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mths.
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Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 1:37 pm

Hi everybody,

I'm quite confused right now. I'm powering a DCDC-converter (3v3-3v3) from the 3v3 Power (Pin 1) and a further DCDC-converter (5v-5v) from the 5v Power (Pin 2) of a Raspberry Pi B+. As each converter needs a base load to supply the specified voltage I added some resistors.

I have searched a lot but couldn't find an answer whether there's a limitation to the current drawn from the 3v3 (Pin 1) and 5v power (Pin 2). Some sources state that the current from 3v3 power is limited to 50 mA while others state nothing like that. I just measured the current drawn by the external circuit and it shows the following:

3v3 power: ~150 mA
5v power: ~ 130 mA

I recognized no strange behaviour of my Raspberry Pi B+ so far but I'm wondering if this circuit could damage (or already has damaged) anything on my Raspberry. I hope someone of you can help me with that.

Best regards
Mathias

klricks
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 2:00 pm

When the RPi is powered by the micro USB socket, the 5V rail is limited to 2A by the input 'polyfuse' F1. RPi + GPIO + USB <= 2A
The 50mA applies to power from /to the GPIO not power rail. But I don't know for sure what the limit is? I have read it is a 1A VR circuit but have not verified it. Of course the RPi needs a portion of that.

Note that the RPF docs state the 50mA applies to the 3V3 rail as well but this info was written for the B model with the linear VR. The B+ and 2B have as completely redesigned switching type supply.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

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mths.
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 2:38 pm

Hi klricks,

the RPi is powered by the microUSB socket. So, if I get anything right - there should be no problem with drawing 150 mA from 3v3 rail and 130 mA from 5v rail of a RPi B+ as long as the sum of total current used by the RPi and all periphery doesn't exceed the 2 A limit of the polyfuse?

Regards
Mathias

klricks
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 5:26 pm

mths. wrote:Hi klricks,

the RPi is powered by the microUSB socket. So, if I get anything right - there should be no problem with drawing 150 mA from 3v3 rail and 130 mA from 5v rail of a RPi B+ as long as the sum of total current used by the RPi and all periphery doesn't exceed the 2 A limit of the polyfuse?

Regards
Mathias
Yes but remember the 3V3 is 'downstream' or in series with the 5V rail... so whatever current the 3V3 uses is already included in the 5V current measurement.
As I mentioned I don't know the max limit on the 3V3 rail?
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

cacti
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 5:45 pm

The 3v3 is produced by a dual switching regulator that can only deliver 50mA.

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mths.
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 7:53 pm

@Klricks
Ok, it seems that I don't really got it right. If the current drawn out of 3V3 is included in the 5V measurement - why do I measure a higher current (150 mA) on 3V3 than on 5V (130 mA)?

@cacti
Why can I measure 150 mA on 3V3 - without the RPi crashing or behaving strange - then?

Confusion is going on... :/

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FTrevorGowen
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 8:25 pm

mths. wrote:@Klricks
Ok, it seems that I don't really got it right. If the current drawn out of 3V3 is included in the 5V measurement - why do I measure a higher current (150 mA) on 3V3 than on 5V (130 mA)?
@cacti
Why can I measure 150 mA on 3V3 - without the RPi crashing or behaving strange - then?
Confusion is going on... :/
cacti wrote:The 3v3 is produced by a dual switching regulator that can only deliver 50mA.
IIRC, what @cacti is referring to applies to A and B Pis, not the A+, (your) B+ and the P2B. Personally, since I usually need to use level-shifters for my application circuits, I tend to use (one of) such that has an on-board 5V to 3.3V regulator to provide an "external" 3.3V supply the current of which being drawn from the main 5V supply.
Trev.
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

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mths.
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Re: Current limitation for 3v3 and 5v power rails

Mon Mar 09, 2015 8:43 pm

Hi Trevor,

buying a 5V-to-3V3 DCDC-converter and wiring it to the 5V power would be my next step if supplying currents above 50 mA from 3V3 power would damage my RPi or shorten its lifetime. But up to now I'm not totally sure whether drawing 150 mA from 3V3 power is harmful for my RPi...!? :/

Regards
Mathias

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Re: Current limitation for 3v3 and 5v power rails

Tue Mar 10, 2015 12:28 am

According to this writeup the RPi VR is a 1A dual switcher: https://learn.adafruit.com/introducing- ... wer-supply
RT8020AGQW http://www.digikey.com/product-search/e ... 020AGQW-nd
What is unknown is how much of the 3V3 current is used by the RPi?
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

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mths.
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Re: Current limitation for 3v3 and 5v power rails

Tue Mar 10, 2015 7:18 am

klricks wrote:According to this writeup the RPi VR is a 1A dual switcher: https://learn.adafruit.com/introducing- ... wer-supply
RT8020AGQW http://www.digikey.com/product-search/e ... 020AGQW-nd
What is unknown is how much of the 3V3 current is used by the RPi?
I don't have a comparable source in English at hand, but according to http://www.elektronik-kompendium.de/sit ... 910071.htm the current consumption of a RPi B+ powering 2 USB devices (keyboard and mouse), connected via HDMI, using ethernet and with a CPU-usage of 100% is about 500 mA (494 mA). So for my understanding there should be no damage done to the RPI B+ by drawing 150 mA (or even more) from the 3V3 power rail as there should be a lot of space up to the 1 A of the RT8020AGQW. Right? :)

Regards
Mathias

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joan
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Re: Current limitation for 3v3 and 5v power rails

Tue Mar 10, 2015 8:06 am

If the Pi is starved of 3V3 it will be unreliable or reboot. If neither of those is affecting you then personally I'd stop worrying. The 50mA figure is often touted. I've seen no evidence it ever applied (but if it did at all it would be on the earlier Pi models).

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mths.
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Re: Current limitation for 3v3 and 5v power rails

Tue Mar 10, 2015 8:18 am

Thank you Joan, Klricks and Trevor for your help. I'll go on like I did and stop worrying! ;)

Regards
Mathias

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