Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 10:45 am

So, as above really. I have a 3B+ as part of a project, which is tricky to get power to via micro USB, so I had a 5V regulator set up using a standard circuit which was detailed in the 7805 technical documentation. It all worked fine and lovely, until, suddenly whilst installing Numpy it failed. It took out the main regulator board, the transistor which was part of the secondary (5v) regulator circuit, and it would seem, the pi too.

So, before I file it in the bin under "Really should pay more attention to cooling on PSU's", is there anything that can be done to salvage it? It was powering through the 5v pins on the GPIO. I've tried plugging in an official Raspberry pi supply to test it, and no lights come on. Might it just be the regulator chip in the pi, or is likely to have taken out other bits too?

drgeoff
Posts: 9598
Joined: Wed Jan 25, 2012 6:39 pm

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 11:53 am

If the 7805 failure resulted in more than about 6 volts into the 5 volt pin on the GPIO header then there is an overvoltage protection diode that comes into action. Normally the current it sinks is coming from the micro-USB power socket and the polyfuse between that socket and the diode has a chance to save the bacon. However, in your case the polyfuse was not in circuit.

In which cases sometimes the diode fails short circuit and gives some non-zero probability of saving the rest of the board but also means that any further attempts to power the RPi (by micro-USB or GPIO header) fail.

The diode is not necessary for a RPi to run. Try removing the diode and see if the board will boot. If yes, consider replacing the missing diode.

Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 12:01 pm

Sadly it wasn't the 7805 that failed, it was the transistor next to it. No idea if that resulted in a voltage of above 5v (it would be 12v if it was), or a short across the pins.

Which one of the myriad of SMD components may be the (now) faulty diode?

User avatar
RaTTuS
Posts: 10377
Joined: Tue Nov 29, 2011 11:12 am
Location: North West UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 12:35 pm

put it down as a bad choice in powering ....
much easier to start a gin wiht a new board TBH
How To ask Questions :- http://www.catb.org/esr/faqs/smart-questions.html
WARNING - some parts of this post may be erroneous YMMV

1QC43qbL5FySu2Pi51vGqKqxy3UiJgukSX
Covfefe

Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 12:47 pm

RaTTuS wrote:
Wed Apr 03, 2019 12:35 pm
put it down as a bad choice in powering ....
much easier to start a gin wiht a new board TBH
New board arrives tomorrow. Would just like to have old board as backup.

And currently redesigning the PSU part.....

drgeoff
Posts: 9598
Joined: Wed Jan 25, 2012 6:39 pm

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 1:06 pm

Apretext wrote:
Wed Apr 03, 2019 12:01 pm
Which one of the myriad of SMD components may be the (now) faulty diode?
D7. https://www.raspberrypi.org/documentati ... educed.pdf

User avatar
davidcoton
Posts: 3940
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 3:59 pm

While redesigning your PSU, you should add a polyfuse (same type as D7) to the output if you need to power the Pi through the GPIO header.

Is there a reason for trying to use a linear regulator? 12V in, 5V out gives about 33% efficiency -- wasting twice as much power as the Pi uses (that's when it doesn't overheat and burn out). Better to use one of the many switch mode regulator modules that are readily available. You still need an extra polyfuse or equivalent if you need to power the Pi through the GPIO header.

If the damaged Pi works with D7 removed, replace it before returning it to regular service. Without D7 it certainly will not survive another 12V attack.
Signature retired

Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 5:16 pm

davidcoton wrote:
Wed Apr 03, 2019 3:59 pm
While redesigning your PSU, you should add a polyfuse (same type as D7) to the output if you need to power the Pi through the GPIO header.

Is there a reason for trying to use a linear regulator? 12V in, 5V out gives about 33% efficiency -- wasting twice as much power as the Pi uses (that's when it doesn't overheat and burn out). Better to use one of the many switch mode regulator modules that are readily available. You still need an extra polyfuse or equivalent if you need to power the Pi through the GPIO header.

If the damaged Pi works with D7 removed, replace it before returning it to regular service. Without D7 it certainly will not survive another 12V attack.
I'm planning on using a LM2596T-5 this time, with a BIG heatsink, and a fan.... That way, in combination with the 12v regulator next to it (I'll run them in parallel this time, input is 15.6V from a laptop power supply), so yes, switched rather than linear this time.

Any idea if there is a THT equivalent for the SMBJ5?

