OpenSprinkler Forums OpenSprinkler Unified Firmware Couple of questions Reply To: Couple of questions


Csaba Toth

When you use one OpAmp for current sensing there is no amplification. Assuming 3 valves the output voltage range is around 0,75A * 0,2ohm = 150mV. Depending on the offset it could be worse. The LSB of ADC is 3,2mV using the 3.3V supply as reference. Full scale swing is 50 LSB for 3 valves, 16 LSB for one valve. This still could be enough for current fault detection, but the resolution is quite low.
In my solution there is an amplification, to make sure the wider swing, the resolution for one valve is 137 LSB what definitely enough for valve diagnostics.
As most OpAmps are dual version, the 2 OpAmp solution has no cost increase, and LM358 vs MCP602 doesn’t make a serious difference.
This circuit was not working well using LM358 due to the limited output swing.
For protection I like the 5V TVSS solution vs. the the BAV199 diodes.