"Maybe use a Galvanically Isolated Current Sensor IC at the front end like ones from Allegro.
The MCU reads the output of sensor and determines the state of the connection based on the expected voltage drop across the sensor."

If you use a current sensor, you could use an MCU pin to read the sensor output voltage.
If the sensor output voltage is above VCC/2 then its DC. if it is AC, it will be biased at VCC/2. You could use the MCU to calculate the AC value.

 

"Maybe use a Galvanically Isolated Current Sensor IC at the front end like ones from Allegro.
The MCU reads the output of sensor and determines the state of the connection based on the expected voltage drop across the sensor."

If you use a current sensor, you could use an MCU pin to read the sensor output voltage.
If the sensor output voltage is above VCC/2 then its DC. if it is AC, it will be biased at VCC/2. You could use the MCU to calculate the AC value.

In theory. My experience with those sensors is they are very noisy and to get a reading off the 330vDC or 120vAC you'd need to pull a fair few mA which means a big dropper resistor.. The smallest of those devices is, from memory, 5A, so 100mA is going to be 50mV output and on 330v that's a 50W resistor....!

 

In theory. My experience with those sensors is they are very noisy and to get a reading off the 330vDC or 120vAC you'd need to pull a fair few mA which means a big dropper resistor.. The smallest of those devices is, from memory, 5A, so 100mA is going to be 50mV output and on 330v that's a 50W resistor....!

they’ve come a long way and should be explored..

 

they’ve come a long way and should be explored..

I've used them a lot - including the latest hall-effect external bus monitor @500A. I'm open to being persuaded, but IMHO there's nothing in Allegro's product range that will work here. Maybe from another vendor?

 

I've used them a lot - including the latest hall-effect external bus monitor @500A. I'm open to being persuaded, but IMHO there's nothing in Allegro's product range that will work here. Maybe from another vendor?

This one claims 10mA(or lower) to 5A range

https://learn.sparkfun.com/tutorials/current-sensor-breakout-acs723-hookup-guide

 

This one claims 10mA(or lower) to 5A range

https://learn.sparkfun.com/tutorials/current-sensor-breakout-acs723-hookup-guide

As always, the devil is in the details. Sparkfun's blurb says: " The amplified breakout board (Low Current) is capable of sensing very small currents down to around 10mA and large currents up to 5A! However, since the output is analog, your usable readings will be limited by noise and the resolution of the ADC reading the output. This sensor is not recommended for current sensing lower than 10's of milliamps. "

Lets look at the datasheet. The 5A part has a basic sensitivity of 400mV/A. So 100mA is 40mV as I previously mentioned.


But that's not the whole story. As I said previously, and the blurb above implied, noise is a factor. Further down the datasheet we read:



So realistically our minimum sensed current, to ensure we are out of the noise floor, is realistically 50 - 100mA on the narrower bandwidth. If thats 50mA @ 120v AC (168v pk) then its 100mA at 330v DC assuming a common resistor and thats 33W! Not practical.

 

You've heard the saying "knows enough to be dangerous", with my limited experience with HV, that's probably me ... literally!
Below shows @Irving 's apparent experience with such circuits:

Unfortunately some of the requirements have changed and now the parts count on my next circuit attempt probably exceeds his.

Since the GND of the device I'm connecting to, with a simple transformered linear supply, is not connected to Earth GND, I felt comparing AC lines would be better. My problem was that Hot and Neutral kept finding their way back to the MCU or even to each other. So I came up with this triac scheme for isolation but if there's an issue or better way of determining Hot or Neutral, please let me know. Also does the SSR (instead of a relay) or the fuse add any additional protection?