rmarques
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What happens to the Pi if I connect a 4.2V voltage source to its power input?

Tue Nov 28, 2017 7:36 pm

I designed a power supply on batteries with a button and a filter that enables a boost converter that boosts the voltage from the battery voltage to ~5V.

So, the voltage output by the power supply is 4.2V if the boost is disabled and the batteries are fully charged. I would like that the Pi didn't try to boot until the power on button of the power supply is pressed to boost the voltage to 5V.

But now I'm concerned that the Pi boots on 4.2V and damages itself OR the SD card. What are my chances? Do I need to completely shutdown the power supply to 0V when I first connect it to the Pi?

klricks
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Re: What happens to the Pi if I connect a 4.2V voltage source to its power input?

Tue Nov 28, 2017 8:30 pm

The RPi may work OK but any connected USB devices likely will not work giving the appearance that the RPi has crashed.
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.

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joan
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Re: What happens to the Pi if I connect a 4.2V voltage source to its power input?

Tue Nov 28, 2017 10:01 pm

From memory I think the Pi will boot if there is slightly more than 3V3 on the 5V rail. I assume the SD card only needs 3V3 to operate (which makes sense as it is connected via the Pi's 3V3 GPIO).

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Ronaldlees
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Re: What happens to the Pi if I connect a 4.2V voltage source to its power input?

Tue Nov 28, 2017 10:56 pm

When back-feeding power thru the 3v3 GPIO - my keyboard, mouse, and WiFi dongle work at ~3.50 Volts, but stop working when I dial the voltage down to ~3.4V. I actually have one AP (access point) - working this way. I don't know what happens at lower voltages, less than 3v3 (probably just doesn't boot).

The reason for back-feeding power is that I burned out the 5V regulator (by attaching a 20V supply by accident :-) ) The regulator blew completely off the board. They don't recommend backfeed of course (and neither do I) - since certain protections are by-passed.

It's not the best idea to run the 3v3 supply on the high side, and so that's not my recommendation either. I think around 3.6V is the theoretical max voltage, but any DC supply's voltage accuracy will move around slightly (due to heating and other factors) - so you don't want to run close to the edge. I pretty much arbitrarily set it to run at 3.5V.

At 3.5V, some USB devices won't work, and the ones that do work may turn off/on randomly. This has happened a few times on the Pi2 AP just mentioned. I basically have a 100 mV "window" (of 3.4V - 3.5V) where the AP works most of the time, because the voltage is just high enough to make USB work (usually), and low enough so that (thus far) - it hasn't killed the SoC.

I've read that the regulator built onto the Pi has an accuracy of plus/minus 100 mV. If true, then I'm running on the high side of that tolerance at 3.5V. If what I read is true, then the recommended voltage to the SoC would be 3.2V - 3.4V, inclusive. Apparently, the Pi when normally powered has overcurrent protection, but that too is gone when the device is back-powered, and is another reason to avoid the idea. My recommendation is to stick with the normal power setup, with the onboard regulator, and hopefully no chips will "blow off the board". This means I should be buying another Pi. Oh well, they're cheap.

This doesn't clearly answer your question, since you're using the regulator. But, I guess it does say something about USB devices in general, which is that some of them work (although maybe less reliably) at surprisingly low voltages.
Last edited by Ronaldlees on Wed Nov 29, 2017 2:19 am, edited 12 times in total.
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rmarques
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Re: What happens to the Pi if I connect a 4.2V voltage source to its power input?

Tue Nov 28, 2017 11:00 pm

Since I want to boot/shutdown the RPi with two buttons it seems like I'll have to hack some components on top of the board. You see, it was already built and assembled since I designed it, forgot this part :oops:

Thanks

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Ronaldlees
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Re: What happens to the Pi if I connect a 4.2V voltage source to its power input?

Wed Nov 29, 2017 12:31 am

Probably the lower voltage on the 5V rail is more likely to cause issues with HDMI, if that's being used, than with some or even most USB stuff. Just guessing.
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