How to switch between two power sources?

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Pro Disma

Joined: 18 Feb 2005
Posts: 2

18 Jul 2005 12:21 How to switch between two power sources?

Dear all,

what is the most power efficient way to choose between two power sources?
To be more precise, I have a solar powered circuit with a dc regulated output and a rechargable battery, and want to be able to automatically select the best of those two upon voltage level conditions (i.e.always keep a 3V output).

Thanks a lot!

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throwaway18

Joined: 09 Jun 2005
Posts: 563
Helped: 75
Location: Kent, UK

18 Jul 2005 12:37 Re: How to switch between two power sources?

If you are using a battery that can be float charged (Ie lead acid) and it dosn't matter if the output voltage can droop then the best way is probably to feed the solar cell output into a constant voltage regulator that charges the battery and also provides power out. If your equipment draws more current than the solar cell can provide it will start drawing surrent out of the battery.

If you want to switch between sources you can buy relays with two coils. The contacts stay in the last postion they were pulled to with no power on either coil.

A comparator circuit to drive such a relay could be made that uses very little power when not changing between power sources.

Last edited by throwaway18 on 24 Mar 2006 13:08; edited 1 time in total

IanP

Joined: 05 Oct 2004
Posts: 6490
Helped: 1542
Location: West Coast

18 Jul 2005 12:49 Re: How to switch between two power sources?

I think relays draw to much current so you should consider p-channel mosfet option which can be driven from a low-power comparator thruogh a couple of npn transistors; some more details you can find here:
http://www.edaboard.com/viewtopic.php?p=467164&highlight=#467164
(my last post)
Regards,
IanP