sharix
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25W Power supply

Fri Dec 27, 2013 10:32 pm

Can I use a 25W (5V, 5A) power supply to power my Pi (through the standard micro usb port)?
This is the PSU in question:
Image
Input: AC100-240V 50/60Hz
Out put: DC 5V 5A 25W
Out Adjustable +/- 10-15%
100% full load burn-in test
Protections: overload/ over voltage/ short circuit

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FLYFISH TECHNOLOGIES
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Re: 25W Power supply

Fri Dec 27, 2013 10:46 pm

Hi,

Yes, you can use it.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

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mahjongg
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Re: 25W Power supply

Fri Dec 27, 2013 10:47 pm

You could try, after you measure if the unloaded output voltage is below 5.25V!

These PSU's are not designed to deliver so little current.
Also If they deliver too much voltage (>6V) and you connect the PSU through the GPIO you stand a good chance to burn out the PI's overvoltage protector device.

sharix
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Re: 25W Power supply

Fri Dec 27, 2013 10:53 pm

Thanks for your quick reply. It does have a regulating potentiometer so I guess I'll turn it down all the way first and then slowly rise the voltage to match 5.00V. As I said, I will connect it to the rPi's micro usb connector so it should be protected by the fuse.

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Re: 25W Power supply

Fri Dec 27, 2013 11:05 pm

Hi,
sharix wrote:It does have a regulating potentiometer so I guess I'll turn it down all the way first and then slowly rise the voltage to match 5.00V.
While doing this make sure that the current will be similar to one burned by RasPi in its final configuration (probably something around 1A).
If you'll do this regulation with RasPi, don't get scared if some USB devices will not be recognized when applying this lowest voltage.
sharix wrote:I will connect it to the rPi's micro usb connector so it should be protected by the fuse.
I'd not count on the polyfuse for overvoltage protection.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

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mahjongg
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Re: 25W Power supply

Sat Dec 28, 2013 1:32 am

The way the over-voltage protection works is this:
Its basically a thyristor switched between the 5V line and GND, it is set to trigger at about 6.0V, so if that happens it shorts the 5V line to GND, even if many Amperes are running through it, until the current drops below the "thyristors" hold current/voltage.
If on a PSU that can deliver (about) 5V even when drawing 20A you can imagine what happens to the poor device!
Normally the polyfuse would immediately blow, and it would survive, but without the polyfuse...... :o :(

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Re: 25W Power supply

Sat Dec 28, 2013 1:53 pm

sharix wrote:Thanks for your quick reply. It does have a regulating potentiometer so I guess I'll turn it down all the way first and then slowly rise the voltage to match 5.00V. As I said, I will connect it to the rPi's micro usb connector so it should be protected by the fuse.
I am using a 5V 10A supply and have no problem running my RPi 24/7 ..... However, as mentioned some larger power supplies do not work well if the load is too small. The PS may not start up and/or regulate properly. If that is the case then the voltage trim pot will not help. You would need to add a dummy load such as a light bulb, fan, or load resistor.
Unless specified otherwise my response is based on the latest and fully updated Raspbian Buster w/ Desktop OS.

sharix
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Re: 25W Power supply

Sat Dec 28, 2013 2:24 pm

Yeah, I remember this from when I was rigging old 300W computer PSUs to homemade 5V and 12V outputs, I always added one lightbulb from the spares of my car's headlights.

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Richard-TX
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Re: 25W Power supply

Sat Dec 28, 2013 2:38 pm

mahjongg wrote:You could try, after you measure if the unloaded output voltage is below 5.25V!

These PSU's are not designed to deliver so little current.
Also If they deliver too much voltage (>6V) and you connect the PSU through the GPIO you stand a good chance to burn out the PI's overvoltage protector device.
I have no idea where you are purchasing power supplies that are unregulated at no load but any power supply that I have had that exhibited that behavior were scrapped as it was broken.

I do have to caution people about making sure that whatever power supply they use, make sure it is configured correctly. I saw one fellow forget to hook up the sense leads. He could not figure out why a brand new +5 volt supply was outputting 11 volts.
Richard
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Re: 25W Power supply

Sat Dec 28, 2013 5:52 pm

Hi,

As I mentioned a couple of times before, the nice thing about this our profession is that typically we can observe/measure values and the result proves which "theory" was applicable for a particular case... Ok, sometime this could be a brave approach (to use positive connotation ;-) ), but this is just a detail in the whole picture... ;-)

So, my first statement above is based on two key assumptions:
- RasPi is not marginal load for the 25W power supply,
- power supply with a trimmer is rather not optimized to supply just one very specific device.


Best wishes, Ivan Zilic.
Running out of GPIO pins and/or need to read analog values?
Solution: http://www.flyfish-tech.com/FF32

b0d
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Re: 25W Power supply

Sun Dec 29, 2013 1:18 am

Shocking how much ignorance of basic physics there is here.
The device's resistance determines how much current is taken, hence a 5v supply of any (maximum) amperage will be ok if the voltage is accurate, because the device takes the amperage required.
Even the largest power supply cannot force through more amps at the given 5volts.

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mahjongg
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Re: 25W Power supply

Sun Dec 29, 2013 1:27 am

Very true,but not the point, the point is that PSU's designed to deliver 25A often do not deliver 5V when the current is less than 2% of that 25A, the regulation may fail when the load is too low, and it may output more than the allowable 5.25V.

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Richard-TX
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Re: 25W Power supply

Sun Dec 29, 2013 4:41 am

mahjongg wrote:Very true,but not the point, the point is that PSU's designed to deliver 25A often do not deliver 5V when the current is less than 2% of that 25A, the regulation may fail when the load is too low, and it may output more than the allowable 5.25V.

