CrossNox
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Joined: Sun Mar 15, 2015 5:38 am

Measuring PSU max current

Sat Aug 13, 2016 4:37 am

I recently bought an external hard drive, to use with a b+, but failed to think about the usb current from the pi to power it. Searching in several forums i found about adding

Code: Select all

max_usb_current=1
to the config file (in my case for openelec+kodi). Having added that, i ssh'ed into the pi and

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lsusb
lists the external hard drive. But it won't load up and beeps every 5 or so seconds. So i started doubting about the psu specs being real (a samsung phone charger that reads 5V, 2A output). So i cut in half a male-female usb cable, conected the male to the PSU, the female to a USB(male)-microUSB and then to the raspi. I joined all the inner cables from the male-female cable except the red one and put an amperimeter in the middle. I plugged the raspi in and after it booted, the amperimeter read 0.35A. Then I plugged the hard drive and the amperimeter read 0.65~0.7A.
Is this a reliable way to assert that this PSU does not output 2A?

mosespi
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Re: Measuring PSU max current

Sat Aug 13, 2016 4:50 am

Your testing and meter setup seems correct. Although to be complete I would want to monitor the voltage as well, to see if it gets dragged down too much.

What you are likely not seeing is the high startup current needed for your hard drive to start spinning. It usually causes the Pi to crash.. but looks like your hard drive isn't too heavy on the Pi and gives up the startup and keeps trying (the 5 second beeps). Most meters are not quick enough to catch these intermittent power spikes.

Try a powered USB hub if you have one, or a different hard drive. Some of the smaller 2.5" based USB hard drives can run on the Pis USB power I believe, the larger 3.5" desktop drives probably not.

EDIT.. You can load up the Pi with something else.. some other USB device, or something other then a Pi even. and see if it works, take a voltage reading too. I would trust the ratings on a Samsung supply.

Regards,
-Moses
Power problems? MoPower UPS for the Pi
http://www.allspectrum.com/mopower/

fruit-uk
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Location: Suffolk, UK

Sat Aug 13, 2016 5:12 am

Inline USB voltage/current displays are available (search for USB volmeter or similar) but possibly not very accurate and won't show instantaneous values such as HDD startup current.

klricks
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Re: Measuring PSU max current

Sat Aug 13, 2016 5:29 am

CrossNox wrote:..... Is this a reliable way to assert that this PSU does not output 2A?.....
No. The RPi as well as any electrical appliance only uses what it needs and no more.
To test the power supply you need a load resistor (or some form of resistance such as a light bulb).
Using Ohms law we find that the load resistance for 5V @2A would be 2.5 Ohms and the power used would be 10 Watts
So to test you would need to measure the output voltage under full load (2.5 Ohms) and verify that it stays in specification 5.00V +-5%. Also measure the current at the same time. Note that a 10 Watt resistor would be very large and expensive. So this is not really feasible.
But you can still be reasonably sure that the power supply is working by just checking voltage under load and make sure it stays in specification.

Did you reboot after changing the config.txt settings?
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

CrossNox
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Re: Measuring PSU max current

Sat Aug 13, 2016 6:08 am

First, thanks to all for your replies.
mosespi wrote:Try a powered USB hub if you have one, or a different hard drive. Some of the smaller 2.5" based USB hard drives can run on the Pis USB power I believe, the larger 3.5" desktop drives probably not.

EDIT.. You can load up the Pi with something else.. some other USB device, or something other then a Pi even. and see if it works, take a voltage reading too. I would trust the ratings on a Samsung supply.
Truth is i can't tell if it is a copycat charger or an original, and if i had to bet, i'g go for the former. The hard drive is a WD elements 1TB, which i think is a 2.5 inches drive.
klricks wrote:Did you reboot after changing the config.txt settings?
Yes, i did. Maybe i should update the openelec version and see if it works then.

fruitoftheloom
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Re: Measuring PSU max current

Sat Aug 13, 2016 6:18 am

Retired disgracefully.....

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FTrevorGowen
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Re: Measuring PSU max current

Mon Aug 22, 2016 9:44 pm

(Slightly OT) FWIW, wrt the Samsung PSU, is it one of the three types I've measured the loading characteristics of?:
https://www.cpmspectrepi.uk/raspberry_p ... ng_TC_E250 (Probably not)
https://www.cpmspectrepi.uk/raspberry_p ... ETA0U10UBE (3 samples, but, also, probably not)
https://www.cpmspectrepi.uk/raspberry_p ... EP-TA11UWE (more like this perhaps?)
Trev.
Still running Raspbian Jessie on some older Pi's (an A, B1, B2, B+, P2B, 3xP0, P0W) but Stretch on my 2xP3A+, P3B+, P3B, B+, A+ and a B2. See: https://www.cpmspectrepi.uk/raspberry_pi/raspiidx.htm

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