Electronics > Beginners
2 DACS / One Arduino Nano / Two Independent voltages
Ian.M:
How are you planning to control the 20 Nanos?
If you just plug them in to USB they'll all share the same ground and USB is *MANY* times harder to isolate than I2C.
20 Arduino Pro Mini boards with galvanically isolated RS485 transceivers on the RX and TX pins of their UART interface could work, with either a master one running a keypad and LCD or OLED to control them, or a PC with a USB RS485 interface controlling them.
robert_south1978:
Since this is a test stand, it will be operated without a PC or USB connections to any NANO.
Each NANO will contain the same program, once programmed, NANO will receive isolated 5vdc on the VIN pin.
Each NANO/DAC will output a variable voltage with simple up / down buttons that an operator will control.
I plan on putting 4 NANOs/DAC on one PCB board, wire wrap the connections, and stack 5 boards, each identical.
( note in fuel cell stack all 'CELLS' are connected in series, so Cell 1(-) to Cell 2(+), Cell 2(-) to Cell 3(+), ect ... )
We will verify each Voltage Channel of our fuel cell is recording the correct DAC/Voltage and we can use the DAC/Voltage to test
our control software to trigger errors / shutdowns when voltage levels are out of range. ( again, during dummy testing, no real fuel cell installed )
Thanks again for your comments!
P.S´. Once this test stand has completed it's mission, I can disassemble it and reuse all the components.
HB9EVI:
some thoughts about that project:
- there's always an issue about i2c vs spi - if we are talking about galvanic isolation, spi clearly wins; it's easy to achieve with e.g. 6N137 couplers - as DACs I'd go with the MCP4921, the dual DAC MCP4922 I used for a dual channel PSU, each channel isolated. Well, the only limit with spi of course is the chip select. I don't have in mind with AVR the Nano uses, for 4 CS it should last.
- maybe no need, but as caveat: even though these DACs are r2r, they are never accurate enough to serve as, what somebody would call a 'voltage reference', they neither reach VSS down to the bottom nor VDD. of course you can calibrate it out to a certain point, but it's not comparable to a real voltage reference.
robert_south1978:
Dear HB9EVI
Thanks for your comments.
I have never tried SPI, I will play with it one of these weekends.
You are correct about the percision of the DACS. But the goal of this project is to simulate 20 cell voltages of a fuel cell. To test the wiring of our FuelCell dev. system.
So that voltage on DAC 1 causes voltage increase on our system channel 1, and so on.
We are developing fuel cell technology and I am writing the control software for the fuel cell system. Also, by adjusting the DAC voltage over/under a control threshold for each channel
will allow me to test my control software reactions. So percision is not too much in focus.
Kind Regards, Robert
Ian.M:
20 separate Nanos with manual controls is going to be a real PITA to use. Seriously look at including isolated comms to a PC so you can run a script on the PC to set up all the simulated cell voltages before a test run and sequence changes to cell voltages during a run repeatably.
Personally, I'm with HB9EVI on this - its easiest to do the isolation between the Arduino and SPI DACs, as then the Arduino can use its standard USB UART interface for PC comms. The 20 SS (/CS) outputs is easy - simply use three 74HC138 3 to 8 line demultiplexors (+ a few inverters for some of the EN inputs) to implement a 5 to 24 line demultiplexor, with SS from the Arduino connected to /EN, then select which DAC you wish to address by outputting a binary number on five I/O pins. The 74HC138 chips could be distributed over your DAC boards - put one on each (five in total), with a 74HC86 quad XOR on each so you could set the board address with a DIP switch.
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