Power supply and battery charger

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gproduct
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Posted by gproduct: Sat. Jul 30, 2016 - 12:02 AM
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Hello!
I've designed this circuit for my small project that uses an RTC,MCU and a display.
My goal is to use a 20mmx20mm pcb and I don't want to include a separate battery for the rtc.
The IN is the 5V from usb and the BAT is the battery 3.7-4.2V. I've connected the 5v to a mosfet and directly to the regulator. My objective is that when I connect the usb that the electronics will run of the 5V and it will disconnect the 3.7 and let it charge.
While switching states (mosfet) will the electronics reset?
The schematic:

Thank you!

 

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Tags:

Learning and Information, General Electronics, battery charger pcb mcu rtc 5v
Last Edited: Sun. Jul 31, 2016 - 09:04 PM
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Someguy22
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Posted by Someguy22: Sat. Jul 30, 2016 - 12:37 AM
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I am not a power supply engineer by any means but I see nothing but failure with that circuit and what you are trying to achieve. I will leave it to talent greater than mine to say what is wrong. 

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ford2go
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Posted by ford2go: Sun. Jul 31, 2016 - 04:59 AM
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Assuming you have your fet biased right, that will disconnect the batt. But, how are you charging it?

 

If you can take the voltage hit, which you probably can't, you could just connect the batt to the regulator through a diode.

 

I got yelled at for this, so I'll menton it to you. You can  embed pics directly in your post. Folks here like that, and it eliminates the the links to 'jaw dropping cheerleaders'.

 

 

 

 

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ka7ehk
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Posted by ka7ehk: Sun. Jul 31, 2016 - 07:15 AM
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Well, it should sort-of-work. The issue is when the external supply is first connected. You can change the gate voltage at the very start ONLY if the battery voltage also changes. In short, it is NOT a good circuit.

 

For that kind of function, I use a TI TPS2112A. It is called "Auto-switching Power Mux" and it does exactly what you are trying to do, only correctly.

 

Jim

 

Until Black Lives Matter, we do not have "All Lives Matter"!

 

 

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