Zoandar
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Absolute max/min supply voltage?

Thu May 02, 2013 9:30 pm

I am looking for some power supply absolute limit specifications. I'm new to the PI, (specs of my hardware are in my signature) and really would like to use a 6ft. USB cable for power, so that I can loom that cable with the other cables running to the Pi (HDMI, LAN, USB from a powered hub) for a neater desk while working with it. However, after researching which 5V power supply and cable might be a good choice (I got the Amazon Basics gold plated 6 ft. cable and their 5V 2.1A USB charger supply) I am still getting the same results as every other 'longer than 3 foot' USB cable I have tried. That being, a voltage drop at the Pi down to around 4.55V, measured from my GertBoard GND pin and 5V pin near its GPIO connection header.

Using the Canakit 3 ft. USB cable and their 1A USB charger supply that came with my Pi, I see a range of 5.1 down to 4.97V while the Pi is operating. It is the only cable of many I have which does not seem to drop several hundred mV on the way to the Pi.

The Pi seems to be working currently at 4.56V, using the Amazon 6 ft. cable and supply I just bought, but the first time I discovered that a longer cable causes this nearly 0.5V drop, it failed to boot properly. So it is likely on the bleeding edge of being 'happy'. I'd like to learn what the safe performance "envelope" is for supply voltage for the Pi, and then build my own regulated power source into which I can plug the Amazon Basics 6 ft cable and feed the Pi. It seems my source will need to exceed 5V a little, but I don't yet know what would be a safe level at which to regulate it. I see references of 4.8 to 5.2V mentioned on the forum, but if my 6 ft. supply cable is going to be dropping nearly half a volt (and I have yet to actually connect anything to the GertBoard) in itself, I think I would like to supply as close to the top end of recommended voltage so it has some 'wiggle room'. That said, what are the absolute maximum and minimum voltages considered 'safe' and reliable when measured at the Pi itself for its supply voltage? Can it use 5.5V? Or even a little more? I would probably use an adjustable regulator with pot in the circuit so I could start out at the low end and carefully increase the supply voltage while monitoring it at the Pi itself, until it shows a consistent 5V while operating.
- Zoandar -
Current:
Pi-Top Notebook with Pi 3 B v. 1.2
Older hardware:
Rpi Rev B - 512 MB with Rev 2 GertBoard

pjc123
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Re: Absolute max/min supply voltage?

Thu May 02, 2013 10:57 pm

The specs for the pi's power supply are 4.75V to 5.25V.

If you are that worried about getting an exact voltage at the pi, then I would stop messing around with usb cables and USB charger power supplies.

Get yourself a real power supply with a voltage trim pot that you can adjust for cable loss so there is exactly 5v at the pi's test points, for instance:

http://us.tdk-lambda.com/lp/products/ls-series.htm

And hard wire the power supply to a USB connector using large AWG wires (Like 22 AWG or larger).

Now for one of my pi's I do exactly that (The real reason is that I need +12V elsewhere that I convert to +5V for the pi), although on the other pi I do use a 6' USB cable with a USB charger power supply. To be honest, I am within the power supply specs for the pi (4.75V to 5.25V) in both cases and I have not had any problems with either one no matter what I attach to the USB ports. The manufacturer of my 6' USB cable (Mediabridge) claims that it has 22 AWG power cable wires. There are also Monoprice brand USB cables that have 24 AWG power lead wires that I am also using successfully, and although mine are 3' lengths, they are also available in longer lengths.
My Raspberry Pi Project Page:
https://www.flaminghellmet.com/launch/

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rpdom
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 5:43 am

You could do what I did, which is cut the ends off my cheap, rubbish, microUSB lead, leaving a short length of lead on each. Stripped back the wires a bit and soldered a length of fairly heavy duty power cable to them. I used some red and black figure-8 power cable, rated 3A (order code XS69 at Maplin).

That was for my main Pi and gets 4.95V on the test points.

For one of my other Pis I had an old 5V (non-USB) PSU from some device that has long died, cut the thin cable from that leaving a stub. I also cut the socket end from a USB extension lead. I joined the two with over 6 feet of the same cable. That is currently powering a Pi via a Kindle microUSB power lead and showing 4.92V on the test points. Total cable length about 13 feet.

W. H. Heydt
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 5:50 am

And for more data... I regularly use 6 ft. USB cables with no problems. The cables I use are good quality (heavier wires, especially on the power leads) than the cheap phone charger cables. In addition, I prefer to use the Adafruit power supplies, which are set to output 5.25v, thus offsetting the losses in many cables.

