raven_squire
Posts: 23
Joined: Fri Jun 29, 2012 8:39 am

Raspberry Pi3v - what size capacitor?

Sat Feb 11, 2017 11:49 am

Hello,

I am using the raspberry pi3 on an Atrix lapdock and want to use a capacitor to make sure the pie will stay powered up when power is removed for 3 seconds (accidental lid close).

A 1000uf capacitor was needed to boot without the lightning bolt coming up on the screen. This tells me that there is not quite enough power being supplied by the lapdock.

Could I use something like a bunch of 4700uf capacitor? At what point would too many capacitors become a problem?

I am also thinking about some getting micro usb break out cables so that I can measure the resistance from my home made power cable just in case that is causing power problems. What resistance value would be troublesome?

mfa298
Posts: 1352
Joined: Tue Apr 22, 2014 11:18 am

Re: Raspberry Pi3v - what size capacitor?

Sat Feb 11, 2017 2:34 pm

raven_squire wrote:Hello,

I am using the raspberry pi3 on an Atrix lapdock and want to use a capacitor to make sure the pie will stay powered up when power is removed for 3 seconds (accidental lid close).

A 1000uf capacitor was needed to boot without the lightning bolt coming up on the screen. This tells me that there is not quite enough power being supplied by the lapdock.

Could I use something like a bunch of 4700uf capacitor? At what point would too many capacitors become a problem?

I am also thinking about some getting micro usb break out cables so that I can measure the resistance from my home made power cable just in case that is causing power problems. What resistance value would be troublesome?
There are calculators that can help you determine that, However the key parts that will impact this are the change in voltage across the capacitor that's allowable and the current being drawn. What you'll likely find is that with a capacitor on it's own you'll need something quite large just for a few seconds (probably into the realm of supercaps), in that setup you can only use a very small portion of the charge stored in the capacitor before the voltage drops too low to power the Pi.

If you added step-up/down circuitry you might manage something smaller. I.e. if you could change the capacitor to a higher voltage and have something provide 5V to the Pi through the whole discharge curve of the capacitor.

FM81
Posts: 518
Joined: Wed Apr 17, 2013 4:33 pm

Re: Raspberry Pi3v - what size capacitor?

Sat Feb 11, 2017 5:20 pm

raven_squire wrote:I am using the raspberry pi3 on an Atrix lapdock and want to use a capacitor to make sure the pie will stay powered up when power is removed for 3 seconds (accidental lid close).
I assume it will go in the range of 5F or more. Im powering a Rpi-zero during shutdown for a three or five seconds during shutdown from 2F and the voltage drops during that from 5,2V to 4,6V. The current of this setup is only about 200-300mA. A RPi3 (and more additionally components) will pull much more of course ...
raven_squire wrote:A 1000uf capacitor was needed to boot without the lightning bolt coming up on the screen. This tells me that there is not quite enough power being supplied by the lapdock.
Could I use something like a bunch of 4700uf capacitor? At what point would too many capacitors become a problem?
Remember that a very big bunch of capacitors will draw a high amount of current in the power-on-moment. Or if you can not deliver this current, it would take a longer time. Remember also, that it will take a very long period to, after "switching off" before these capacitors are discharged enough, to leave all your components in a defined state.
raven_squire wrote:I am also thinking about some getting micro usb break out cables so that I can measure the resistance from my home made power cable just in case that is causing power problems. What resistance value would be troublesome?
If you can measure it under "normal home conditions" it will be probably to high! 0,05 ohms in plus and 0,05 ohms in minus give at 2A a drop of 0,2V. And it is not easy to measure 0,05 ohms with a normal multimeter ... There exists milliohm-meters for such. (See also four-lines-method.)

A step-up will also not help very much in this case ... if capacitor discharges, voltage goes lower ... to keep output of step-up constant it will draw higher current (Because you can not make power from nothing!) ... higher current means faster discharging again ...

Greetings, FM_81
A: What does the command 'cat /dev/urandom', can you tell me please?
B: Yeah, that's very simple: It feeds your cat with radioactive material!

User avatar
Burngate
Posts: 5508
Joined: Thu Sep 29, 2011 4:34 pm
Location: Berkshire UK
Contact: Website

Re: Raspberry Pi3v - what size capacitor?

Sat Feb 11, 2017 5:33 pm

For the capacitor:
The lightning bolt will appear when the 5v rail has dropped to ~4.65v. If the supply starts at 5.00v, then you've got 0.35v leaway.
If your Pi is drawing 1A (it could well be more) and you want to run for 3 seconds, your capacitor will need to supply 3 coulombs.

Q = CV

So C = 8.6 Farads

For the cable:
If your supply is giving 5.00v, and your Pi is using 1A, and you don't want the lightning bolt, again you can stand 0.35v

V = IR

So R = 0.35 Ohms

Return to “Beginners”