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This circuit is an upgrade to a battery mod on a DSO150 oscilloscope.
Updated to put the MOSFET the right direction hehe. Still works.
All of the modifications I found removed or disabled the 9V jack. What happens if your in a situation where the battery is dead and you don't have a micro USB cable handy. OK, I know you might be able to possibly use the the device while it's charging. Or maybe not, the battery is 3.8V going through a converter to bring it up to ~8.5V, the USB charger is ~5V. So, I guess I should test that and see what the converter output is when the battery is charging.
Irregardless, what if you have a 9V supply and wanted to use it? That's where this circuit comes into play. Once you plug in the jack the MOSFET shuts off the flow from the battery and the internal diode on the FET keeps it from trying to charge the battery.
The first one I created. http://everycircuit.com/circuit/5087440930078720
Here is the second. http://everycircuit.com/circuit/6528990264426496
Reduced the part count to only 3. Was able to remove 2 diodes, 1 NPN transistor, and 1 resistor. Circuit uses a single surface mount P MOSFET. P/N - NTD25P03L that costs about $1.25 USD, one 100k and one 10k 2010 surface mount resistors, which are like $0.60ish each. I only made them that large so I could see the little buggers to solder them. Next I am going to make the artwork and a prototype pcb.
The Gate voltage comes from a wire that you solder to the + pad on the jack of the DSO150
The drain comes from the + side of the converter.
The source goes to the lower center pin on the main power switch, you do have to make a single trace cut between the two center pins, and of course it's under the switch if you have already installed it.
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