knute
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Documentation for 3.3 volt pin?

Fri May 14, 2021 1:24 am

Is there any documentation that specifies how much current one can draw from the 3.3 volt pins? I want to power an RFM69 radio and that can take as much as 130ma when transmitting. I've looked at the docs and can't find any reference. I've seen mentioned in articles and comments as much as 800ma to don't use any.

drtechno
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 5:20 am

the regular is 1.5A continuous, 2.5A peak. By the data sheet. you most likely would have the headroom.
https://www.maxlinear.com/ds/mxl7704.pdf

Btw, I made a nice power switch just using a resistor array ic , a diode, and a 2-way switch. works nice on my 4B. PI 3+ is the same regulator, but didn't build the circuit out. so a nice power switch could be put on a 3+ but it will be a couple of parts more because the circuit isn't build out according to the schematics.

aBUGSworstnightmare
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 5:44 am

Which Pi do you use? The CM4 3.3V output is ratet at 600mA max (refer to CM4 data sheet). 130mA should be no problem.

gordon77
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 6:29 am

"the regular is 1.5A continuous, 2.5A peak. "

Where is that data from ?

There has always been debate about how much current you can get from the 3.3v pins, and much less than those figures, maybe a few mAs ?
Last edited by gordon77 on Fri May 14, 2021 9:06 am, edited 1 time in total.

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rpdom
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 7:13 am

I agree with gordon77. The general advice is to not use the 3.3V pin for powering circuits or peripherals greater than a couple of tens of mA at most. If you need 3.3V, use a separate voltage regulator running from the 5V pins.
Unreadable squiggle

gordon77
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 9:18 am

rpdom wrote:
Fri May 14, 2021 7:13 am
I agree with gordon77. The general advice is to not use the 3.3V pin for powering circuits or peripherals greater than a couple of tens of mA at most. If you need 3.3V, use a separate voltage regulator running from the 5V pins.
It's still not clear as some have tested to 800mA https://raspberrypi.stackexchange.com/q ... imitations, whilst it's regularly been quoted as the max is 50mA from the gpios. It clearly also depends on model of pi.
It would be good if there was a definitely answer.

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davidcoton
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 10:11 am

gordon77 wrote:
Fri May 14, 2021 9:18 am
rpdom wrote:
Fri May 14, 2021 7:13 am
I agree with gordon77. The general advice is to not use the 3.3V pin for powering circuits or peripherals greater than a couple of tens of mA at most. If you need 3.3V, use a separate voltage regulator running from the 5V pins.
It's still not clear as some have tested to 800mA https://raspberrypi.stackexchange.com/q ... imitations, whilst it's regularly been quoted as the max is 50mA from the gpios. It clearly also depends on model of pi.
It would be good if there was a definitely answer.
This, particularly model dependency. Early Pi models had a far less capable 3V3 regulator. Note that the original Pi3B+ had a somewhat fragile regulator, again I've not seen a definitive statement about whether newer Pi3B+ are better. (The Pi4B is.)

I would still be cautious about adding a load which varies to the 3V3 pin, thus introducing electrical noise back into the Pi. I would incline towards a separate regulator until there is definitive clarification from RPT engineers.

Ignore drtechno's post. That relates to the component spec, not the Pi implementation which may have different (lower) limitations. In any case we don't know how much current is drawn onboard at 3V3. The second para is pure waffle which is unrelated to your issue.
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B.Goode
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 11:59 am

knute wrote:
Fri May 14, 2021 1:24 am
Is there any documentation that specifies how much current one can draw from the 3.3 volt pins? I want to power an RFM69 radio and that can take as much as 130ma when transmitting. I've looked at the docs and can't find any reference. I've seen mentioned in articles and comments as much as 800ma to don't use any.

Is your question related to the 3.3volt fixed voltage power rail exposed at Physical Pins #1 and #17 on the 40-way headeer? https://pinout.xyz/pinout/3v3_power


Or to the program-controllable GPIO Pins? Presented at 26 positions on the 40-way header?
https://www.raspberrypi.org/documentati ... /README.md

LTolledo
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 12:14 pm

on my setups, if a 3.3v device will draw more than 40mA... it gets its own 200mA LDO regulator or its own 2A buck converter....

better safe than sorry.....
dont want to see any of my RPis dying due to my careless bravado! ;)

meanwhile.... in a similar topic....
viewtopic.php?t=284104#p1720141
"Don't come to me with 'issues' for I don't know how to deal with those
Come to me with 'problems' and I'll help you find solutions"

Some people be like:
"Help me! Am drowning! But dont you dare touch me nor come near me!"

