Posts: 18
Joined: Tue Aug 06, 2013 4:56 am

Power supply issue

Sat Aug 17, 2013 8:20 pm

What specs in power supply should I look for? I originally got 5v 1a with 2 usb ports, it works until there is some load especially from the hdd, it causes the pi to restart. so I got very powerful one supposedly 2.1A each port, but strangely the pi slows down rapidly, and it seems the usb psu gets hot. is this a bad psu, or is there something else I'm missing? I thought it is all about enough mA and 5v

User avatar
Posts: 2027
Joined: Tue Jan 01, 2013 9:33 pm
Location: East Midlands, UK

Re: Power supply issue

Sat Aug 17, 2013 8:31 pm

How is your hdd powered?

Ideally, it should have its own power supply. Second best, it could be usb-powered if it was connected via a usb hub that has its own power supply.

Trying to power a high-demand device like an hdd from the usb port on the RPi is likely to produce the symptoms you are describing.

(If you have a multi-port powered usb hub, perhaps you could power the RPi from one port, and run the hdd from the other?)
How To Ask Questions The Smart Way:
How to Report Bugs Effectively:

Posts: 7137
Joined: Sat Jan 12, 2013 3:01 am
Location: Grants Pass, OR, USA
Contact: Website

Re: Power supply issue

Sat Aug 17, 2013 11:53 pm

When the RPi is powered via the micro USB power plug the current first passes through a 'poly' fuse. The poly fuse is a self resetting type. When the fuse is tripped it will self heal if left alone and powered down for a few hours.

The fuse designated F3 on the board can only handle about 750mA of current. Any higher than that the F3 will start to heat up and increase resistance. The increased resistance will cause the voltage in the RPi to drop and cause unstable operation. Any currents above 1A or so will cause the fuse to fully trip within a few seconds.

In theory a 750mA power supply is all you need. But many power supplies have trouble holding the voltage to 5V when at their maximum output load. Also some manufactures of cheap power supplies have been known to overstate their specifications. So getting a 1A to 2A rated power supply is usually recommended. I am running a 10A which is way 'overkill'.

The RPi itself uses 300mA-400mA. Mouse and keyboard use another 150mA or so. There not much left over for power hungry USB devices.

As far as I know there are no USB hard drives which will work in the RPi if directly connected and powered by the RPi USB port.
You must use a powered hub OR a hard drive enclosure which has it's own power supply.
Unless specified otherwise my response is based on the latest and fully updated RPiOS Buster w/ Desktop OS.

Posts: 18
Joined: Tue Aug 06, 2013 4:56 am

Re: Power supply issue

Sun Aug 18, 2013 3:38 am

the hdd has a secondary usb cable, that is meant to distribute the load, I connected that to the usb power supply which has 2 ports. but based on your explanations I guess it might be the hard already adding more strain on raspberry. each port on this usb can deliver up to 2.1A.

the hard works well with raspberry if it has enough power, I switched back to other usb adapter, but connect the extra cable to PC which I don't want to do it as permanent solution.

Do you think if I get 2A powersupply 1A on each port, and connect the extra usb cable of hard to one port, that might solve the problem, or it will be same? since you said 750mA is the start of the trouble.

Another thing, I connect a usb hub but it is not self powered, if I connect usb cable from one port on the hub to the power supply, would it work in relieving the load on raspberry ? I'm not sure if safe for the hub or psu.


W. H. Heydt
Posts: 12442
Joined: Fri Mar 09, 2012 7:36 pm
Location: Vallejo, CA (US)

Re: Power supply issue

Sun Aug 18, 2013 3:52 am

The spec for USB 2.0 is for a maximum of 500mA per port, so a 4-port hub should have 2+A power supply. if it is backfeeding (and most appear to do so...damnit), one would expect it to need at least 2.5A.

(The reason for the "damnit" is that I prefer to power Pis with the Adafruit power supplies, which output 5.25V, and when you connect a 5V powered hub to a USB port God only knows how much current is going to be flowing in which direction. If I only had an adapter that was M-F USB A with no power lines in it, I could defeat such ill behavior without having to chop up cables. Or if powered USB hubs had a way to deselect power on their uplink side. I am not a hardware hacker.)

Posts: 18
Joined: Tue Aug 06, 2013 4:56 am

Re: Power supply issue

Sun Aug 18, 2013 8:04 am

I know I probably squeezed the raspberry. though the usb hub only two devices connected to it, mouse which I rarely use but connected most of the time and bluetooth

the hdd is the worst of all, particularly this one, even on older pcs struggle.

I checked config.txt and more about overclocking, and tweaked it , and now not just it is much snappier, this usb power supply now seems to work fine. I removed overvoltage which was put by default raspi-config overclocking when choosing 950

I removed overvoltage, reduced gpu_mem to 64
core. I simply made it same as one of the posters on this thread: ... 35&t=24636


now emulationstations even dosbox works like a charm. it was too slow.

Still not sure why the 2.1A/ea port usb psu can't handle overclocking when overvoltage=6 while 1A can handle that but have to connect hdd extra power to PC or else it resets when under heavy hdd work. Both of them has 5v written on them as output.

The 2.1 one still gets hot quickly. normal?

tl;dr summary: now works after removing overvoltage from config.txt, but usb psu still gets hot, hope it is normal and doesn't explode

Return to “General discussion”