julyjim
Posts: 117
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Powering exteranl 3.3V device

Wed Jul 19, 2017 1:30 pm

Where can I find (reference please) how much load can Raspberry Pi 3B supply on external (GPIO connector ) 3.3 V bus.
I am using DDS AD9850 and will power it from 3.3 V .
The inputs are OK, in uA range, but output DAC can supply up to 30 mA.
Thanks

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mikronauts
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Re: Powering exteranl 3.3V device

Wed Jul 19, 2017 3:18 pm

I've read that the original A/B had about 50mA of elbow room left from the on-board 3v3 regulator.

Newer Pi's may allow more, however if the 3v3 regulator draws too much, the 5V can be pulled too low, leading to the dreaded lightning symbol indicating low voltage.

How much you can draw depends on:

- your 5V power supply
- quality of USB cable used to provide power
- what other USB peripherals / add-on boards you are driving
- draw off the 3v3 regulator by other devices

Generally for most of my designs I add my own 3v3 regulator (however tempting it is to leave off) and sometimes provide for an external 5V supply.
julyjim wrote:Where can I find (reference please) how much load can Raspberry Pi 3B supply on external (GPIO connector ) 3.3 V bus.
I am using DDS AD9850 and will power it from 3.3 V .
The inputs are OK, in uA range, but output DAC can supply up to 30 mA.
Thanks
http://Mikronauts.com - home of EZasPi, RoboPi, Pi Rtc Dio and Pi Jumper @Mikronauts on Twitter
Advanced Robotics, I/O expansion and prototyping boards for the Raspberry Pi

mosespi
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Re: Powering exteranl 3.3V device

Wed Jul 19, 2017 7:47 pm

Julyjim,

The schematic of the Pi 3 lists a PAM2306AYPKE as the regulator for the 3.3v and 1.8v rail. If designed to spec.. the datasheet for the device lists a max output of 1A per rail. Although there are other variables to consider as well, the information so far tells me that pulling 30ma or so from the 3.3v rail should be fine.

Regards,
-Moses
Power problems? MoPower UPS for the Pi
http://www.allspectrum.com/mopower/

julyjim
Posts: 117
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Re: Powering exteranl 3.3V device

Thu Jul 20, 2017 12:15 am

Moses, thanks for actually answering my question as asked.
Appreciate than very much.
Just out of curiosity the PAM2306 should be abler to handle BOTH DC-DC converters at 1 A each.
It looks as 1.8V is not available on P8 , but so far I do not need s 1.8 V externally anyway.

julyjim
Posts: 117
Joined: Tue Jan 31, 2017 5:04 am

Re: Powering exteranl 3.3V device

Thu Jul 20, 2017 1:06 am

mikronauts wrote:I've read that the original A/B had about 50mA of elbow room left from the on-board 3v3 regulator.

Newer Pi's may allow more, however if the 3v3 regulator draws too much, the 5V can be pulled too low, leading to the dreaded lightning symbol indicating low voltage.

How much you can draw depends on:

- your 5V power supply
- quality of USB cable used to provide power
- what other USB peripherals / add-on boards you are driving
- draw off the 3v3 regulator by other devices

Generally for most of my designs I add my own 3v3 regulator (however tempting it is to leave off) and sometimes provide for an external 5V supply.


Sir, please read the next post by Moses. He answered exactly what I asked for , your post did not.
Posting misleading "here say" or "I read" without identifying at least design spec or device data-sheet is of little benefit to general public.


- your 5V power supply if rated power supply is bellow 2.5 A and 5 V draw exceeds that
- quality of USB cable used to provide power -it losses in the power cable (suppling 2.5 A @ 5V) is a real a real issue, I hope it is not, - using USB 3.0 standard cable is wrong design choice
- what other USB peripherals / add-on boards you are driving - what that got to do with capacity of the 3.3V source?
- draw off the 3v3 regulator by other devices - that was NOT the question , however knowing the actual 3.3V draw by INTERNAL circuitry would be helpful.

julyjim wrote:Where can I find (reference please) how much load can Raspberry Pi 3B supply on external (GPIO connector ) 3.3 V bus.
I am using DDS AD9850 and will power it from 3.3 V .
The inputs are OK, in uA range, but output DAC can supply up to 30 mA.
Thanks

klricks
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Location: Grants Pass, OR, USA
Contact: Website

Re: Powering exteranl 3.3V device

Thu Jul 20, 2017 4:10 am

julyjim wrote:Moses, thanks for actually answering my question as asked.
Appreciate than very much.
Just out of curiosity the PAM2306 should be abler to handle BOTH DC-DC converters at 1 A each.
It looks as 1.8V is not available on P8 , but so far I do not need s 1.8 V externally anyway.
Moses reply was based on the best available information but does not actually answer your question:
how much load can Raspberry Pi 3B supply on external (GPIO connector ) 3.3 V bus.
If we assume the 3V3 regulator can supply 1A then the available current = 1A - (what the RPi uses).
What the RPi uses (3V3 bus) is unknown and is probably variable depending on CPU load etc.

In order to determine that, someone would need to either lift the 3V3 reg output pin and insert an ammeter -OR- load down the 3V3 bus until it chokes and calculate the difference.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

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