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A question of power...

Posted: Sat Jun 07, 2014 3:18 pm
by frak
Hi!

I am attempting to make a light scythe based on the Mechatronics Guy's project: https://sites.google.com/site/mechatron ... tscythe-v2 and my work so far can be found at http://frak.github.io/LightScythe

I have purchased a Li-Ion battery pack that is capable of outputting 5800 mA/h which seems more than reasonable for what I need. This is output through a USB socket and I guess I can just connect that to the Pi. However, in the project the power is given to the Pi through the GPIO header and I was wondering why this was? It would obviously be much simpler for me to provide the power through the mini USB socket, but I wonder if I am risking damage to my Pi if I do this?

Thanks in advance...

Re: A question of power...

Posted: Sat Jun 07, 2014 3:25 pm
by FLYFISH TECHNOLOGIES
Hi,
frak wrote:This is output through a USB socket and I guess I can just connect that to the Pi.
Yes. Through power micro USB connector.
frak wrote:However, in the project the power is given to the Pi through the GPIO header and I was wondering why this was?
Shortcut.... which is not recommended, because you bypass internal overvoltage & overcurrent protections.
frak wrote:It would obviously be much simpler for me to provide the power through the mini USB socket, but I wonder if I am risking damage to my Pi if I do this?
Not at all. The risk is increased if you'd supply the RasPi through GPIO.


Best wishes, Ivan Zilic.

Re: A question of power...

Posted: Sat Jun 07, 2014 3:30 pm
by frak
This all makes sense, but the over current protection you mention - won't that stop the current getting through to the GPIO header that will power the LEDs?

Re: A question of power...

Posted: Sat Jun 07, 2014 3:42 pm
by FLYFISH TECHNOLOGIES
Hi,
frak wrote:This all makes sense, but the over current protection you mention - won't that stop the current getting through to the GPIO header that will power the LEDs?
Overcurrent protection is activated when the overall current is exceeded significantly. The protection is RasPi-wide and not per-output-pin.
In case of nothing but LEDs attached, the GPIO outputs' limit are met before the overcurrent protection is activated.


Best wishes, Ivan Zilic,

Re: A question of power...

Posted: Sat Jun 07, 2014 3:52 pm
by frak
FLYFISH TECHNOLOGIES wrote:In case of nothing but LEDs attached, the GPIO outputs' limit are met before the overcurrent protection is activated.
I am not sure I fully understand this. From what I have read about the LED strip https://learn.adafruit.com/light-painti ... i/hardware, it requires about 2A of power per metre, and I have two metres, thus needing about 4A for the LEDs. Are you saying that this will pass the 4A through to the GPIO?

Re: A question of power...

Posted: Sat Jun 07, 2014 3:57 pm
by FLYFISH TECHNOLOGIES
Hi,
frak wrote:
FLYFISH TECHNOLOGIES wrote:In case of nothing but LEDs attached, the GPIO outputs' limit are met before the overcurrent protection is activated.
I am not sure I fully understand this. From what I have read about the LED strip
You haven't mentioned LED strips... just LEDs.
frak wrote:Are you saying that this will pass the 4A through to the GPIO?
No, it will not "pass".
You need a separated power for LEDs strip.


Best wishes, Ivan Zilic.

Re: A question of power...

Posted: Sat Jun 07, 2014 4:05 pm
by frak
If you had looked at either of the links in my question you would have seen what is involved in the project ;)

OK, so I need to power the Pi through the USB and provide a separate 5V supply to the strip. However, the schematic here https://learn.adafruit.com/light-painti ... i/hardware seems to suggest that powering the Pi from the GPIO is ok with a 2A supply, and they even hint you can connect a 10A supply in this way - wired this way, which seems to be in parallel, wouldn't the current go mostly to the LEDs and the Pi would only draw what it needed?

Re: A question of power...

Posted: Sat Jun 07, 2014 4:15 pm
by FLYFISH TECHNOLOGIES
Hi,
frak wrote:If you had looked at either of the links in my question you would have seen what is involved in the project
After some time spent here, you'll find out that people are jumping to all kind of subjects within a single thread. ;)
frak wrote:However, the schematic here seems to suggest that powering the Pi from the GPIO is ok with a 2A supply
It's up to you to make a decision which information to follow...
frak wrote:wouldn't the current go mostly to the LEDs and the Pi would only draw what it needed?
My statement about protections is (still) applicable.


Best wishes, Ivan Zilic.

Re: A question of power...

Posted: Sat Jun 07, 2014 4:20 pm
by frak
I think on reflection I will go the extra mile and not power the LEDs through the GPIO pin and give them a separate connection to the battery. As you have made clear, there is a risk to the Pi and as cheap as it was to buy, I feel a real connection to it and wouldn't want to destroy it through negligence. Thanks for your advice my friend :)

Re: A question of power...

Posted: Sat Jun 07, 2014 9:34 pm
by frak
One follow up question - I have 5800mAh coming out of the battery, and the amount of current the strip draws will vary with what is being displayed - how do I protect the LED strip from too much current?

Re: A question of power...

Posted: Sat Jun 07, 2014 10:03 pm
by FLYFISH TECHNOLOGIES
Hi,
frak wrote:how do I protect the LED strip from too much current?
Basic misunderstanding...
You need to take care that:
- the voltage of source (power supply) equals to load's required voltage and
- the current a source can provide is higher or equal to the maximum current burned by the load.

You're on the safe side only when the declared power suppliy current is higher than the current required by the load.


Best wishes, Ivan Zilic.

Re: A question of power...

Posted: Sat Jun 07, 2014 11:17 pm
by frak
Ah OK, I mistook the need for accuracy on voltage for a corresponding need to care about the current.

Thanks again.