shinji2009
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add electrolytic capacitor on power?

Tue Jul 18, 2017 10:31 am

hi. i know that rpi is very picky to the power adapter. i look to the board and schematics and see that theres no big electrolytic capacitor nowhere in the power. and what i think is what if adding the big electrolytic capacitor on the +5v can prevent some power issues or even make possible to use not very powerful PSU? does anybody know anything about it? try it?
and for example if solder a cap. what + point i should use? pp1 or pp35?
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Burngate
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Re: add electrolytic capacitor on power?

Tue Jul 18, 2017 11:01 am

shinji2009 wrote:... theres no big electrolytic capacitor nowhere in the power.
If you look just past the "ideal diode" (U14 et. al.), there's a 47μF capacitor.
... what if adding the big electrolytic capacitor on the +5v can prevent some power issues or even make possible to use not very powerful PSU?
If the PSU can't supply the current the Pi and its peripherals require, then a cap isn't going to cure it.
To supply, say, an extra 1A for 1sec, with the voltage drooping only 1V, you'll need 1,000,000μF
and for example if solder a cap. what + point i should use? pp1 or pp35?
If you were to connect your million-microFarad capacitor after the fuse, you would risk blowing the fuse with the inrush current. Also, if a fault developed on one of your peripherals, the fuse wouldn't protect anything - the energy stored in the cap would still be enough to destroy something.

You're trying to improve the supply, so connect your capacitor where the supply enters the board.

RDS
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Re: add electrolytic capacitor on power?

Tue Jul 18, 2017 6:34 pm

Why not just use the genuine Pi Power Supply, 2.5A I think.

shinji2009
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 7:10 am

thank you for answers
Burngate wrote: If you look just past the "ideal diode" (U14 et. al.), there's a 47μF capacitor.
this is quite small capacitance. for a device with such theoretical total power consumption. haven't seen anything like this before.
Burngate wrote: If the PSU can't supply the current the Pi and its peripherals require, then a cap isn't going to cure it.
when i add a cap it is become a part of rpi and maybe help it to get better power on at least. i just measure you know my pri eats like 400mA max when working. but it don't even power on from a 5V 1A adapter. so it is power on the moment when adapter is tested for power. so i wondering what if cap do the trick.
Burngate wrote: To supply, say, an extra 1A for 1sec, with the voltage drooping only 1V, you'll need 1,000,000μF
brownout can occur on 0.1V drop in some circuits and rpi work on a high frequency and processes are faster than 1sec so your calculations is very sarcastic yes) i was thinking about something more real like 2200uf.
Burngate wrote: You're trying to improve the supply, so connect your capacitor where the supply enters the board.
it looks not so good outside of the casing. but there's pretty much free space in the casing. and maybe it won't help outside cince it must be part of the rpi to work.

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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 7:20 am

shinji2009 wrote:hi. i know that rpi is very picky to the power adapter
Your initial statement is wrong. If it was true there would be huge out cry from the (however million it is now) users.
If you use a cheap PSU that can't meet the PI's requirements then it will cause problems, same as with any piece of equipment that runs from "wall warts". That's not being "picky".
The solution to the vast majority of power related problems is to get a better PSU that can maintain the voltage when supplying the needed current.

PeterO
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shinji2009
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 7:50 am

PeterO wrote: Your initial statement is wrong.
shinji2009 wrote: pri eats like 400mA max when working. but it don't even power on from a 5V 1A adapter.

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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 7:58 am

shinji2009 wrote:
PeterO wrote: Your initial statement is wrong.
shinji2009 wrote: pri eats like 400mA max when working. but it don't even power on from a 5V 1A adapter.
Sorry, but that just shows you don't understand as much as you think you do. Why do you think the recommendation is for a psu rated to be able to provide 2.5A at 5V ?
PeterO
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Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
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shinji2009
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:03 am

i was thinking this is because there's no big electrolytic capacitor in the rpi power circuit, and because of that rpi eats a lot of curent during power on and because of that you need to use 2a adapter even if there's no such power needed afterwards.

