Electrical – Multimeter shows different voltages for positive and negative voltage(interchanging multimeter probes) for same source


I have designed a 5v and a 3.3v regulator getting same source of 12v battery.
When I measure, the 5v regulator output shows -4.67v for + on black probe and - on red probe, and 5.12v when red is connected to + and black to -.

Similarly for 3.3v regulator…

Now I have an Arduino project and it includes a flow sensor. This project works fine if the serial port is connected to a laptop. But if I only connect an external power supply as described above, the sensor gives some output even if there is no flow…..

[The second problem involving a flow sensor has now been asked in a separate question here.]

Best Answer

Different positive and negative readings on a dual-slope ADC can be caused by the maker using a crummy integrating capacitor with a large amount of dielectric absorption. It's also possible the battery is dying and the integrator is saturating on one side but not the other.

Your other issue sounds like a grounding problem.