I am working on a small 1-12 vdc, 1 amp max power supply containing two LM317 regulators (one in current limit configuration and the other for voltage set and regulation). Power to my power supply is floating 24 vdc from a bridge rectifier. I saw a design that accomplished the current limit function with an op amp circuit thus eliminating the need for the "current limit" LM317. The current sensing portion of the circuit works by placing a 1 ohm resistor (appropriate wattage) into the circuit prior to the LM317 regulators. Input signals to a differential op amp are obtained through voltage dividers placed on the incoming and outgoing current ends of the resistor. The op amp I am using required a split supply (+ and -) since it would not allow op amp outputs (nominally 0 to 1 volt) close enough to the grounded rail in a single supply.configuration. This was accomplished using 4 9 volt batteries with center as ground. I breadboarded this current sensing portion of the op amp circuit and interfaced it with my previously described power supply circuit. This of course required joining the commons for both circuits. When I did this I noticed the output of the differential amplifier was -230 mv. The circuit worked as expected in terms of sensing current via a correlating voltage at the differential op amp output except for having to mentally add 230mv to the value I was reading on my DMM to account for the -230mv offset. Through trial and error I was finally able to reduce the offset to +5 mv, or thereabouts, by adding a 100 ohm resistor between the two floating supply commons at the point where they joined.
Obviously the two floating supply "commons" were at sightly different potentials. How should I have recognized this would be a problem before I interfaced the circuits? Why was the differential mode op amp output affected by this?
Obviously the two floating supply "commons" were at sightly different potentials. How should I have recognized this would be a problem before I interfaced the circuits? Why was the differential mode op amp output affected by this?