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Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 10:26 am
by geffers
Folks,

Accidentally connected to my Pi3 using Serial Connector whilst the normal USB power was still connected :cry:

Now, this was a couple of minutes at most and when noticed naturally I pulled off the red power lead to GPIO pin 2 which supplies 5v

Now, Pi3 is still working fine and am wondering, what potential damage, if any could have been caused and are the two power sources actually in parallel (no problem) or series (eek!)?

Geffers

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 11:13 am
by FTrevorGowen
geffers wrote: Folks,
Accidentally connected to my Pi3 using Serial Connector whilst the normal USB power was still connected :cry:
Now, this was a couple of minutes at most and when noticed naturally I pulled off the red power lead to GPIO pin 2 which supplies 5v
Now, Pi3 is still working fine and am wondering, what potential damage, if any could have been caused and are the two power sources actually in parallel (no problem) or series (eek!)?
Geffers
They would have been in parallel. I suspect the USB port feeding the serial dongle (or the dongle itself) would be the "weaker link". (I've only "serially powered" a P0 myself, w/o any extra USB devices connected that might draw too much current than can be supplied by the USB-to-serial device**)
Trev.
** http://www.cpmspectrepi.uk/raspberry_pi ... Powered.22

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 11:44 am
by rurwin
If the power supplies were different by a fraction of a volt then there could have been large currents flowing through the power leads and the power tracks on the PCB. That could have made them hot enough to break. The self-resetting "fuse" on the power connector will have seen that current and may have triggered. However, so far as I can see, no other components should be affected.

If you can still power the Pi "normally" and you can see 5V on GPIO pin 2, then you have almost certainly got away with it.

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 12:13 pm
by sparkyhall
rurwin wrote:
Thu Jan 18, 2018 11:44 am
If the power supplies were different by a fraction of a volt then there could have been large currents flowing through the power leads and the power tracks on the PCB. That could have made them hot enough to break. The self-resetting "fuse" on the power connector will have seen that current and may have triggered. However, so far as I can see, no other components should be affected.

If you can still power the Pi "normally" and you can see 5V on GPIO pin 2, then you have almost certainly got away with it.
That's extremely unlikely because most power supplies can only source, not sink, current.

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 12:43 pm
by hippy
sparkyhall wrote:
Thu Jan 18, 2018 12:13 pm
That's extremely unlikely because most power supplies can only source, not sink, current.
Any difference in voltages at the two ends of a conductor will cause current to flow in that conductor.

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 12:53 pm
by PeterO
hippy wrote:
Thu Jan 18, 2018 12:43 pm
sparkyhall wrote:
Thu Jan 18, 2018 12:13 pm
That's extremely unlikely because most power supplies can only source, not sink, current.
Any difference in voltages at the two ends of a conductor will cause current to flow in that conductor.
I don't think you understand the difference between source and sink.

In general, current can only flow out of a power supply. When you put a load on a power supply it provide just the right current to maintain its output voltage. If the voltage drops the PSU will provide more current (upto its current limit). If the load changes and the voltage rises the PSU provides less current to reduce the voltage. This is just appliaction of ohm's law.

But if some other power supply is providing too much voltage a PSU will not sink current in a attempt to reduce the voltage.

PeterO

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 1:48 pm
by geffers
Pi still working fine.

On noticing my mistake I immediately disconnected the red power lead to pin 2 of GPIO - I hadn't at that point even made a connection with the Pi as being on Windoz I couldn't find the Com port. I then rebooted in to Linux Ubuntu and connected to the PI.

So at that point it was being powered by a PSU into the micro usb power socket on the Pi.

The Pi is powered by a 2amp PSU which shows as supplying 5.15v at 0.52amps when the Pi is running normally. It has an ADS-B tuner (TV stick) plugged in to USB port and that is all so the .52amps is with it all running. It was at this point I accidentally plugged in the USB plug with the red power lead still connected to pin 2 of GPIO. Obviously the USB from the computer supplies whatever that is set for which should be 5v 500 mA

But as said, Pi working fine.

Geffers

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 2:39 pm
by hippy
geffers wrote:
Thu Jan 18, 2018 1:48 pm
Pi still working fine.
That's good to hear. What has probably happened is that one supply was weaker than the other, had its voltage pulled up or down to what the other supply was delivering, and any 'fighting each other' and raising the potential for damage was minimised. Or any adverse situation did not last long enough to cause serious damage.

Luckily an "oops" in this case rather a fatal mistake.

Re: Whoops! - Two Power Sources

Posted: Thu Jan 18, 2018 4:33 pm
by davidcoton
PeterO wrote:This is just appliaction of owm's law.
I'm not sure I know that one. Is it the derivation of Ohm's law that applies when it hurts? :lol:

Re: Whoops! - Two Power Sources

Posted: Sun Jan 21, 2018 5:28 pm
by Paul Hutch
It is generally true that powers supplies are designed to only source, not sink, current.

It is definitely true that without knowing the specific circuitry in each power supply the result of connecting the outputs in parallel can not be known.

Sometimes both supplies will be OK
Sometimes one supply will be damaged
Sometimes both supplies will be damaged