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Overload DAC Mod

Many client want to replace the analog potentiometer on OverLoad to a DAC for current controlling, and Client now show us a successful hack : )

He is using our Over­load electronic load for single cell testing. And he remove the pot and hack it for programmatic cur­rent control. He developed a daughterboard that uses the J1 header I2C signaling and the Microchip MCP4725 12-bit DAC to reach digital controlling.

“24 over­load boards are simultaneously used during modules testing in 2 sets of 12. 12 daughterboards feed serial data to a Zig­Bee radio. See a future post for more info.

Here’s our tar­get measurement 2.45V@3.5A.

Here’s a list of over­load modifications we per­formed.
  • Replaced R6 with .25ohm resistor, allows for higher cur­rent at lower volt­age
  • Removed F1, allows for higher cur­rent
  • Changed R8 to 3.3K, gives more accuracy CH1 volt­age range now 0-4V
  • Modified Arduino serial library to disable TX when not transmitting, allows for pseudo multi-drop communication with­out RS-485 driver
  • Add large proportional fonts to PCD8544 library, easier to read out­side
  • Added piece-wise linear interpolation routines to linear CH0 and CH1 reading, dramatically improving accuracy (less than 0.5% over range of inter­est). 2 50 value calibration tables are stored in AVR 328’s EEPROM * Removed the barrel power connector, powering over bus cabling instead.

Notes: MOS1’s biasing circuit is highly non-linear (the op amp’s virtual node volt­age is rarely 0 volts), which is fine for our application. Our inebriation routines remove these non-linear affects.”

You can find more information about this in rethinksun blog.