Dummy Load / Digital PSU

A power supply is must have for any electronic project but some times a dummy load is also useful. So, I started to design a power with a dummy load option based on this specs.

Power supply

Vout: 2.5V - 15V

Iout:  0A - 1A

Dummy load

0 - 1A

The power supply is based on the LM317 voltage regulator it goes from a minimum output of 1.2V to 30V at 1.5A, as for the dummy load see the video from  Dave's  he explains how it works.

The below schematic is only the analog circuit and it has two working modes, power supply and adjustable load, this modes are selected by a relay that switches the output pins connections in this configurations

Power supply                Adjustable Load
B+ -> LM317              B+ -> Mosfet
B- -> Mosfet                B- -> GND

The working principle for the voltage ajdjustment is very basic, a pwm signal is applied to the non inverting input of the OA and this one creates a reference voltage on the adj pin of the LM317
For the current adjustment it uses the same principle but is better explained on Dave's video 

For the digital circuit I used an PIC16F73, this mcu is used to generate two pwm signals, read three analog inputs and display information on a spi lcd.
On the software side I kept it simple as the final project is intend to use a an Android tablet instead of the pic and lcd, but for now on power mode it displays Voltage, Current and preset's, in load mode it displays Voltage, Current and Power it also plots a small graphic with the discharge curve.

Power supply mode, charging Li-ion battery (in this image battery is already charged)

Load mode, discharging Li-ion battery @3W

The pcb was designed with eagle and manufactured by OSH Park as this is rev 1 it has some issues like the relay footprint is wrong and diodes footprint is small, on the power dissipation side the heat sink is too small and should use an TO-220 mosfet instead of three so-8 mosfet

project files can be found here