kurairaito
Posts: 2
Joined: Sat May 26, 2018 4:58 pm

need help to customize a component to power the pi

Sat May 26, 2018 5:19 pm

Hi,

I bought the screen you can see on this wiki : http://wiki.sunfounder.cc/index.php?tit ... spberry_Pi

it have a usb output dedicated to power the pi. it worked fine with the pi 3B, but I wanted to upgrade to pi 3b+ and the power fail. the new pi power on for about 3 seconds ans then bothled shut down. The same pi works fine with the official power plug.

on this picture http://wiki.sunfounder.cc/index.php?tit ... creen5.jpg , you can see that the output was given for 5V and 2.5A, the same as the official plug, but it doesn't works.

in fact, the little component that does the regulation is the component I highlighted in the attached picture.

this component is a mp1484 (https://www.mouser.com/ds/2/277/MP1484-1186021.pdf) and is given for 4.75-18V and up to 3A continuous current. So it seems to be enough.

added to that, you can see on this picture : http://wiki.sunfounder.cc/index.php?tit ... creen4.jpg that there is a 470uF (polarized) capacitor.

I don't know how much current the pi3b+ drag, but it seem to be a big amount for a long time (I tested without any usb peripheral plugged, not even the touchscreen controller, nor the hdmi cable)

I would ask to you electronician, and hardware designer, what would you do to make this power works for the pi 3b+
1/ change capacitor for a higher value? (which value)?
2/ change the mp1484 for a better reference that would not drop it's voltage under load (wich one?)
3/ create my own power from the 12V input of the card ? (once again which regulator I should take as even the mp1484 can't do it)

Thank to all for your hints!
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Touch_Screen5_reulator.jpg
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wh7qq
Posts: 1131
Joined: Thu Oct 09, 2014 2:50 am

Re: need help to customize a component to power the pi

Sun May 27, 2018 5:49 am

Although it is not pretty or convenient, probably the best bet is to have separate supplies for the 3B+ and the display. The B+ with its onboard wifi, will draw more power than the 3B and that probably taxes the ability of the mp1484 and the 2A supply. Bigger caps won't help and unless you have the ability to do pc board repair, you will probably ruin the display board by trying to change the chip...even if that would help. By the time you add the mouse and keyboard and what other USB things you use to the 3B+, you have probably exceeded the system's current capability. A higher current 12V supply (3-4 amp) might help, or not.

kurairaito
Posts: 2
Joined: Sat May 26, 2018 4:58 pm

Re: need help to customize a component to power the pi

Sun May 27, 2018 7:01 am

Hi,

Thanks for your answer, I tried an old netgear power supply (12v, 3.5A).

The pi shut down after some seconds, but come back a few seconds later and then everything is fine. I will keep it like that.

It's for a wall mounted screen, so it will always be powered on once started (and still be able to restart alone if a power cycle occur).

Ps : I have all the tool to unsolder component like the mp1484, even if the back pad is soldered (soldering station with heat gun). The component might die, but the rest of the board would still be fine :)

drgeoff
Posts: 8633
Joined: Wed Jan 25, 2012 6:39 pm

Re: need help to customize a component to power the pi

Sun May 27, 2018 11:25 am

wh7qq wrote:
Sun May 27, 2018 5:49 am
Although it is not pretty or convenient, probably the best bet is to have separate supplies for the 3B+ and the display. The B+ with its onboard wifi, will draw more power than the 3B and that probably taxes the ability of the mp1484 and the 2A supply. Bigger caps won't help and unless you have the ability to do pc board repair, you will probably ruin the display board by trying to change the chip...even if that would help. By the time you add the mouse and keyboard and what other USB things you use to the 3B+, you have probably exceeded the system's current capability. A higher current 12V supply (3-4 amp) might help, or not.
The MP1484 is a switching regulator. When fed from 12 volts it should only require about half the current that the RPi is taking at 5 volts. It is unlikely that the MP1484 itself is the problem which would mean that changing it will not help.

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Imperf3kt
Posts: 1386
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Location: Australia

Re: need help to customize a component to power the pi

Sun May 27, 2018 10:21 pm

Is your USB cable rated for the current required? 95% of the ones on the market are only rated for 0.5A

Disclaimer: number pulled from thin air
Stop plugging your fan directly into the GPIO 5v
https://www.electronics-tutorials.ws/power/transient-suppression.html

drgeoff
Posts: 8633
Joined: Wed Jan 25, 2012 6:39 pm

Re: need help to customize a component to power the pi

Mon May 28, 2018 10:27 am

Imperf3kt wrote:
Sun May 27, 2018 10:21 pm
Is your USB cable rated for the current required? 95% of the ones on the market are only rated for 0.5A

Disclaimer: number pulled from thin air
Despite the disclaimer (which I suspect is intended to cover the "95%") the "rated for 0.5A" is meaningless.