I presume it's the circled component here?
IMG_1256.jpg
IMG_1256.jpg (83.67 KiB) Viewed 810 times

User avatar
NGC6543
Posts: 75
Joined: Thu Feb 28, 2019 7:30 pm
Location: Lancashire, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 7:21 pm

For a through hole, SA5.0A is pretty close or maybe a 1N6373.

I'm going to be buying some myself for my PSU soon, along with some fuses :D

User avatar
Imperf3kt
Posts: 2544
Joined: Tue Jun 20, 2017 12:16 am
Location: Australia

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 8:40 pm

This is one of the reasons I prefer to step up the voltage, rather than down.
Worst case scenario, you corrupt the SD card and have to reflash.

Is using a lower voltage supply an option?
55:55:44:44:4C
52:4C:52:42:41

Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 9:04 pm

Imperf3kt wrote:
Wed Apr 03, 2019 8:40 pm
This is one of the reasons I prefer to step up the voltage, rather than down.
Worst case scenario, you corrupt the SD card and have to reflash.

Is using a lower voltage supply an option?
Sadly no, I need 12V for some stepper motors.

User avatar
NGC6543
Posts: 75
Joined: Thu Feb 28, 2019 7:30 pm
Location: Lancashire, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 9:35 pm

Imperf3kt wrote:
Wed Apr 03, 2019 8:40 pm
This is one of the reasons I prefer to step up the voltage
You know, I never thought of that! Not that it's an option for what I'm doing right now, but next time...

User avatar
davidcoton
Posts: 3940
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Wed Apr 03, 2019 10:37 pm

Imperf3kt wrote:
Wed Apr 03, 2019 8:40 pm
This is one of the reasons I prefer to step up the voltage, rather than down.
OTOH, lower voltage means higher current means either thicker wires and/or more voltage/power lost.

Horses for courses. With a properly designed power system (not bypassing protection components) a 12V source for conversion to 5V for a Pi is not unreasonable. If using batteries (which may not be the case here), 12V lead acid batteries are also more economical (and in some ways safer) than LiPo or similar technologies. NiMH is also worth considering.
Signature retired

Apretext
Posts: 9
Joined: Wed Apr 03, 2019 10:37 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Thu Apr 04, 2019 10:19 am

Ok, so I've found D7, and it is indeed shorted. So, Time to remove it and see if it'll work again (before I then order a new diode....)

alphanumeric
Posts: 1903
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Thu Apr 04, 2019 10:16 pm

A diode when forward biased will be almost a short? A 0.3 or 0.7 volt drop across it is normal. Reverse biased it should be an open.
The poly fuse on the other hand, will be a short when normal and an open circuit (high resistance) when tripped.
And I do believe the poly fuse is on the other side on the circuit board.

User avatar
rpdom
Posts: 14722
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 6:24 am

alphanumeric wrote:
Thu Apr 04, 2019 10:16 pm
A diode when forward biased will be almost a short? A 0.3 or 0.7 volt drop across it is normal. Reverse biased it should be an open.
The protection diode is not like a normal diode. I believe it is bi-directional and shouldn't conduct in either direction until the voltage hits 6.0V (in this case). It's a bit like two Zener diodes connected back to back.

alphanumeric
Posts: 1903
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 8:23 am

rpdom wrote:
Fri Apr 05, 2019 6:24 am
alphanumeric wrote:
Thu Apr 04, 2019 10:16 pm
A diode when forward biased will be almost a short? A 0.3 or 0.7 volt drop across it is normal. Reverse biased it should be an open.
The protection diode is not like a normal diode. I believe it is bi-directional and shouldn't conduct in either direction until the voltage hits 6.0V (in this case). It's a bit like two Zener diodes connected back to back.
Ok, now it makes sense. I should have researched it a bit before posting. :oops:

User avatar
davidcoton
Posts: 3940
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 9:49 am

rpdom wrote:
Fri Apr 05, 2019 6:24 am
The protection diode is not like a normal diode. I believe it is bi-directional and shouldn't conduct in either direction until the voltage hits 6.0V (in this case). It's a bit like two Zener diodes connected back to back.

There are TWO versions of the SMBJ5.0.


1) The SMBJ5.0A is uni-directional, and will act to clamp reverse voltage at a low value. This is the part specified for Pi computers, AFAICT.
2) The SMBJ5.0CA is bi-directional, and would provide no protection against reverse polarity supply. While some models had a perfect diode to block reverse polarity, the Pi3B+ does not. So this is not suitable for use on a a Pi3B+ (or Pi3A+, I suspect).

