Raspberry Pi 2 "5 V Power Supply" Notes

Certainly when I was learning Electronics with the good old TTL and CMOS chips along with the CPU/ ROM/ GPIO chips of the time, it was all 5V +/- 5%, not 10% but having looked at a few of the common PDF details for some chips I'm using now, they are indeed all 10% (1970s - 1990s) compared to 2015

This does not imply ALL chips are now 10%, just that some are, so I would still be cautious of assuming 10% is good for all and as has already been stated, without schematics no-one can be sure

On a separate note, this is a USB connector and the PSU can be (And at some point will be) used on other devices. the USB specification clearly states +5%, - 11% so between 4.45 and 5.25V MAX. but up to 5A in the case of a charging interface so a wall brick with USB interface outputting more than 5.25V is OUT OF SPECIFICATION.

Hope this helps

Peter

Certainly when I was learning Electronics with the good old TTL and CMOS chips along with the CPU/ ROM/ GPIO chips of the time, it was all 5V +/- 5%, not 10% but having looked at a few of the common PDF details for some chips I'm using now, they are indeed all 10% (1970s - 1990s) compared to 2015

This does not imply ALL chips are now 10%, just that some are, so I would still be cautious of assuming 10% is good for all and as has already been stated, without schematics no-one can be sure

On a separate note, this is a USB connector and the PSU can be (And at some point will be) used on other devices. the USB specification clearly states +5%, - 11% so between 4.45 and 5.25V MAX. but up to 5A in the case of a charging interface so a wall brick with USB interface outputting more than 5.25V is OUT OF SPECIFICATION.

Hope this helps

Peter

Oh do not use a cheap USB powered hub. I did the other day and it blew out the laptop I was using. The battery is so dead, it will not power the laptop while inside. Luckily no battery works.

Clem

Yeah the older generations of 5V parts seem much wider and it's the newer ones that insist on the 5% in fact 5% on many of the 3.3/1.8... parts.

I assume regulation has got better and designers rely on it!

I've had that happen before, It's horrible!!!

The issue with most of these chargers ... are just that.

They are designed to charge the battery of the device they were supplied for.

A 3.7/4.1v battery doesn't really care what voltage it gets so long as it is enough for the charging circuit.

Sadly some of the recommended power packs supplied are not up to the job.

I haven't tried to find a 2A version, so I can't say which one will work.

Mark

Similarly, most of the 'pi is designed to run off a 3.7V lithium battery. So also the 'pi doesn't care what voltage it gets.

This argument breaks down for:
* HDMI: The pi should provide some 5V power on the connector. The HDMI->VGA converters rely on this.

* USB: The pi should provide 5V to the USB devices. If this voltage is outside spec, some USB devices refuse to work.

The first 'pi had a "lowsy" 3.3V regulator. It required a voltage drop of at least 1V to operate. This means a 5V on the pi of 4.3V was required to keep the 3.3V on the pi in regulation. I expect that a new pi2 has a much lower dropout on the 3.3V and will operate just fine at 3.4V on its 5V line.... (But I haven't tried it yet, so I might be wrong).

Let me back up my claims and I'll report back today with measurements, ok?

Afraid to say that you are, the PI2 even has built in voltage monitor and it triggers around 4.65ish volts, this is indicated by the small rainbow square showing up in the top right corner of the console and indicates a low input voltage is detected.

On several of my PI2's I have observed issues even with standard USB adapters but poorer USB leads triggering this indicator and in some cases the PI2 even will hang. The PI2 may well have better regulators (Buck vs Linear) but it is actually worse for tollerating lower input voltages

I will do a video on this soon showing a formal test of input volts over stability , jsut need to get a few other things out of the way first

Peter you are right about the newer Raspberry Pi are voltage tolerant as long as 5 volts +/- 10%. Otherwise the artefact will occur in upper right corner warning about low voltage. I first saw this on my Pi 2.

Clem

OK.

I ran the tests......

When slowly decreasing voltage, I seem to be able to reach voltages BELOW 3.3V, and keep the 'pi running.

When booting, a voltage of about 3.5V seems to be required.

I've spent a good part of the last two weeks debugging power-problems on one of my own designs. So I've gathered a bit of experience. I'm guessing that during boot a short power-spike causes things to drop even further. My previous measurements on the first 'pi indicated that the LAN chip is the critical part. It requires 3.3V +/- 10%, and will continue to work (as specified) down to about 3.0V. As predicted the dropout of the switching regulator for the 3.3V is a lot less than the dropout of the 1117 on the first pi. And about 0.2V sounds about right for the dropout of such a regulator. Current spikes while booting probably cause a drop below 3.0V.

My pi currently runs at 0.25A at 5V (1.25W), while it booted at 3.5V and ended up with 0.35A (1.22W). So the difference in power consumption is very small.

Note that I've taken care to remove all possible 5V-requiring peripherals.

Also note that by lowering the voltage below 3.3V I've risked damaging my SD card. The SD card also runs from 3.3V.

One funny thing: My power led has not come back on now that the voltage is back at 5.0V after booting at 3.5V.....

So..... For battery applications, you are better off running the pi directly from a lithium ion battery than transforming the battery voltage to 5.0V (probably at 80% efficiency). Then, if you have 5V peripherals, you should arrange to feed them separately: you're saving about 0.2W by not passing the power for the pi itself through the step up converter.

[update: The pi boots just fine with only 3.2V, I was using bad leads to power it.....]

[update2: Still boots with 2.93V (doesn't boot with 2.7V, even though it remains running at that voltage)..... ]