Zoandar
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 2:57 pm

Thank you all for the suggestions. Although I have all the components here to approach this from several angles, and I was planning to build an LM317 regulator circuit for a solution later today, to be used with one of several types of 12V supplies on hand, the option to actually "build" a beefier power cable by replacing its central run of cable with heavier leads is looking like my best option for several reasons, mostly having to do with where the supply would be sitting in relation to the Pi if I made a regulated line dedicated to the Pi. The place where the current USB style wall charger resides in a small outlet strip cannot accomodate a larger power supply.

Using the USB 'A' and 'micro' ends of one of my existing cables (I have some inexpensive 10 foot cables I am willing to sacrifice) would also prove interesting, to learn if it is just the wire run over 10 feet, (or 6 feet, in the case of the cable I wanted to use) or whether the cheap connector ends themselves are in some way also contributing to the 0.5V of line loss I am seeing. I am guessing the 'figure 8' cable one of you mentioned you have at Maplin in the UK is what we in the US call 'zip cord'? If so, I have some of that in black and red trim in both 24 AWG and 18 AWG, the latter of which I would use here. Doing so should also allow me to continue using the 2.1A USB charger I just bought for the Pi. And I can thus customize the length of the cable to fit the intended loom route perfectly (6 feet is actually a little longer than I need).

I wanted to mention that during my testing with different cable lengths and supplies last night, I saw the Pi boot and appear to be quite stable on as little as 4.25V. But since I have also seen it spit out a screen full of "waiting for hardware interrup" messages and fail to boot under similar low voltage circumstances, it is the best of stability I seek. So providing genuine 5V at the Pi seems desirable. All aspects of the Pi, learning Python, etc. are new to me, so I am trying to eliminate as many of Murphy's ill fated surprises as I can here. Because with my luck, I'll see every one of them before I reach my goals. ;)

Thanks!
- Zoandar -
Current:
Pi-Top Notebook with Pi 3 B v. 1.2
Older hardware:
Rpi Rev B - 512 MB with Rev 2 GertBoard

obcd
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 3:37 pm

I use 4.85V as the lower treshold. The reason is I had 3 identical adapters. One of them it's voltage was a little lower than the others. (Still had 4.81 on TP1 - TP2). I used a hdmi 2 dvi adapter and had the Pi connected to a pc monitor like that. The supply with the lower voltage showed strange interference in the image. I could fix it with the hdmi_boost in the /boot/config.txt file, but still, I feel it shouldn't be like that. I soldered another 1 Amp polyfuse above the one already on the board. This also raised the TP1 - TP2 voltage with 0.12V. It's not much, I know, but it can make the difference between stable operation and "not so stable" operation.
With 4.85V or more, if something doesn't work, at least you know it's not your supply you should suspect.
My power adapters come from RS and have the supply cable with mini usb connector fixed attached to the adapter. The wires seem solid and it delivers 5V at it's connector under normal load conditions. They are cheap, and I can recommend them.

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rpdom
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 4:50 pm

It's not so much the Pi, which can run on quite a bit lower than the specified voltage, but the USB peripherals connected to it, including the USB/LAN chip on the Model B.

Zoandar
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Re: Absolute max/min supply voltage?

Fri May 03, 2013 9:34 pm

OK, I hacked a cheesy Chinese USB to micro USB cable, and added 6 feet of nice 18AWG wire into it. I used only about 3" of the Chinese cable on each end to make the connections. It has extremely fine wire. I am not sure of the gauge, but I use a pair of Klein 11057 precision wire strippers to work with this fine wire, and it has notches for all the way down to 32AWG. I stripped the first tiny wire with those 32 AWG notches just as a test, and I do not think I cut the copper core. I moved up to the 30 AWG to do the other 3 wires.

After soldering and heat shrink assembly for support of the tiny wire connections I am seeing between 4.88 to 4.92V on the Gertboard 5V pins. (It obscures one of the Pi power TPs, so it is easier to clip micro clip leads to the pins on the Gertboard itself for monitoring).

Based on all your advice, this will likely work OK.

One added note I discovered in this project - The Amazon Basics 6 ft. gold connector cable I just bought has stamped right on it "28 AWG/1P AND 28AWG/2C". This indicates that ALL the wires within the cable, both for power and data, are 28AWG wires.