[Deleted incorrect info]
Last edited by davidcoton on Fri Apr 05, 2019 12:22 pm, edited 1 time in total.
Signature retired

User avatar
Gavinmc42
Posts: 3429
Joined: Wed Aug 28, 2013 3:31 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 10:25 am

I have used many switch mode modules in the 7805 pinout footprint on older Pi's.
The isolated versions for solar panel monitor Pi's, they also get isolation on i2c, just in case.
Even used those Cigarette socket car USB chargers, sometimes they are on special at $2.
All these step down.

Have not used any for the 3B+'s yet, they get the official supplies ;)
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges

User avatar
NGC6543
Posts: 75
Joined: Thu Feb 28, 2019 7:30 pm
Location: Lancashire, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 11:00 am

davidcoton wrote:
Fri Apr 05, 2019 9:49 am
The polyfuse is on the top side of every Pi I have checked (B+ 2B 3B 3A+) except the original B. Not fitted on Pi0 of course.
Note that the component number varies between models.
This is it underneath on the 3B+
polyfuse2.jpg
polyfuse2.jpg (124.27 KiB) Viewed 566 times

alphanumeric
Posts: 1903
Joined: Tue Jan 19, 2016 2:17 pm
Location: Sydney, Nova Scotia, Canada

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 11:31 am

NGC6543 wrote:
Fri Apr 05, 2019 11:00 am
davidcoton wrote:
Fri Apr 05, 2019 9:49 am
The polyfuse is on the top side of every Pi I have checked (B+ 2B 3B 3A+) except the original B. Not fitted on Pi0 of course.
Note that the component number varies between models.
This is it underneath on the 3B+

polyfuse2.jpg
Its in that same spot on the back side of my 3A+ also. And its on the back side of my A+'s.

User avatar
davidcoton
Posts: 3940
Joined: Mon Sep 01, 2014 2:37 pm
Location: Cambridge, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 12:24 pm

:oops: Thanks guys, I got confused between polyfuse and TVS. :oops:
I blame morning caffeine deficiency.
Earlier post corrected.

Polyfuse is underneath on all models I checked.
TVS underneath on original B, on top on B+ 2B 3B 3A+.
TVS conponent number varies.
No polyfuse or TVS on Pi0 models.
Signature retired

User avatar
rpdom
Posts: 14722
Joined: Sun May 06, 2012 5:17 am
Location: Chelmsford, Essex, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 12:50 pm

davidcoton wrote:
Fri Apr 05, 2019 9:49 am
rpdom wrote:
Fri Apr 05, 2019 6:24 am
The protection diode is not like a normal diode. I believe it is bi-directional and shouldn't conduct in either direction until the voltage hits 6.0V (in this case). It's a bit like two Zener diodes connected back to back.

There are TWO versions of the SMBJ5.0.


1) The SMBJ5.0A is uni-directional, and will act to clamp reverse voltage at a low value. This is the part specified for Pi computers, AFAICT.
2) The SMBJ5.0CA is bi-directional, and would provide no protection against reverse polarity supply. While some models had a perfect diode to block reverse polarity, the Pi3B+ does not. So this is not suitable for use on a a Pi3B+ (or Pi3A+, I suspect).

[Deleted incorrect info]
I sit corrected. I should have checked instead of relying on faulty memory.

User avatar
NGC6543
Posts: 75
Joined: Thu Feb 28, 2019 7:30 pm
Location: Lancashire, UK

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Fri Apr 05, 2019 3:59 pm

davidcoton wrote:
Fri Apr 05, 2019 12:24 pm
:oops: Thanks guys, I got confused between polyfuse and TVS. :oops:
I blame morning caffeine deficiency.
:D I had to call the doctors as soon as I got up this morning. The receptionist asked my date of birth... it took me a good fifteen seconds to remember it! And then I wondered momentarily if I was right.

More coffee required :lol:

User avatar
Gavinmc42
Posts: 3429
Joined: Wed Aug 28, 2013 3:31 am

Re: I suspect My Pi is bricked, due to a failed PSU, but how do I check?

Sat Apr 06, 2019 4:41 am

I blame morning caffeine deficiency.
I try to avoid doing anything until the 2nd coffee ;)
The 1st is just enough to get to work, the kettle goes on while Windows boots.
1/4 of the cup has gone before Win10 has decided to be ready to work.
I'm dancing on Rainbows.
Raspberries are not Apples or Oranges

Return to “Troubleshooting”