your PSU can't deliver the power, the zero won't either.

julyjim
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Re: How much external power from Zero?

Sun Apr 22, 2018 9:40 pm

I guess you have more then one magic finger since NOBODY else could come up with schematic.

Few things stand up from "schematic"
There are two 3,3V pins on header - nice
The "processor" runs on 1.8 V - so how does it manage to put 5V on GPIO ?
Or does it or better yet - has to?

Perhaps one has to make sure the I/O devices can function with 1.8V logic.


And since there is no 5V regulator the 5V pins can indeed supply as much current as you reasonable ( in mA! not Amps ) put on the USB "power" port.
Thanks for all the work you have done , kudos.

MarkR
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Re: How much external power from Zero?

Sun Apr 22, 2018 9:54 pm

julyjim wrote:
Sun Apr 22, 2018 9:40 pm
There are two 3,3V pins on header - nice
And all other Pi models too.
The "processor" runs on 1.8 V - so how does it manage to put 5V on GPIO ?
No, it does not. The soc "high" level for the gpio is 3v3 on all Pi models. It does not output 5v on GPIO, the GPIO *header* has a couple of pins attached to the 5v net, which just comes from the positive lead of the power connector.
Perhaps one has to make sure the I/O devices can function with 1.8V logic.
No, the gpio pins use levels of approximately 3v3 and 0v. They aren't 5v tolerant.

The 1v8 power is used only internally and you shouldn't ever "see" it normally on outputs.

Mark

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rpdom
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Re: How much external power from Zero?

Sun Apr 22, 2018 9:58 pm

julyjim wrote:
Sun Apr 22, 2018 9:40 pm
Few things stand up from "schematic"
There are two 3,3V pins on header - nice
This is a well known fact. The layout of the 40 pin header is standard across the Raspberry Pi range (other than the early models with 26 pin headers and the CM modules).
The "processor" runs on 1.8 V - so how does it manage to put 5V on GPIO ?
It doesn't. The GPIOs run at 3.3V. The 1.8V is for signals inside the chip only. The pins marked 5V are direct 5V feeds from the power input.
Perhaps one has to make sure the I/O devices can function with 1.8V logic.
No. They have to use 3.3V logic.

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mahjongg
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Re: How much external power from Zero?

Mon Apr 23, 2018 9:28 am

julyjim wrote:
Sun Apr 22, 2018 9:40 pm
I guess you have more then one magic finger since NOBODY else could come up with schematic.
Huh, its just where you would expect it to be, no "magic finger" needed.
just follow the help section. /documentation/hardware/raspberrypi/schematics

julyjim
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Re: How much external power from Zero?

Mon Apr 23, 2018 12:56 pm

Guess what, OF been there, but the memory got overwritten with more recent stuff.
Appreciate your help.

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