Not in my experience. I have had 5 volt 500 amp (2500 watt) regulated supplies running with no load on them at all and the output voltage was exactly 5.0 volts. If they were any different they would have been sent back for repair. When a $3 million dollar computer is dependent upon the voltage to be exactly 5.0 volts, one does not mess around with a power supply that goes outside of spec regardless of the load. The same applies to every regulated power supply I have ever had in my possession. If the no load voltage is any higher than what it is rated at at full load, it gets trashed or returned.
Richard
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The 9-25-2013 image of Wheezy can be found at:
http://downloads.raspberrypi.org/raspbian/images/raspbian-2013-09-27/2013-09-25-wheezy-raspbian.zip

grayhair
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Re: 25W Power supply

Sun Dec 29, 2013 9:05 pm

Richard -
Like you, I have been at this stuff for a long time, most of my experience is with embedded controllers. In that realm, and in the inexpensive power supply world, a switching power supply typically requires "some" load current for proper regulation. The trick is to find out what "some" really means. This is a well known characteristic of power supplies, as shown by the infallible Wikipedia :) ( http://en.wikipedia.org/wiki/Load_regulation ): "Typically, a linear supply is capable of handling lower loading (higher resistance). Switching supplies usually cannot operate without some load current."

Regards

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Re: 25W Power supply

Sun Dec 29, 2013 9:32 pm

I've warned about this in the past too. But all the power supplies I have read the datasheet of recently, regulate down to zero current, or maybe 10mA. Even big PC supplies (that I have checked) regulate well within the power taken by a RaspPi.

It is always worth checking the datasheet, but it is not the problem that it once was.

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Tage
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Re: 25W Power supply

Sun Jan 05, 2014 4:13 pm

BBZ.png
BBZ.png (23.27 KiB) Viewed 6591 times
it is always worth checking any power supply before connecting it to the Pi. with a large power supply I would measure the output voltage at no load, and even check that the voltage does not shoot up when the Pi is unplugged, as this causes overvoltage to the Pi if it is plugged back in while the unloaded power supply output voltage is still above the normal value.
and I would be very careful with buying and ac/dc power supply from the internet without being absolutely sure it has passed electrical safety tests!

some comments on overvoltage protection:

as we all know by now, the polyfuse and the overvoltage protection diode will offer some protection against brief overvoltages, but it is only a very crude protection that is far from 100%.

when heated up by an overcurrent, the polyfuse will limit the current into the overvoltage diode (which is basically a large zener diode with low resistance, not a thyristor) and this makes it appear as if the overvoltage protection diode triggered, as the voltage drops. but what actually happens is that the current into the Pi is large enough that it keeps the polyfuse activated. once the polyfuse has triggered and there is still a significant current flowing through it, it will remain activated.

you have to unplug the power supply and let the polyfuse cool down to "reset" the fuse. even then, the stress on the polyfuse may have been large enough to alter its characteristics, so in the future it is more likely to trigger even at normal current levels. the polyfuse is actually selected with a bit too narrow margin to the actual load current, but it is a tradeoff between having some protection and having nothing. and the polyfuse will also prevent a fire in case someone connects a battery the wrong way to the power connector..
once it has triggered the voltage drop across the fuse may also become larger than what it was before. so it may be necessary to replace the polyfuse if it has been activated even once, because otherwise the 5V on the Pi side of the polyfuse becomes too low and the result is that there are all kind of issues with USB functions.

I looked at one of my Pi boards, and it has an overvoltage protection diode from ST Microelectronics, with the marking BBZ. the datasheet has the following information (above): the standoff voltage is 5V. the breakdown voltage is 6.74V (typical value with 10mA current). so don't count on it to handle a power supply that is set at 6V. for example. if the current pulse from the power supply is 68A the device will clamp the voltage to max 9.2V. (68A may seem high, but you can easily get that amount of current from an output capacitor in a power supply. the polyfuse will only briefly allow this high current)

in other words, the overvoltage protection diode does help to some degree but cannot really protect the Pi against overvoltage from a bad or wrongly adjusted power supply. the voltage can get high enough to damage the Pi permanently. this is more likely to happen if the power supply is connected to the GPIO pins so the polyfuse is bypassed.

daggoni
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Re: 25W Power supply

Sun Jan 05, 2014 7:31 pm

Thanks all for hints on this PSU. I also wanted to buy such one, but ended up in (cheaper?) solution.
Here's what I did
- bought 12V 2A AC adapter
- bought DC-DC regulated converter easily available on ebay. Output is regulated.

I connected this regulator and set output voltage to 5,25V for RPI.
I connected a second regulator for USB hub.
What it gave me ?
Very stable voltage for RPI.
Great voltage for USB hub and it's devices. Bought 10-poty Manhattan hub, put few 100uF capacitors, and voila. HDD, three network cards, two SD card readers. Wifi card for access point purpose.

The only drawback is that both devices boot at the same time (RPI,hub) so before devices or hub itself initializes, RPI tries to detect its devices, so I have to boot both and reboot RPI once as devices are already up'n'working so RPI can easily detect them on hub USB bus.
Anyone can recommend some solution ? Use some relay switch and turn power to usb hub once RPI booted ?

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Re: 25W Power supply

Tue Jan 07, 2014 2:03 pm

Some combination of boot_wait=n (n>1) to allow the hub + attached devices to settle (in /boot/config.txt) and add rootwait
to cmdline.txt if your / is on the HDD. (ie root=/dev/sda2 rootwait)
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