5V power with lithium battery backup

hramrach
 
Posts: 3
Joined: Wed Mar 18, 2015 1:56 pm

5V power with lithium battery backup

Wed Mar 18, 2015 3:40 pm

Hello,

I would like to power vocore from a 5V source such as USB port with a lithium battery (such as CR2032 or CR123A) for backup.

I am sure that powering off vocore, switching the power source, and powering it back on should work.

It would be cool to be able to unplug the 5V source or reboot your PC without vocore powering off.

I suspect that connecting the output of the 5V->3V3 regulator together with the battery is not wise but just switching from one to the other might cause a spike since the battery can have anywhere from 3.2 to 3.7V.

I am not an electrical engineer so I might try a thing or two and if it works that's good and if not whatever.

If somebody has a solution that is *supposed* to work I would like to hear.

Thanks

Vonger
 
Posts: 840
Joined: Sun Oct 19, 2014 6:00 am

Re: 5V power with lithium battery backup

Thu Mar 19, 2015 5:55 am

As I know, if you want a power switch, you must have a big cap :)

How about this solution:
Use a mobile battery, output port 5V connect to VoCore, and another battery charge port connect to power.
Mobile battery contains a stable solution and you do not have to do any hardware, it will work.

Greenwire-Elektronik
 
Posts: 101
Joined: Thu Dec 04, 2014 6:31 am

Re: 5V power with lithium battery backup

Thu Mar 19, 2015 6:22 am

Problem is easy to solve - you can connect a flip coin cell or whatever battery to 3.3V. However, use a diode between battery and 3.3V pin - it protects the battery from being accidently loaded and also that the battery will be drained if 5V is connected.

Only important thing is the voltage drop over the diode.
Buy now - breakout board for VoCore to easy adapting your idea!

Follow us on Twitter or facebook!

hramrach
 
Posts: 3
Joined: Wed Mar 18, 2015 1:56 pm

Re: 5V power with lithium battery backup

Thu Mar 19, 2015 10:26 am

Hello,

Yes, adding a cap was one thing that came to my mind. I have a box of caps of varying properties so I could add some at random. Unfortunately, I don't have anything that would show the voltage in realtime to determine if the cap is large enough or fast enough or whatever.

As for diode I expect that it would prevent the battery from being 'charged' from 3V3 and possibly exploding when 5V is connected but would not prevent the battery draining when the battery is new and its voltage is higher than 3V3.

So my best idea was to add a transistor that would switch off the battery supply when 5V is provided. Details TBD because I need to estimate the required current and figure out which pin would connect where.

Thanks

Greenwire-Elektronik
 
Posts: 101
Joined: Thu Dec 04, 2014 6:31 am

Re: 5V power with lithium battery backup

Fri Mar 20, 2015 5:46 am

Your concern against the diode is unnecessary depending on your battery voltage. If you have 5V connected, you have 3.3V on the rail, adding a shottky diode forward voltage your battery need >3.7V to be drained ...
Buy now - breakout board for VoCore to easy adapting your idea!

Follow us on Twitter or facebook!

Return to VoCore & VoCore+Dock

Who is online

Users browsing this forum: No registered users and 2 guests