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solar3000
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Dual power supply

Thu Jul 07, 2016 8:21 pm

Has anyone thought of dual power supplies?

I was dreaming up something while daydreaming one day.

Real simple, a 'Y' connector to the pi from two 5 volt sources.
Antikythera

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davidcoton
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Re: Dual power supply

Thu Jul 07, 2016 8:27 pm

Don't even dream. Unless the PSUs are designed to tolerate it, a voltage mismatch (even within the +/- 0.25V tolerance) could destroy one or both PSUs.

Just get a decent PSU with adequate output, like the "official" 2.5A, or bigger if required for power-hungry peripherals (but don't draw more through the Pi).

If you are talking about fault tolerance, well it really isn't "Real simple".
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stderr
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Re: Dual power supply

Thu Jul 07, 2016 8:41 pm

davidcoton wrote:If you are talking about fault tolerance, well it really isn't "Real simple".
Some SBC type boards purport to be capable of being run on various power sources and that they pick the available source that is the highest at that point. In theory, that could mean battery back up support and support for using two power supplies is basically built into the board.

Aydan
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Re: Dual power supply

Fri Jul 08, 2016 11:21 am

stderr wrote:Some SBC type boards purport to be capable of being run on various power sources and that they pick the available source that is the highest at that point. In theory, that could mean battery back up support and support for using two power supplies is basically built into the board.
Those SBCs usually have a PMIC that monitors available supplies and switches accordingly.
Regarding running PSUs in parallel: It depends on the type of reguulator that is used. If it can only source and not sink (meaning pull current when the outside voltage is higher than the PSU nominal voltage, even if the PSU is not supplied by mains), then there's no problem. The PSU with the higher voltage will source the current. If the PSUs are close together, they'll share the load/current. I assume most wallwarts implement this type of output stage.
If on the other hand, the PSU is source/sink, then you're screwed, because the PSU with the lower nominal voltage will try to bring down the voltage of the other one.
If you want to do it properly, look into active diodes. This is basically a FET+Controller which closes with current flowing in one direction, and opens with current flowing in the other direction.

Regards
Aydan

Anecdotally: I had my Cubox-i4pro running with a powered hub connected (HDD on Hub). This worked quite well for some time (read months), untill the PSU of the powered hub gave out spectacularly (meaning emission of toxic fumes and the mains fuse blowing). Seems the cubox has no separation between the USB5V and the 5V input barrel connector, and the PSU of the hub supplied the hub, HDD and the cubox, which probably was too much.

Massi
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Re: Dual power supply

Fri Jul 08, 2016 12:13 pm

solar3000 wrote:Has anyone thought of dual power supplies?
thought.. yes..

solved.. no :)

my idea was to build "something" to joint a solar power source with a "normal" power source (this could be also a battery + a normale..), to be able to use solar when available and normal when not available.
i could not find a solution..

Edit to add:
well', i had never seen this

http://www.dx.com/p/ina219-diy-dc-curre ... 3-kYfmLTIV

that seems quite good.
Ideally, having one of these attached to every power source of che circuit could allow you to "choose" the best one
but in any case i think you would need a "buffer" (a quite big capacitor?) in the circuit, to:
- guarantee current to the circuit while you switch from a source to the other
- give you time to sense the current once switched the power source and, eventually, go back

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solar3000
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Re: Dual power supply

Fri Jul 08, 2016 2:52 pm

Ouch-ouch-ouch-ouch! Ouuuiiww!
Put down the clubs!
Last edited by solar3000 on Fri Jul 08, 2016 2:57 pm, edited 1 time in total.
Antikythera

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solar3000
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Re: Dual power supply

Fri Jul 08, 2016 2:55 pm

What about when you have powered hubs. Aren't you already putting two power supplies together? ....1. power from micro USB. 2. power from powered hub.
Antikythera

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piglet
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Re: Dual power supply

Fri Jul 08, 2016 3:25 pm

No, they should just join the grounds.

mfa298
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Re: Dual power supply

Fri Jul 08, 2016 3:54 pm

solar3000 wrote:What about when you have powered hubs. Aren't you already putting two power supplies together? ....1. power from micro USB. 2. power from powered hub.
The powered hub shouldn't be feeding power back into the Pi. It should only power devices plugged into the hub.

Aydan
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Location: Germany, near Lake Constance

Re: Dual power supply

Fri Jul 08, 2016 6:54 pm

mfa298 wrote:
solar3000 wrote:What about when you have powered hubs. Aren't you already putting two power supplies together? ....1. power from micro USB. 2. power from powered hub.
The powered hub shouldn't be feeding power back into the Pi. It should only power devices plugged into the hub.
should is the word here. But most hubs don't separate uplink and downlink power. The 5v pins of all connectors are simply wired together. It's just cheaper that way.
And as I laready stated in my earlier post: THis will work, as long as all power supplies are up to the job of supplying all devices, or the USB host blocks backfeeding. Otherwise, as has happened to me, the hub power supply will die eventually due to overload.

Regards
Aydan

GordonTGopher
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Location: Letchworth Garden City

Re: Dual power supply

Fri Jul 08, 2016 8:17 pm

Something like this https://shop.pimoroni.com/products/mopi-mobile-pi-power may help.

Regards

Gordon

mosespi
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Re: Dual power supply

Mon Jul 11, 2016 6:18 am

Dual supplies, sure. A simple setup would be 2 wall warts (9v, 12v, etc) and 2 diodes feeding a DC-DC converter. You are diode ORing two power supplies.. you can keep repeating for more supplies, batteries, solar, etc. Higher voltage wins.

If you need to feed in two 5v supplies, pick ones that are 5.1v or 5.2v and go with low drop diodes. If you can't stand the diode drop, then you may be able to build (or buy?) one of those zero drop diode FET circuits.

Regards,
-Moses
Power problems? MoPower UPS for the Pi
http://www.allspectrum.com/mopower/

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