Wanted: 3.7v regulator + backfeed protection circuit.
AwesomeCronk
Posts: 1,055
in Accessories
I am including an Adafruit Feather on a PCB with a propeller several propellers.
Adafruit Feather has two power options: USB or 3.7v/4.2v, 500mAh LiPoly battery.
I would like a technique for powering the Feather that does not involve stumping a bunch of USB cables and soldering about two inches of wire to the board. Or gluing a battery to the board. I notice in the PCB schematic for the Feather that the BAT pin is connected directly to the positive terminal for their battery.
My idea: Use a 3.7v or 4.2v regulator to power the Feather via the BAT pin. Include a feedback protection circuit. Maybe a brownout detector holding a transistor closed(current going through), listening to the line and disconnecting the regulator when more than 3.7v is detected?
Adafruit Feather has two power options: USB or 3.7v/4.2v, 500mAh LiPoly battery.
I would like a technique for powering the Feather that does not involve stumping a bunch of USB cables and soldering about two inches of wire to the board. Or gluing a battery to the board. I notice in the PCB schematic for the Feather that the BAT pin is connected directly to the positive terminal for their battery.
My idea: Use a 3.7v or 4.2v regulator to power the Feather via the BAT pin. Include a feedback protection circuit. Maybe a brownout detector holding a transistor closed(current going through), listening to the line and disconnecting the regulator when more than 3.7v is detected?
Comments
Wouldn't your "main" power supply need to be connected to the 120VAC wiring either directly or via a wall plug? If you want it to be in the same enclosure as the feather that can be done, but it will need some form of input power or charging if it is a battery. We are not at the stage where we have a "main power supply" that can provide power for unlimited periods of time without some form of input energy.
What I do in cases like this is to purchase a 120VAC in module that outputs the desired DC voltage and current, mount it in the enclosure, wire it to the board, and add a 120VAC cord to connect the power supply to the wall plug. For prototyping I may break open a USB wall wart and use the board and cable to power my project board. Thrift stores have wall warts by the dozens if not hundreds so there is no problem finding one for the desired output voltage.
Then for the sake of safety you should use an approved wall wart or other modular encapsulated power supply that does not have 120VAC exposed.
This was the idea. Power the board off this dude. Convert 12vdc down to 3.7vdc. Likely use boost converter/buck converter, such as to not lose amperage. Use another converter to power props, sd cards, USB ports, EEPROMs, and XBee.
That's a reasonable approach, and I have used it on a few occasions where I have needed multiple voltages. Start with a switcher for the highest and use switchers or linear regulators for the lower ones as needed. Typically that would be 12V for relays or solenoid valves, 5V and 3.3V for electronics, and occasionally another for sensors or analog circuitry. Just make sure the primary supply can handle the total current load.
If you are not using a battery you don't need a charging circuit, so no protection required. Since the battery voltage/current also comes to the input of the 3.3V regulator via Q3 the only way the charging voltage could damage the rest of the circuit would be if the regulator shorted out. Very little chance of that happening with the protection circuitry built in to modern regulator IC's.
If you do want a battery the charging voltage/current comes from the VBUS, and so does the voltage/current input to the 3.3V regulator. That means that you will probably need to provide 4.8-5V if you use a 3.7V battery.
Personally, I would use a 5V 5A supply (typical Raspi supply) to power this unless I needed the 12volts for something. That would eliminate the intermediate 12V to 5V supply. The simpler you can make something the more reliable it is.
I can’t change the charging circuit, but if I can’t make a protection circuit between the module and my circuits, then I may need to look into using a different module.
https://www.mouser.com/Texas-Instruments/Semiconductors/Power-Management-ICs/Voltage-Regulators-Voltage-Controllers/LDO-Voltage-Regulators/_/N-5cgac?P=1z0zls6Z1z0wd6tZ1z0shv5&Keyword=reverse&FS=True
if you need 3.7, this not so common one
https://www.mouser.com/ProductDetail/ABLIC/S-13R1B37-M5T1U3?qs=/ha2pyFaduiC3a3erjhtvwwiW2HQdL4EoA%2BUiQUiLTo=
Discharge shunt function, maybe be good or bad depending of your dual supply.