Hi
I'm using an LM311N comparator to switch a relay. I have a single rail +12V supply. I want to use it in a non-inverting configuration, so I have programmed a reference voltage at the inverting input of 50mV (240k and 1k potential divider from the +12V supply). The inverting input is connected (via 1k resistor) to a sense resistor (0.068ohms) in the ground return path of a load circuit. My idea is that the comparator will activate the relay (connected between +12V supply and the comparator output) when the sense resistor current is greater than 0.75A or so. My thinking is that current > 0.75A will cause the inverting input to go > 50mV and drive comparator output low, turning on the relay.
But all attempts fail - the comparator output stays stubbornly high.
Is it not working because I am too close to the negative supply rail (i.e. 0V)?
I realise there are special purpose current sensing comparator devices, but I just wanted to mock up a prototype initially using some 311s I had to hand.
I have tried adding hysteresis (580k between output and non-inverting input), no effect.
Tony