drtechno
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 1:08 pm

aBUGSworstnightmare wrote:
Fri May 14, 2021 5:44 am
Which Pi do you use? The CM4 3.3V output is ratet at 600mA max (refer to CM4 data sheet). 130mA should be no problem.
Yes, looking at the datasheet for the model 4 600mA is left out of the 1.5A regulator it has. It shows it here on pg 27 https://datasheets.raspberrypi.org/cm4/ ... asheet.pdf

drtechno
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 1:50 pm

gordon77 wrote:
Fri May 14, 2021 9:18 am
rpdom wrote:
Fri May 14, 2021 7:13 am
I agree with gordon77. The general advice is to not use the 3.3V pin for powering circuits or peripherals greater than a couple of tens of mA at most. If you need 3.3V, use a separate voltage regulator running from the 5V pins.
It's still not clear as some have tested to 800mA https://raspberrypi.stackexchange.com/q ... imitations, whilst it's regularly been quoted as the max is 50mA from the gpios. It clearly also depends on model of pi.
It would be good if there was a definitely answer.
800mA is probably a peak rating tested. as the circuit build causes limiting factors due to the current rating of the 5V_sys going into the mxl7704 and the current limiting of the series inductor out of its buck output. Since the supply is 15W from the standard usb power supply, I'm thinking that is what is limiting the regulator as a 25W supply would power this chip fully with power headroom. In the user's manual it says 600mA. Which would be the amount left from what ever else is powered by 3.3V. The 50mA rating would be total sink/source current rating of the i/o but that would be split between all of the gpio pins and pg 16 says 4mA per gpio pin: https://datasheets.raspberrypi.org/cm4/ ... asheet.pdf
I'm thinking the boadcom GPIO is sharing global pull up and down resistors inside the gipio section for them to say its 50mA max on GPIO. Overall, to me, it looks like the best driver to use with this GPIO is mosfets instead of transistors, and build out the outputs to 5V TTL by using n channel mosfet drivers when needed. I would recommend a 5V 5A supply instead of the stock 3A supply if anyone was running a bunch of usb hard drives.

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davidcoton
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 4:09 pm

drtechno wrote:
Fri May 14, 2021 1:50 pm
I would recommend a 5V 5A supply instead of the stock 3A supply if anyone was running a bunch of usb hard drives.
That would be pointless as the USB 5V output (all four sockets) is limited to 1A2. Unless, of course, you are powering the drives from the same PSU but not through the Pi.
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 5:29 pm

davidcoton wrote:
Fri May 14, 2021 4:09 pm
drtechno wrote:
Fri May 14, 2021 1:50 pm
I would recommend a 5V 5A supply instead of the stock 3A supply if anyone was running a bunch of usb hard drives.
That would be pointless as the USB 5V output (all four sockets) is limited to 1A2. Unless, of course, you are powering the drives from the same PSU but not through the Pi.
Putting that much current through the PCB would be ill advised as well. There are good reasons why we limit the current.
Principal Software Engineer at Raspberry Pi (Trading) Ltd.
Working in the Application's Team.

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bensimmo
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 5:42 pm

I have a RFM96W wired directly to the pins on a PiZero and it worked in Lora and Lora WAN mode, as did another to a Pi3

I believe similar is done for HAB balloons too.

Is that the sort of thing you are trying?

It transmitted about 23miles distance so far
IMG_20210514_183657_copy_604x726.jpg
IMG_20210514_183657_copy_604x726.jpg (200.65 KiB) Viewed 550 times

knute
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Re: Documentation for 3.3 volt pin?

Fri May 14, 2021 9:08 pm

5.4. Regulator Outputs
To make it easier to interface to the CM4 the on board regulators ( +3.3v and +1.8v ) can each supply 600mA to devices
connected to the CM4. The loads on these outputs isn’t taken into account in the power consumption figures.

Thanks for pointing this out. So assuming a Pi4 is similar to the CM4 then I'm not worried about my 130 ma any more. I am curious if it means the 600ma is not included in the overall rating or just adds to it.

My project is using a Pi4 to control some indicator lighting and a Lidar sensor. The radio is to tell the Pi to go to work.

Bensimmo I'm curious what library you used on your Pi to talk to the RFM69?

Thanks for all the input everybody.

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bensimmo
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Re: Documentation for 3.3 volt pin?

Sat May 15, 2021 8:48 am

Just realised you're talking 69 and I'm 96, but requirements as similar from memory.
So I just have added Lora.

I did use some C code when I started, but Adafruit brought it out with CircuitPython so I started to use that.

Stick Adafruit RFM69 or RFM96 and you'll end up there.
It should also mean you could move to the cheaper less power-hungry Pi Pico without too much effort. One tick... https://learn.adafruit.com/adafruit-rfm ... -for-rfm69

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