For our domotic projects we have the need of an eclectic power supply device.
- It should take the input from a PC USB port .
- It should have 5 V output.
- It should have 3,3 V output.
- It should be transportable.
- It should defend the USB port. O_o
So we have designed such a device.
The power supply device is very simple and at the same time it is very versatile.
It uses a Microchip regulator which generates 3,3 starting from 5V (from the USB port), It also uses a PPTC (POLYSWITCH .20A RESET FUSE SMD – MINISMDC020F-2 ) to protect the USB port from absorption above 250 mA.
A PPTC is a passive electronic device (polymeric positive temperature coefficient), commonly known as resettable fuse, it is normally used to protect electronic circuits from over currents faults. As soon as the maximum absorption of the PPTC (400 mA in this case) is overcome, the device disconnects the circuit from the USB, bringing it back to the maximum allowed (250 mA), thus saving the PC's USB. Below the PPTC, the user has the 5-volt to 3.3 volt. The 3,3 volts is generated through an LDO (low dropout) produced by Microchip (MCP1700). The regulator accepts input voltages up to 6 volts.
There are also a led (to see that the circuit is working properly) and some capacitors that are needed from the mcp1700.
Last but not least there is a diode to prevent the return of currents to the PC's USB.
The circuit has been designed to use USB-A-H or USB-PH connectors:
Here you are the schematic:
And here you are some rendered devices:
And now the Bill Of Materials
| C1 | 1u | Ceramico 1206 |
| C2 | 10u/16V | Elettrolitico (2.5) |
| C3 | 1u | Ceramico 1206 |
| D1 | diodo | schottky SOD123 |
| IC1 | MCP1700 | MCP1700 TO93 |
| J1 | pin | Header (2 pins) |
| J2 | pin | header (2 pins) |
| L1 | LED3MM | |
| P | MINISMDC020F-2 | PTC 1812 |
| R1 | 3.3K | Resistenza (1/8 Watt) |
| X1 | connettore | USB |
Gg1