Hi all.
It's been a LONG time since electronics in university so please be gentle. I'm working on a project that I would like to run from any of 3 power sources, namely ; 12 volt wall wort, external 12 volt battery, or internal 7.4 volt Li-Ion. There is a charging circuit inside to handle keeping the Li-Ion topped up.
My thought is that if the device loses power during operation it can run on the Li-Ion , or if all else fails hook it up to a car battery via a "cigaret lighter" plug. Here is the circuit I was playing with using my VERY fuzzy recollection of that electronics course. Any advice/guidance would be appreciated.
The file is in the attached PDF although it's all in Eagle is that is better for everyone. Thanks in advance.
Grumpy_Mike,
Thanks for the feed back. To your observations:
The diodes on the grounds:
My logic was that I don't think that there is a guarantee that the ground of the power supply being run on 110 AC is actually at the same potential as the ground of the external battery. That said, since in all probability there is no path from the battery back to the power plant providing power to the the power supply you are probably correct about those not being needed.
As to switching the power between the internal battery and any external supply:
I thought that if either of the external supplies were present that the voltage divider connected to the base of the PNP Q1 would be held high with about 10 volts. if the external supply is removed then the base goes to ground and the battery current is allowed to flow through the transistor. Seemed logical at the time but the number of things I don't know about how transistors work is legion.
Grumpy_Mike,
I think I see one reason you may be confused by the battery . Looks like I have the Vcc symbol on the power to the battery charging board backwards. Oops. Vcc should defiantly be an input to the charging board. I'll fix the schematic and repost
All grounds should be connected together. This is important for DC power as well as input signals, so your scheme of diodes to ground is unlikely to work correctly.
I can't make much sense out of the transistor circuit, partly because of the nonstandard way it is drawn, but the way I interpret it, it won't work. For an NPN transistor to conduct, sufficient base current has to flow from a positive source.
I suggest to spend more time looking at the many examples on the web of battery/external power supply switching circuitry. Try googling "schematic battery external power supply"; there are many useful schematics in the images.
ellisco1:
:
The diodes on the grounds:
My logic was that I don't think that there is a guarantee that the ground of the power supply being run on 110 AC is actually at the same potential as the ground of the external battery. That said, since in all probability there is no path from the battery back to the power plant providing power to the the power supply you are probably correct about those not being needed.
Not only not neeeded but they will stop that part from working.
Thanks for clearing up the transistor switch, now I can say difinitavely it will not work. There are many things wrong with it basically transistors do not work like that. Look at the power switching on the arduino uno's schematic for a good example of this sort of circuit.