I am switching a 230-240 V load with a TRIAC and relay while also monitoring the presence of voltage on both ends of the load (i.e. not measuring the amplitude -- just that the switch successfully opened/closed). Previously, I had a positive power supply referenced to L2, and life was peachy.
(The 'MON' signals drive a small P-FET that switches optoisolator LEDs.)
This is when the TRIAC -- SW1 -- was optically isolated from the drive circuit. Now, however, I'd like to move to a hot-side microcontroller to control SW1 directly, which means converting to a negative supply in order to (easily) drive the TRIAC in quadrants II and III.
This presents a challenge since the 5 V source will now have to sink current if the existing monitoring circuits are used. This source is a IRM-03-5 that is not rated to sink current like this, and simulations show the monitor circuit behavior breaking down if an inline diode is placed at its output.
Is there a simple circuit that would accomplish this presence monitoring given the negative supply? Its output isn't too important -- either a pulse train, high, or low -- as long as it is differentiated between the phase being on or off.
Note: I also found an ST app note about driving TRIACs in QII and QIII with a positive supply, but would like to exhaust my options with a negative supply first.