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davidcoton
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:38 am

shinji2009 wrote:i was thinking this is because there's no big electrolytic capacitor in the rpi power circuit, and because of that rpi eats a lot of curent during power on and because of that you need to use 2a adapter even if there's no such power needed afterwards.
A capacitor will not help at power on, because:
  1. It starts discharged, so it makes things worse while it charges
  2. A normal size capacitor does not store enough charge to provide the Pi's level of current for a significant period.
A capacitor might help avoid problems when a USB peripheral is hot-plugged. Most newer Pis (except the Zeros) have USB current limiting to avoid this problem anyway.

Just because your PSU is abelled 5V 1A (unfortunately) does not mean that it can supply 1A at 5V. It may do 5V off load, but at 1A it may only be 4V. :o :(

tl;dr. Adding a capacitor is the wrong approach.
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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:40 am

i was thinking this is because there's no big electrolytic capacitor in the rpi power circuit, and because of that rpi eats a lot of curent during power on and because of that you need to use 2a adapter even if there's no such power needed afterwards.
So when you connect the PSU it now has to provide the current to power the PI AND to charge up the capacitor. That makes the situation worse not better !

If the PSU can't provide the peak current itself, adding capacitance is only delaying the inevitable voltage droop when current demand exceeds current supply.
What if you decide to run an application that uses all four cores "flat out" for 10 seconds, how big a capacitor will you need to make up for the deficiency in your 1A PSU ?
The answer is to get a PSU that can continuously provide the needed current.

PeterO
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davidcoton
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:44 am

PeterO wrote: The answer is to get a PSU that can continuously provide the needed current.
while maintaining 5V at the Pi, so the cable needs to be short and thick, too.
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Burngate
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 4:33 pm

shinji2009 wrote:... so your calculations is very sarcastic ...
I'm sorry. My mother-in-law also berates me for being sarcastic, with good reason. I'll try to be less sarcastic in future.

What I was trying to point out has been explained far more succinctly by the others on this thread, so just ignore me.

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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 5:59 pm

davidcoton wrote:
PeterO wrote: The answer is to get a PSU that can continuously provide the needed current.
while maintaining 5V at the Pi, so the cable needs to be short and thick, too.
I though from the previous posts that was obvious :lol: :lol:
PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:17 pm

PeterO wrote:I though from the previous posts that was obvious :lol: :lol:
I've learnt over time that what is obvious to the transmitter of information (and therefore is not transmitted) is far from obvious to the receiver.
Murphy's Law applies. Obvious information that is transmitted will already be obvious (and therefore superfluous) to the receiver.
But in most circumstances I'd rather be Murphied than misunderstood. :roll: :o :shock: :? :lol: :twisted: :ugeek:
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boyoh
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 8:59 pm

shinji2009 wrote:hi. i know that rpi is very picky to the power adapter. i look to the board and schematics and see that theres no big electrolytic capacitor nowhere in the power. and what i think is what if adding the big electrolytic capacitor on the +5v can prevent some power issues or even make possible to use not very powerful PSU? does anybody know anything about it? try it?
and for example if solder a cap. what + point i should use? pp1 or pp35?
asdcdsdf.GIF
Your lack of understanding o f what the electrolytic capacitor is for on a power supply is
Your problem
It is called a smoothing capacitor, Full wave rectification of the AC,will leave you with
100hz (UK) ripple on you DC out put .this is smoothed with a capacitor , If the capacitor
Is to big ,it will take to much current from the power supply when charging, If the capacitor
Is to small it will not cancel all the ripple out, So care is taken when selecting the capacitor

So fitting a another capacitor will only make the problem worse, The power supply will have
To charge the extra capacitor and supply the out put load current at the same time
BoyOh ( Selby, North Yorkshire.UK)
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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 9:10 pm

boyoh wrote: Your lack of understanding o f what the electrolytic capacitor is for on a power supply is Your problem.
But still living in the 1960s seems to be your problem Boyoh. Switchmode PSU's are the norm these days.

PeterO
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Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
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PeterO
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 9:12 pm

davidcoton wrote:But in most circumstances I'd rather be Murphied than misunderstood. :roll: :o :shock: :? :lol: :twisted: :ugeek:
Or just confused judging by all those smilies ! :roll:
PeterO
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Interests: C,Python,PIC,Electronics,Ham Radio (G0DZB),1960s British Computers.
"The primary requirement (as we've always seen in your examples) is that the code is readable. " Dougie Lawson

boyoh
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Re: add electrolytic capacitor on power?

