1

I'm having trouble powering my pi from 12v 2a battery. I made a circuit using 1117 linear regulator, it's supposed to give 1a current and 5v. Pi powers on normally but any peripherals that I connect do not work i.e. WiFi USB or keyboard. Powering pi from USB charger works OK. What is the problem with power? Isn't 1a enough?

PS I have model b rev. 2

Improve this question
3
  • 1A should be enough depending on what your powering. But are you sure that it is 1A?
    – Vincent P
    Commented Jan 17, 2013 at 9:43
  • From latest test I'm not sure it's 1a, maybe taking another regulator can help, or where can I connect an amperemeter?
    – Viktor
    Commented Jan 17, 2013 at 9:50
  • You cannot connect a amp meter. But I've seen that you can connect a volt meter to the TP1 and TP2 points. If the amp's drop then the voltage will drop. I've tried this, but found it to not be very accurate.
    – Vincent P
    Commented Jan 17, 2013 at 9:53

3 Answers 3

Reset to default
2

At a current of 1A, the LM1117 is dissipating (12-5)*1 = 7W. Unless you have a big heat sink, the thermal limiting will shut the regulator down.

You would be better off to use a Buck converter. There are some on ebay quick cheaply based on the LM2596. The more efficient conversion will double your battery life

Improve this answer
5
  • I've tested common 7805 and LM2575. Both work. 7805 had a heatsink size of a shoe that got pretty toasty while the switcher barely heats up without one. The old 2575 hits the coil pretty hard with a loaded PI, but newer ones will do better. And you can get cheap modules easily these days.
    – XTL
    Commented Jan 22, 2013 at 8:06
  • suntekstore.com/… This would be a particularly cheap part, but needs a few components (fb R, bulk cap).
    – XTL
    Commented Jan 22, 2013 at 8:09
  • suntekstore.com/… Here's a ready one for example.
    – XTL
    Commented Jan 22, 2013 at 8:14
  • @XTL, that looks like the ones on ebay, but ebay is about 1/2 the price
    – John La Rooy
    Commented Jan 22, 2013 at 8:51
  • Yes. There's a lot of sellers for these if you shop around.
    – XTL
    Commented Jan 22, 2013 at 10:11
1

I am sorry but there is no such thing as a 2A or any amp battery. There is amp-hour rating symbolised as Ah,AHr,A·h. When you say 2 I assume you mean 2Ah with the 12volt battery. It does not mean you can use 2A maximum. You can draw 5Amps from the battery if you want.

Some more information can help

  • What revision board is it?
  • Do you plug the stick in when the Pi is on? Does it reboot?
  • Do you have it plugged in and does the Pi boot normally- not safe mode?
  • What is the 1117 amp rating?

Like you said the Pi uses maximum 1A which which should at the worst case give you 2 hours battery time. Now why the peripherals do not turn on might be caused by the line converter you are using. Looking at a generic spec sheet of a LM1117 it is rated for 800ma - These things are smart now a days with current limiting, short circuit protection, etc.

Possible cause

What you can be experiencing is the current limiting feature on the regulator? Maybe the peripherals does try and turn on but because it does not have enough current the internal circuit of the peripheral gives up and does not turn on again (like a polyfuse) so you have to take it out and try again. It happens so quick that the Pi does not experience a dip and does not restart but too quick for you to notice anything happen.

Improve this answer
3
  • 1
    amp hour, not amp per hour
    – John La Rooy
    Commented Jan 17, 2013 at 10:14
  • Thanks- I always thought it was amp per hour because of Amp/Hour - But in fact that is something else and batteries indeed symbolised Amp-hour. Learn something new everyday :)
    – Piotr Kula
    Commented Jan 17, 2013 at 10:21
  • If it was A/h, you'd get more hours out of the battery by drawing more amps.
    – XTL
    Commented Jan 22, 2013 at 8:01
0

The linear regulator is definitely the sticking point here. A switched mode regulator is much more efficient and therefore won't take a chunk out of your battery life in and of itself. The MoPi board provides one in an easy interface for connecting batteries like yours to the Pi, and also lets you know about charge status so that the Pi shuts down nicely. I'd estimate a 12 V, 2 Ah battery to run for around 12 hours.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.