I am trying to make a circuit to measure the resistance of a section of soil, using an H-bridge circuit to reverse the direction of current in order to prevent electrolysis and a differential ADC to measure the voltages of a shunt resistor and the probes in the soil.
When using the H-bridge the measured voltage and resistance change between directions, although for each direction the measurements are consistent. I don't have an issue with the voltages being different (differences in transistor values etc.) but the ratio of the shunt to test resistor voltages are noticeably different, often producing differences of 10 to 500 ohms.
The difference is magnified the bigger the measured resistor (eg a 470 ohm resistor generally measures as 470 with about .2 ohms difference between flips, but a 4.7k ohm resistor will vary by three to five ohms.
When measuring the same resistors in a circuit without transistors, the voltage is generally consistent, even when it doesn't match the expected values. I have tried an h-bridge made from separate transistors, an H bridge chip and a maker drive H-bridge board, and although the maker driver board initially seem to produce consistent results, it has since shown the same problems as the other set-ups.
Is there anything that would cause an inconsistency like this, and is there any way that I can fix it?
When using the H-bridge the measured voltage and resistance change between directions, although for each direction the measurements are consistent. I don't have an issue with the voltages being different (differences in transistor values etc.) but the ratio of the shunt to test resistor voltages are noticeably different, often producing differences of 10 to 500 ohms.
The difference is magnified the bigger the measured resistor (eg a 470 ohm resistor generally measures as 470 with about .2 ohms difference between flips, but a 4.7k ohm resistor will vary by three to five ohms.
When measuring the same resistors in a circuit without transistors, the voltage is generally consistent, even when it doesn't match the expected values. I have tried an h-bridge made from separate transistors, an H bridge chip and a maker drive H-bridge board, and although the maker driver board initially seem to produce consistent results, it has since shown the same problems as the other set-ups.
Is there anything that would cause an inconsistency like this, and is there any way that I can fix it?
Statistics: Posted by TTSB — Fri Apr 26, 2024 10:00 pm — Replies 0 — Views 24