Hey everyone.
I was asked by a friend to design a low cost digital controlled power supply where one could apply pre-sets etc to voltage. Now although it is easy to add the code (most is already there) I decided to keep the current measurement off this plan for simplicity. This power supply has two outputs and is designed to be cheap, easy to build and fully Arduino Controlled.
Specification for PSU.
- 1.25 to 5.00v Adjustable via digital control.
- 1 amp current available per channel.
- Use Cheap parts that are easily available.
- Have some way of displaying voltage.
Now my original design shown below in simulation mode was a little expensive as it was using a larger LCD etc so I decided to use the cheapest LCD that I can find, the 16x2 HD44780 LCD.
Above circuit includes 20x4 LCD but this was a little expensive for my friends taste so instead we opted for the 16x2 as can be seen in below diagram.
The way this setup works is that the Arduino will control the 2 x I2C digital to analogue convertors via the same two lines as each DAC will be addressed differently using the A0 pin to preset address.
These DAC’s will output between 0 and 5 volts depending on the value that you issue to them from 0 to 4095. 0 being 0v and 4095 being 5.0v.
Now the LM317 or 338’s are well known and easily available. The LM317 used in the above has a minimum preset voltage of 1.25v. This can be overcome by apply a negative voltage but this is not really needed for a cheap PSU. 1.25v is pretty much low enough for most experiments. Now when you apply a voltage to the control pin of the LM317 it will output that same control voltage + 1.25v (being the internal reference voltage of the regulator). So if you tell the DAC to output 0v the LM317 will output 1.25v. If however we tell the DAC to output 2v then the LM317 by its design will output 2+1.25v = 3.25v. Easy to understand.
Now in our design above you will notice that the supply can technically go above 5.00v. In fact in this configuration it can achieve 5+1.25v = 6.25v. However this voltage is not really of any use and I have decided to code the control software to limit to 5.00v. You can however add voltage dividers and add the extra 1.25v if you like. It just means two more resistors and some time to change the formula. Most of the code is already there you just have to bring it into the math.
I have also in software pre-programmed the PSU to start with the two common voltages used by Arduino experimenters and digital people.
The relays are there to keep things in check. The DAC runs at around 2.5volts when it has no input control signal and for this reason I have designed the Micro (Arduino) to keep the output turned off until it takes control of the output regulators via the DAC’s.
Next week I will breadboard this design and do a video on this PSU and how to get yours up and running.
Keep yours eyes on www.evione.co.za to see the follow up video.
Cheers for now
Mad Scientist.