Wed Jul 19, 2017 11:09 pm

PeterO wrote:
boyoh wrote: Your lack of understanding o f what the electrolytic capacitor is for on a power supply is Your problem.
But still living in the 1960s seems to be your problem Boyoh. Switchmode PSU's are the norm these days.

PeterO
What fun it was in the 60s, No Google No Plug & Play, Plenty of components to send for
I wish I was still in the 60s, and not in my 80s, At least he will have some Idea of old
Power supply's now
Regards Boy Oh
BoyOh ( Selby, North Yorkshire.UK)
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rpdom
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Re: add electrolytic capacitor on power?

Thu Jul 20, 2017 5:26 am

boyoh wrote:What fun it was in the 60s, No Google No Plug & Play, Plenty of components to send for
I wish I was still in the 60s, and not in my 80s, At least he will have some Idea of old
Power supply's now
Yep, those good old linear power supplies that kept your house warm and cosy ;-)

One day I will learn to make my own custom smps, but currently it is cheaper to just buy a ready made one for most things.

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PeterO
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Re: add electrolytic capacitor on power?

Thu Jul 20, 2017 5:57 am

boyoh wrote:What fun it was in the 60s,
Oh I know. While not being old enough to have experienced it first hand, I look after this https://www.youtube.com/watch?v=Wa7KVU_e8U8&t=5s so I have great respect for 1960's technology !
And just to come back on topic (nearly) We've had problems with capacitors in power supplies !
Image
PeterO
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rpdom
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Re: add electrolytic capacitor on power?

Thu Jul 20, 2017 6:21 am

PeterO wrote:Oh I know. While not being old enough to have experienced it first hand, I look after this https://www.youtube.com/watch?v=Wa7KVU_e8U8&t=5s so I have great respect for 1960's technology !
Cool video! I had one of those plotters once (or very similar Calcomp one). I hooked it up to a BBC Micro to draw things with :-)

boyoh
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Re: add electrolytic capacitor on power?

Thu Jul 20, 2017 9:09 pm

PeterO wrote:
boyoh wrote: Your lack of understanding o f what the electrolytic capacitor is for on a power supply is Your problem.
But still living in the 1960s seems to be your problem Boyoh. Switchmode PSU's are the norm these days.

PeterO
Despite the disadvantages, switch mode power supply technology is the major form of power supply technology used for a whole variety of applications especially those included in computers. For applications where very low noise are required, linear regulator technology is still widely used.
BoyOh ( Selby, North Yorkshire.UK)
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ProDigit
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Re: add electrolytic capacitor on power?

Wed Sep 11, 2019 2:14 am

I actually had exactly the same idea.
The OP was 1 year ahead of me there.
But then again, I only have my Pi 3B+ now for 2-3weeks.

The Pi 3B+ uses +1.5A of power draw in fractions, not continuous. (and yes, power is in Watts, not Amps, but it hardly matters (unless your name is PeterO, then it'd be enough to start a flame war)).
This is very visible when connecting a low power (2A) PSU, and an LCD hat on it.
The hat dims as the CPU does calculations.
And surely, a cap would diminish that effect. Just to level out voltage fluctuations.
A 1.5-2A 5V provides sufficient voltage, but a long thin lead can cause a slight voltage drop at the Pi's input.
While the voltage drop can't be raised easily, the voltage dips caused by the CPU's demand for more power during calculations, can be leveled off, as long as the CPU isn't continuously draining power.
In such case a capacitor wouldn't benefit, but wouldn't harm either.


The initial charge time for the capacitor should not be an issue either, as the pi would be booting bios, reading from the card reader the first few seconds anyway, before it starts booting, so if a cap that can fully charge in 1 or 2 seconds at 5V 2A can be found, such cap would probably be the best option for the Pi.

I myself am thinking of installing one, either on the GPIO pins, or soldering on the bottom of the power plug (micro USB).

jdb
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Re: add electrolytic capacitor on power?

Wed Sep 11, 2019 9:57 am

- Don't necro old posts
- Don't flamebait other forum users.
Rockets are loud.
https://astro-pi.org

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