User avatar
boomonster
Posts: 77
Joined: Fri May 11, 2018 11:18 pm

Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 1:09 am

Just curious... :) The title says it all.
Can I? Or will it fry my pi? :(

Thanks and sorry if I put my post in the wrong forum,
Boomonster

PhatFil
Posts: 1303
Joined: Thu Apr 13, 2017 3:55 pm
Location: Oxford UK

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 1:38 am

Should Be OK A higher amp rating on the psu is not a danger, the pi will draw what it needs so as long as the psu has enough capacity any overhead is just going to be unused.

Its underpowered psus that can present a danger of overheating/ burning up with a device attempting to draw more power than it can supply..

the critical thing is the 5V voltages stability, as long as the psu can supply 5V and keep supplying 5v without dropping or surging.

User avatar
KevinA
Posts: 64
Joined: Tue Oct 27, 2015 3:50 pm
Location: Park Valley, Utah

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 1:49 am

There are a bunch listed on ebay that have switches built in, got one, it works with the RPI 3+, the lighting bolt is gone...

W. H. Heydt
Posts: 10565
Joined: Fri Mar 09, 2012 7:36 pm
Location: Vallejo, CA (US)

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 2:26 am

The danger in a high powered PSU--and let me note that a 3A unit does *not* rise to the level I'm talking about--is that any momentary problem has the potential to put far too much current to handle through the board. You should be just fine with a 3A supply, as that is only 500mA over the maximum amount the Pi3B+ can draw. If you were asking about--say--a 50A supply, then it wouldn't be a good idea.

User avatar
KevinA
Posts: 64
Joined: Tue Oct 27, 2015 3:50 pm
Location: Park Valley, Utah

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 3:17 am

W. H. Heydt wrote:
Fri Dec 21, 2018 2:26 am
The danger in a high powered PSU--and let me note that a 3A unit does *not* rise to the level I'm talking about--is that any momentary problem has the potential to put far too much current to handle through the board. You should be just fine with a 3A supply, as that is only 500mA over the maximum amount the Pi3B+ can draw. If you were asking about--say--a 50A supply, then it wouldn't be a good idea.
F-MSMF250/X Holds at 2.5 amp trips at 5 amp - This is on the USB micro connector voltage line, all current to the RPI goes through that Polymeric PTC Resettable Fuse. The micro USB connector is the limiting factor, they are rated from .5 to 1.8 amp per contact, 5 contacts with -D, +D, Ground, Voltage (Vbus +5), ID.
You could hook an arc welder to the Vbus with 300 amps, the PI draws the current, current isn't 'pushed' into the PI.
I've had power supply problems with the RPI Zero, if the 'switching regulated' power supply is noisy the Zero doesn't work, same supply on a PI1~3 works fine. When you hook a battery to a PI it has the potential of dropping all the current in one instant but because of ohms law it doesn't.
I can only hope that Raspberry PI X (next generation) has a C type connector...

User avatar
Imperf3kt
Posts: 2527
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 8:35 am

True, but if there's a fault in the Pi and it suddenly draws a lot of current, it can cause a fire. A lower power, power supply, leaves less chance for a fire in the event of a fault.
The micro usb socket can deliver more current than it is rated for, the rating is just the safe limit at which it will deliver five volts.
55:55:44:44:4C
52:4C:52:42:41

User avatar
boomonster
Posts: 77
Joined: Fri May 11, 2018 11:18 pm

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 7:07 pm

Ok... I am not using a standard PSU, I am using a power bank which gives 2.1A out of one port, and 1A out of the other. So I thought of joining
the two outputs by clipping and soldering 2 micro usb cables to make 3A. I am doing this because my pi 2 gives me the power warning (lightning)
symbol when I use it off the 2.1A port alone.

So this whole thing about PSUs going on fire? Could that happen to the battery? Or am I fine?

Boomonster

jbudd
Posts: 873
Joined: Mon Dec 16, 2013 10:23 am

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 11:07 pm

I am using a power bank which gives 2.1A out of one port, and 1A out of the other. ... my pi 2 gives me the power warning (lightning) symbol when I use it off the 2.1A port alone.
2.1A should be plenty for a Raspberry Pi 2! I suspect that you have a voltage rather than a current problem. Some power banks don't reliably supply 5V.

To make the best of the situation use a high quality, thick and short USB cable to avoid further voltage loss in the cable.

User avatar
davidcoton
Posts: 3930
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 11:10 pm

boomonster wrote:
Fri Dec 21, 2018 7:07 pm
So I thought of joining the two outputs by clipping and soldering 2 micro usb cables to make 3A
You should never join two power sources in parallel unless you know and understand how the power is regulated. There is the possibility of the two sources "fighting" which can cause instability, and just possibly overheating of one or both.
Signature retired

drgeoff
Posts: 9587
Joined: Wed Jan 25, 2012 6:39 pm

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 11:34 pm

Not all dual output power banks are the same but its not uncommon for the power wires of the 2.1 amp and 1 amp outputs to be directly paralleled inside and fed from one DC-DC converter. The difference in the two sockets is merely the wiring of the data lines which are frequently sensed by phones and tablets to determine how much current the port is able to deliver. For a RPi (which has no connections from the data lines of the micro-USB power socket) such sensing is irrelevant.