Some USB cables, including the 3 foot cable that ships with the CanaKit version of the Pi, have the following marking : "28AWG/1P + 24AWG/2C". These cables use 24AWG wires to carry the power in the cable. One reason it has less line loss. :) What would really be nice would be to find a USB cable that would have even heavier gauge wire. But I don't know if anyone makes such a product.
- Zoandar -
Current:
Pi-Top Notebook with Pi 3 B v. 1.2
Older hardware:
Rpi Rev B - 512 MB with Rev 2 GertBoard

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stevejferry
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 9:59 am

Guys,

I'm wanting to power a Raspi with rechargeable NiMH cells. These are rated at 1.2V so 4 cells would be 4.8V and 5 would be 6V however, I believe that a fully charged cell has a higher voltage and that 1.2V is the nominal voltage.

Has anyone else used rechargeable batteries and did you use 4 or 5 cells and if 5, did you need a voltage regulator or is the Raspi OK with 6V?

Thanks, Steve.
There are 10 types of people. Those who understand binary and those who don't.

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joan
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 10:17 am

At 6V and over you are definitely into danger territory. I'd guess some Pi's might survive (depending on the tolerances of their individual components) but most would die.

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penguintutor
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 10:28 am

stevejferry wrote:Guys,
Has anyone else used rechargeable batteries and did you use 4 or 5 cells
I have run with 4 x AA (Hi Capacity Hybrid batteries).
Despite nominal voltage of 1.2v per battery you actually get slightly more on fresh batteries.

This is enough to run the Pi and power small motors:
See my battery powered robot vehicle

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redhawk
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 10:30 am

A better solution would be to use 5v step-up buck regulator providing the input voltage does not exceed 5v i.e. 4.8v

Richard S.

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mahjongg
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 11:23 am

rpdom wrote:It's not so much the Pi, which can run on quite a bit lower than the specified voltage, but the USB peripherals connected to it, including the USB/LAN chip on the Model B.
Actually NO! The LAN9512 is powered with 3V3, so it is impervious to lower voltages too!
Its really ONLY the USB devices that need a voltage above 4.75V, at least some USB devices do!
When they do not get it many USB devices start to fail, and that crashes the PI!
Without USB the PI would probably be content with any voltage that the 3V3 LDO regulator is content with, although the Broadcom SoC does use 5V directly for its built in switcher that provide the core with a voltage around 1.5V, but that probably won't fail before the 3V3 regulator fails, but it might draw more current when fed with say 4V than with 5V.

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stevejferry
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 2:16 pm

The Raspi has 3 on board regulators, a 3V3 one (RG2) an inch along from the power socket (that's 25mm to you young'uns), a 1V8 regulator (RG1) directly opposite RG2 beside the GPIO pins and RG3 half way between RG1 and the video out which is too small for my eyes to see what it is. I would have thought that the 3V3 regulator would protect the sensitive components and that any higher voltage applied via the supply socket would only go to power any USB devices.

However, I think better safe than sorry and for a few extra quid, I will opt to use 6V with a 5V0 regulator.

Thanks for all your answers, Steve.
There are 10 types of people. Those who understand binary and those who don't.

Zoandar
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Re: Absolute max/min supply voltage?

Fri Jul 04, 2014 2:46 pm

Any time something is powered with batteries, the batteries will slowly decrease in voltage available as they discharge. And of course we also know that most batteries will yield more than their rated cell voltage when fully charged. For these reasons I think using a 5V regulator circuit would be very prudent. That way you could actually use more cells than trying to exactly put together a 5V source, and power would last much longer until the supply voltage from them goes too low for practical use. You could even use some kind of 12V battery, or RC hobby lithium polymer batteries (lipos), so long as the regulator you choose can handle whatever input voltage you opt for. :)
- Zoandar -
Current:
Pi-Top Notebook with Pi 3 B v. 1.2
Older hardware:
Rpi Rev B - 512 MB with Rev 2 GertBoard

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beagle2017
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Re: Absolute max/min supply voltage?

Sun Jul 23, 2017 1:28 am

I noticed often times the issue is with the USB cable being too thin and/or too long, also the usb mini connector has a lot of drops.

I used a regular 1A 5V phone charger with 3 ft USB cables with custom 18AWG wires for GND and 5V. The voltage measured at the mini USB is nearly identical to the voltage measured at the charger output USB port, but the voltage at TP1/TP2 was 4.4V!!!! The mini usb connector dropped 0.6V with the pi just idling.

This made me solder the 18awg wires directly on the 5V input (+wire on F3 pin1, -wire any nearby ground). I can now plug in any accessories, and there is no more voltage drop whatsoever.

EDIT: I take back what I said above. There's also a voltage drop of 0.46V on F3, a 5V 1A SMD fuse. Even with direct wire solder onto VIN, TP1 still measure 4.5V

I am using the Raspberry Pi 1.0.
Raspberry-Pi-Schematics-R1.0.pdf Date 17Apr12

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