Dual Pwr switching between AC-DC and batteries

[ericmar]

dumy load ac/dc

Hi,

Just want to know if my design for dual power switching between an AC-DC adapter and batteries as shown below is correct. My intention is to allow the system to draw power from the AC-DC adapter instead of batteries when AC-DC adapter power is available.

Appreciate if there is any feedback on this circuit.

Thanks.

Regards,
Eric

**broken link removed**

Added after 3 hours 59 minutes:

hi IanP,

I receive notification of your reply but the post isnt here. Thanks for your reply anyway. Appreciate if you can re-post so that I can see your attached picture.

Message below:===============
As it is, you may have problems with switching the MOSFET off, especially if the DC voltage is 9V and battery voltage is 12V .. Try to add another transistor, can be any general purpose BJT or MOSFET (2N7000) .. see attached picture ..
Rgds, IanP
===============

Thanks.
Eric

 

[IanP]

Different idea:

[You may have problems with switching off/on the P-MOSFET ..]

Try to add ≥100kΩ between [G] and , replace R35 with a 3.3V Zener (Vdc=9V, Vbat=12V) - [K]-to-[G] and [A]-to-Vdc and add a dummy load for Vdc [1kΩ - 4.7kΩ] ..

Rgds,
IanP

 

[ericmar]

Different idea:

[You may have problems with switching off/on the P-MOSFET ..]

Try to add ≥100kΩ between [G] and , replace R35 with a 3.3V Zener (Vdc=9V, Vbat=12V) - [K]-to-[G] and [A]-to-Vdc and add a dummy load for Vdc [1kΩ - 4.7kΩ] ..

Rgds,
IanP

Hi IanP,

Thanks again for your reply.
Sorry but I am not quite clear with your suggestion.
What is the use of the 3.3V Zener and the dummy load fo Vdc? I probably will reduce the number of alkaline batteries to just 4-cells, which would end up with +4.8V (about +1.2V if using re-chargeable batt.) ~ +7.2V (about +1.6V if using batt. like Energizer Lithium). So the input voltage of batteries will definitely be lower than +9V of AC-DC adapter.

Please advise. Thank.

Regards,
Eric

 

[IanP]

If the battery voltage is ≤ Vdc then you don't need Zener (and 100kΩ), but dummy load for Vdc may be usefull as the output configuration of the AC/DC is unknown and you need a dc path between [G] and [GND] to switch on the MOSFET ..

Rgds,
IanP