User avatar
ab1jx
Posts: 867
Joined: Thu Sep 26, 2013 1:54 pm
Location: Heath, MA USA
Contact: Website

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Fri Dec 21, 2018 11:39 pm

Voltage drop in a cable is a bear. OTOH normal desktop computers have power supplies good for 30 amps or more on the 5 volt bus. If the USB current limit isn't too strictly enforced plugging a USB-microusb cable into a spare USB port on the desktop then plugging that into a Pi's power connector should work. Too much current? Well, a 25 cent fuse will take care of that problem, but you might have to solder your own adapter. The Pi has its own fusing and so do desktops, it's only a potential short in your connecting wire that could cause a problem.

User avatar
Imperf3kt
Posts: 2527
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Sat Dec 22, 2018 7:22 am

jbudd wrote:
Fri Dec 21, 2018 11:07 pm
I am using a power bank which gives 2.1A out of one port, and 1A out of the other. ... my pi 2 gives me the power warning (lightning) symbol when I use it off the 2.1A port alone.
2.1A should be plenty for a Raspberry Pi 2! I suspect that you have a voltage rather than a current problem. Some power banks don't reliably supply 5V.

To make the best of the situation use a high quality, thick and short USB cable to avoid further voltage loss in the cable.
No, it is a current issue.
You'll get 5.25v out of a powerbank, but as you try to draw higher currents out of it, the voltage drops in order to keep the current up.
So yes, it's a voltage problem, but it's caused by not having enough current.
55:55:44:44:4C
52:4C:52:42:41

User avatar
davidcoton
Posts: 3930
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Sat Dec 22, 2018 10:10 am

Imperf3kt wrote:
Sat Dec 22, 2018 7:22 am
jbudd wrote:
Fri Dec 21, 2018 11:07 pm
I am using a power bank which gives 2.1A out of one port, and 1A out of the other. ... my pi 2 gives me the power warning (lightning) symbol when I use it off the 2.1A port alone.
2.1A should be plenty for a Raspberry Pi 2! I suspect that you have a voltage rather than a current problem. Some power banks don't reliably supply 5V.

To make the best of the situation use a high quality, thick and short USB cable to avoid further voltage loss in the cable.
No, it is a current issue.
You'll get 5.25v out of a powerbank, but as you try to draw higher currents out of it, the voltage drops in order to keep the current up.
So yes, it's a voltage problem, but it's caused by not having enough current.
The voltage drops along the cable because of Ohm's Law. Nothing to do with "keeping the current up". Yes, it's a voltage problem. Caused by trying to draw too much current through thin wires. The "not enough current" is not really helpful, just a consequence of too much resistance. In fact the current can even increase as the voltage-starved switching regulators on the Pi try to keep up.
Signature retired

Brandon92
Posts: 773
Joined: Wed Jul 25, 2018 9:29 pm
Location: Netherlands

Re: Can I use a 5 Volt 3 Amp power supply for my RPi2?

Sat Dec 22, 2018 11:13 am

davidcoton wrote:
Sat Dec 22, 2018 10:10 am
Imperf3kt wrote:
Sat Dec 22, 2018 7:22 am
jbudd wrote:
Fri Dec 21, 2018 11:07 pm

2.1A should be plenty for a Raspberry Pi 2! I suspect that you have a voltage rather than a current problem. Some power banks don't reliably supply 5V.

To make the best of the situation use a high quality, thick and short USB cable to avoid further voltage loss in the cable.
No, it is a current issue.
You'll get 5.25v out of a powerbank, but as you try to draw higher currents out of it, the voltage drops in order to keep the current up.
So yes, it's a voltage problem, but it's caused by not having enough current.
The voltage drops along the cable because of Ohm's Law. Nothing to do with "keeping the current up". Yes, it's a voltage problem. Caused by trying to draw too much current through thin wires. The "not enough current" is not really helpful, just a consequence of too much resistance. In fact the current can even increase as the voltage-starved switching regulators on the Pi try to keep up.
Well I think it also depends on what the control loop is inside the device and how fast it can react, but lets not go down that route.

However, If you have a "bad" powerbank. It could deliver the current that is promised, however the voltage will drop significant. For example, as showed in this video.
On the other hand, if you have a powerbank that can deliver the promised output current. And will regulate the voltage is such way that will stay 5V, no matter what the output current is. And if you apply a bad cable to it, it will indeed react as a resisitor in series with the Rpi. And there will be to much voltage across that bad cable. But, at the output of the powerbank it is still the nice 5V, but the Rpi will not see that.

Return to “General discussion”