October 21, 2013
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So I worked more on the schematic… some changes.
Surprisingly , the hardest thing now is to search for each component and to check if it matches the footprint. This will take more days.
Regarding the case I think I will use a DVD case . the buttons are matching well the function that I want to use.
The button function will be :
[-] [+] [MENU]
[down] [up] [OK]
[ON/OFF ] [ OK ]
I tried also a routing of the board… well it is hard. manual routing is a pain in the ass, . I discovered now Freerouting and I will have to play a little bit with it to see if can give good results.
October 17, 2013
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I just started a new project. It is a lab power supply. The main target is to do it with a microcontroler (PIC) qnd to have good enough accuracy.
My spec is 5mV per step for output voltage and 1mA per step for current. The max voltage is 20.48V and max current is 1.024A.
For this I will have a LCD screen ( 16×2 characters) and the panel will be with push burrons. I do not want to use rotary encoders.
The Lab Power Supply will use a AC-DC converter from 220V AC and will provide also some other fixed voltages like : 5V, 3.3V and maybe others (12V).
For the moment I have two old AC-DC converter from some printers (HP and Canon).
For the case I think I will use a old DVD player case since I have there all the necessary buttons and front panel.
I target to use PIC16F1789 since it is powerfull enough and has many IO pins. The ADC reading will be made with MCP3201 12bit and the DAC wil be MCP4922 dual 12bit DAC.
For the reference I intend to use MCP1541 but here I am not sure. I may also use a trimmable reference since I want to be precise for the 4.096V value.
For the current measurement I wanted to use INA210 or similar from TI but they are not cheap and hard to find. Also TI sample program is a nightmare. So I decided to use a opamp and a PNP to read the high-side current.
The opamp that I want to use is MCP6022 (2 OPA per package) . I need to use this opamp since it needs very low offset and full rail-to-rail input/output.
The other opamps that I have are not well suited for this job (some have bigger offset or/and not rail-to-rail in/out).
The ILIM loop and voltage loop are made fully in analog domain. I do not want to make it in software since may create issues later and also because I have more experience with the analog than PIC software. The other reason is that it is easier to debug and they are modular.
For the voltage divider I use a buffer since the ADC require low inpedance input source and a simple resistor divider will not work well.
I added below the full schematic that I have now. It is still a work in progress so I do not encourage anyone to think that is fully functional or that is tested. It is just a schematic made based on my knowledge.
LabPicSupply version 1 (pdf file)
LabPicSupply BOM list