michaelp
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Joined: Mon Aug 13, 2012 3:38 pm

4.6v. problem

Mon Aug 13, 2012 3:57 pm

I set up my nice Pi a few days ago. It seems to be going fine, despite me only measuring 4.6 volts between TP1 and TP2.
I bought a D-link DUB H4 which I am using to power the Pi via a 'Y' cable and for the keyboard and mouse (I should have got the 7 port version though).
The open circuit output voltage of the hub is 4.94 volts, whether it's measured at the psu, port or at the end of the 'Y' cable.
The keyboard and mouse are 100mA and 50mA so the hub supply is well over-specified.
Where could my voltage be going please?
Many thanks,
Michael

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RaTTuS
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Re: 4.6v. problem

Mon Aug 13, 2012 4:03 pm

poor quality Y cable?
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pwinwood
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Re: 4.6v. problem

Mon Aug 13, 2012 4:40 pm

michaelp wrote: The open circuit output voltage of the hub is 4.94 volts, whether it's measured at the psu, port or at the end of the 'Y' cable.
I suggest that you measure the loaded output voltage of the hub. My USB hub came with a power supply that measured 5.5 volts open circuit that fell to 4.5 v with a load drawing 450mA. This was despite the power supply being rated at 2A. I had a spare better quality power supply module that delivers 5.05 volts at all load currents.

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rurwin
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Re: 4.6v. problem

Mon Aug 13, 2012 5:58 pm

You wont get a meaningful figure measuring the output of the cable open-circuit. It only starts to drop voltage when current is flowing.

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Montala
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Re: 4.6v. problem

Mon Aug 13, 2012 6:13 pm

It is surprising just how much the voltage can drop, once you connect a couple of peripherals, such as wi-fi adapters for example, even if the RasPi is being supplied directly from the hub.

As has already been mentioned, this could be down to one (or more) factors, namely a sub standard power supply (avoid cheap EBay ones!), or thin poor quality power cables. The quoted rating printed on the PSU plug can't always be relied upon either, unfortunately.

It is always best to check the voltage between TP1 and TP2 under load, with everything connected, and preferably with a live internet connection, and it may well be lower than you anticipated... mine certainly was!

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Re: 4.6v. problem

Mon Aug 13, 2012 7:05 pm

I suspect that there can be other problems as well....

I got in two Pis. Using the same power supply and cable, with no peripherals attached (just power and SD card...the same SD card in both cases), one Pi tests at 4.87v. The other tests at 4.42v.

The one that tests at 4.42v starts to boot and then shuts down. I have put in to have it replaced.

michaelp
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Re: 4.6v. problem

Tue Aug 14, 2012 8:03 pm

Thank you for the replies.
So it could be the power supply, the cable or the RasPi.
I have swopped the 1m. Ex-Pro 'Y' cable I had got (it was recommended on the forum), for a StarTech 300mm 'Y' cable and the board voltage has risen to 4.71v.
I got the D-Link DUB-H4 (not some eBay unknown psu) - this was recommended on here and the Wiki and it has a 2.6A supply.
The voltage on an unused hub port remains at 4.93 when the Pi is running. So the supply isn't dropping under light load?
What are these good quality power supplies and where from? I thought paying out for the D-Link hub was a neat solution and would provide me a decent supply.

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