## Too high voltage? (PSP PSU 5.4 V)

alias_neo
Posts: 108
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Location: London, England
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### Re: Too high voltage? (PSP PSU 5.4 V)

alexeames wrote:
alias_neo wrote:This would chain tot he regulated PS we're creating above with a voltage of 7.4V (2x 3.7v cells).
Don't forget that's the "empty" voltage of lipo cells. They're 4.2 V/cell charged and they don't drop off much under light load. I still don't understand why you're even considering a linear regulator when using batteries though. Surely a switching reg would give you much better efficiency?

Good point, and they aren't to drop below 3V or they become damaged.

I suppose I could go for a switching regulator but even a low drop linear reg is reasonably efficient.

I'm making it for... and giggles more than any practical, purpose, but I suppose if I'm going to share the design I should at least do it right for those who will use it properly >_<
http://2byt.es - Just my two bytes - RPi - Android - Arduino - Blog

redhawk
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### Re: Too high voltage? (PSP PSU 5.4 V)

I found this on eBay which might prove useful it's a DC to DC converter step up / step down http://www.ebay.co.uk/itm/290691738832? ... 1423.l2649
From what I understand the input voltage can be between 3.5v and 35v but can still output a constant 5v whereas normal dc to dc step down converters require a higher voltage at the input to produce the step down voltage as much as 1.5v more.
The same is true for 7805 regulators for them to regulate 5v you need the input to be at least 6.5v or higher if you try an regulate something like 6v the 7805 will output roughly 4.6v.
So basically with you have approximately 7v to 12v+ to play with but a DC to DC converter step up / step down would pump out a lot more juice especially with AA rechargeable batteries.

Richard S.

alias_neo
Posts: 108
Joined: Fri Jul 29, 2011 8:55 am
Location: London, England
Contact: Website

### Re: Too high voltage? (PSP PSU 5.4 V)

redhawk wrote:I found this on eBay which might prove useful it's a DC to DC converter step up / step down http://www.ebay.co.uk/itm/290691738832? ... 1423.l2649
From what I understand the input voltage can be between 3.5v and 35v but can still output a constant 5v whereas normal dc to dc step down converters require a higher voltage at the input to produce the step down voltage as much as 1.5v more.
The same is true for 7805 regulators for them to regulate 5v you need the input to be at least 6.5v or higher if you try an regulate something like 6v the 7805 will output roughly 4.6v.
So basically with you have approximately 7v to 12v+ to play with but a DC to DC converter step up / step down would pump out a lot more juice especially with AA rechargeable batteries.

Richard S.

The problem with this is that step up would require a higher current than we can provide, or be unable to source enough current to run the Pi.
http://2byt.es - Just my two bytes - RPi - Android - Arduino - Blog

alexeames
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### Re: Too high voltage? (PSP PSU 5.4 V)

alias_neo wrote:Good point, and they aren't to drop below 3V or they become damaged.
Some of the RC plane guys, hammering their lipos at 20C use a cut-off on the ESC of about 2.9V/cell, but that's under seriously heavy load. The cells recover quickly to 3.7V+ when not under load.

We'll want the Pi to last several hours on a charge, so a lot less than 1C. Much lower loading, so I would have thought a cut-off nearer 3.7 would be more appropriate.
Alex Eames RasPi.TV, RasP.iO

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