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Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 2:25 am
by Codyak
I had a quick question for those who had a better understand wiring/electricity and what not. I was hoping(more of just a possibility) to mess with using a standard old computer power supply to power a few Raspberry Pi's.

I stripped a micro usb cable, hooked it to the 5v and ground on both ends to the PSU, tested the voltage and it came out at 5.05 and then tried it in my phone. According to what I am reading, it's only getting about 60mA to the phone(that came with a 1A charger), which obviously isn't enough to do just about anything. Is there a way to boost this as the total rating for the PSU is 15A for 5v? Am I possibly doing something wrong? Is there possibly another combination of cables I could use to help?

Any ideas/suggestions would be much appreciated!

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 2:45 am
by error404
The USB spec requires that devices draw < 100mA from the bus before they negotiate for more from the host, so with no communication from the host, that's what it will do. Try shorting the data lines together. This is the standard way to signal to the peripheral that it is connected to a dumb power provider, rather than a smart host, and should allow it to charge.

Depending on your phone though, some require particular voltages on the data lines to specify exactly how much current is acceptable (some tablets can charge at a couple amps, for example), and might not (*cough* iPhone *cough*) charge at all without this special undocumented signalling.

RPi shouldn't have either of these issues though, just hook it up and it will work fine.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 2:57 am
by Codyak
Ah, well that makes perfect sense. Now, the cable I am using only has a red and black cable, no data cables that I can tell, any other way to set that "dumb" signal with that setup? I actually cut an old USB wall to charger I had sitting around. The wall charger was charging the phone with 700mA(ish) before I cut it and hooked it to the PSU. Thanks again.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 3:12 am
by Codyak
Kind of a side note/info that may help. I noticed that when just idling, it was using 9 Watts, when I plugged the phone in, it jumps to 14.3. Now maybe I am wrong here but 5.3 watts/5.05 Volts should be just around 1 amp?? It easily takes twice as long to charge with the psu hookup than the 1 amp wall wart I was trying however.

Any suggestions??

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 4:42 am
by error404
If this two lead charging cable worked for you with a charger, I don't really have an answer for you. The ATX supply should work just as well, since any signalling stuff must be in the plug itself. Have you measured the voltage out of the charger that was working? I've heard of some devices wanting closer to 6V to charge at full rate, but I wouldn't think it would refuse to charge at all without that…

You'll lose some power in inefficiency, especially at such a low load, but yeah, you should still be getting at least 500mA into the phone.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 6:14 am
by shiftyphil
For some ATX supplies you may need to put a dummy load on the 12V line as well.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 9:10 am
by Tozzi
It should be a sticker on the ATX with spec on every line (12V line, 5V line, 3,3V line and etc info with A on every).

Do you have that sticker? If it would work, you could plug in many many USB connections on the box (no wiring outside beside USB cable to the boards).

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 9:18 am
by dukla2000
Codyak said:


I noticed that when just idling, it was using 9 Watts, when I plugged the phone in, it jumps to 14.3. Now maybe I am wrong here but 5.3 watts/5.05 Volts should be just around 1 amp?? It easily takes twice as long to charge with the psu hookup than the 1 amp wall wart I was trying however.

Any suggestions??


The ATX PSU is most likely running well below 50% efficiency at low power outputs. You say it is old – if it is anything other than reputable maker (FSP, … there are several) or more than 2 years old it may be around 20% efficient at that low power.

BTW – I too am planning to power my Pi from my PC. But figure on getting some pins onto the GPIO place and jumpering 5V straight in. Figure one of the old FDD 3-pin headers looks perfect with the middle pin not connected (on the Pi side).

PS - as per shiftyphil having a load only on 1 rail of an ATX PSU is likely to be a poor config: many more recent PSU specify a min load on 12V (and 5V) to get them to run the way they were intended.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 9:55 am
by bredman
An ATX PSU will not supply current unless it is properly loaded.

Google for "ATX PSU as bench supply".

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 8:49 pm
by arm2
One ATX PSU manufactured told me their PSU's need 2A minimum power drain on 12V,5V and 3V3!

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 9:19 pm
by WereCatf
arm2 said:


One ATX PSU manufactured told me their PSU's need 2A minimum power drain on 12V,5V and 3V3!



Sounds perfectly logical, though, ATX PSUs aren't designed with very low loads in mind. I personally wouldn't use one for RPi because all the waste power, it is not worth it. I've got multiple cell-phone chargers here that I can use, they're a lot better choice for such low loads as RPi.

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 9:23 pm
by gruetzkopf
The Pi should be able to run from the Standby 5V easy enough.

(Like the name says, there is always power on it to keep Power Controller and other stuff online even if switched off)

Re: ATX PSU as Power supply

Posted: Fri Jan 27, 2012 11:29 pm
by hippy
gruetzkopf said:


The Pi should be able to run from the Standby 5V easy enough.


Excellent point. From the ATX12V Power Supply Design Guide ( Version 2.2 )

http://www.formfactors.org/dev.....ic_br2.pdf

"The +5 VSB output should be capable of delivering a minimum of 2.5 A at +5 V ± 5% to
external circuits"

However version 1.3 states 2.0 A, version 1.1 states 1.0 A. Not sure about original ATX. I don't know if that's usually stated on the PSU label.