Two effects of passing current through a cable are:

1. the resistance of the cable causes a voltage drop. Without knowing what voltage drop is acceptable it is nonsense to mention a specific value of current as its rating.

2. the resistance of the cable causes heat to be generated.. However it is extremely unlikely that passing 0.5 amps though any USB cable on the market would generate enough heat to warrant concern.

Heater
Posts: 10021
Joined: Tue Jul 17, 2012 3:02 pm

Re: need help to customize a component to power the pi

Mon May 28, 2018 12:18 pm

Certainly cables have ratings. They have insulation with a breakdown voltage. They have a current carrying capacity. However that is rated.

That is why the cooker in your kitchen is not connected to the mains with a lamp cord and the cables on your car battery are huge.

A cable is basically a long fuse. Fuses certainly have current ratings.

Pendatism aside, Imperf3kt's comments were apt, in layman's terms. A lot of USB cables out there are so thin that there will be significant voltage drop when drawing the expected current.

It they are not officially "rated" in some way, then we can rate them. Fit for the job or not.

drgeoff
Posts: 8633
Joined: Wed Jan 25, 2012 6:39 pm

Re: need help to customize a component to power the pi

Mon May 28, 2018 12:48 pm

Heater wrote:
Mon May 28, 2018 12:18 pm
Certainly cables have ratings. They have insulation with a breakdown voltage. They have a current carrying capacity. However that is rated.

That is why the cooker in your kitchen is not connected to the mains with a lamp cord and the cables on your car battery are huge.

A cable is basically a long fuse. Fuses certainly have current ratings.

Pendatism aside, Imperf3kt's comments were apt, in layman's terms. A lot of USB cables out there are so thin that there will be significant voltage drop when drawing the expected current.

It they are not officially "rated" in some way, then we can rate them. Fit for the job or not.
None of that is at odds with what I posted.

Imperfekt cited a current not voltage rating. Current rating of a cable can really only be defined by the temperature aspect. That is independent of the cable length (in the normal use case of the cable being uncoiled) whereas voltage drop does depend directly on length and its significance depends on the operating voltage. A wire tasked with carrying x amps in a vehicle with 12 volt electrical system is generally thicker than a wire of the same length tasked with carrying the same x amps in a 240 volt mains situation.

A cable will indeed act as a fuse if the current is high enough but it should not be the fuse that operates.

I don't disagree with what imperfekt was trying to say. Some USB cables are not good enough to use with a RPi. But what he actually wrote was not technically accurate.

Heater
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Joined: Tue Jul 17, 2012 3:02 pm

Re: need help to customize a component to power the pi

Mon May 28, 2018 2:14 pm

We agree then. Not technically correct. But it layman's terms good enough to get the point across.

Problem is, all this technical correctness does not help, is it even possible for a consumer to find a USB cable with any kind of specification. Like ohms/meter, total resistance over it's length etc?

We have to rate them ourselves. Either they are fit for powering a Pi or they are not.

The only way I managed to stop a Pi 3 from complaining about voltage dips with any USB cable I have managed to find is to get a power supply with some slight voltage adjustment. Wind it up to 5.25V at it's output and the Pi is then happy!

W. H. Heydt
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Location: Vallejo, CA (US)

Re: need help to customize a component to power the pi

Mon May 28, 2018 5:54 pm

It occurs to to ask... The input rating is 12v at 2A (24W). How much power does the display itself require? USB 2.0 (that USB port has a a white alignment block--so 2.0) is specified to deliver 500mA at 5v (2.5W). The board may be configured to limit the output current to 500mA. If that is the case, it won't provide enough current to run anything more that a Pi0/Pi0W or A+. *Maybe* it would run a B+.

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Imperf3kt
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Location: Australia

Re: need help to customize a component to power the pi

Mon May 28, 2018 9:29 pm

Gee, pedantic much.
I'll reword myself then - most USB-A to USB micro-B cables are rated to supply 5v at no more than 0.5A

Your Pi wants significantly more than 0.5A @ 5v, so these cables will not suffice unless you accomodate for massive voltage drops - which may be enough to overload the cable.


Also, just for the record, the last usb cable I purchased (from a reputable company), was rated by amps (@5v)
Stop plugging your fan directly into the GPIO 5v
https://www.electronics-tutorials.ws/power/transient-suppression.html

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