Sensing AC voltage

[ploutone]

In a project I'm working on (Totem pole PFC) I need to sense the input AC voltage, scale it down, rectifing it/adding an offset and then send to the ADC of the digital controller to use in the control loop (average current control)

I tried a number of circuit, such as a voltage sensing transucer (LEM LV-25), but it has 3 problems:it's expensive, it's slow (<10kHz BW) and it has accuracy problems as the large power resistors heat up.

So I thought about using a differential amplifier and detect the line voltage between L and N, set ground to half ADC reference, i.e. 1.25V and have the output voltage equals 1.25 + k Vac, where k is the differential amp gain

This circuit :



Is this circuit going to work properly? how can I add isolation? and for the ground for (V-) and 1.25V voltage reference, will it be the GND of the MCU or the totem pole? sorry for the silly questions.

Thank you very much.

[fcb]

100K on the inputs to the opamp - probably take that up by a factor of 10x to 50x if you're sensing mains.

Also, be careful where you measure it from - you'll get different results if you measure after your front end noise suppression.

[fourtytwo42]

In a project I'm working on (Totem pole PFC) I need to sense the input AC voltage, scale it down, rectifing it/adding an offset and then send to the ADC of the digital controller to use in the control loop (average current control)

I tried a number of circuit, such as a voltage sensing transucer (LEM LV-25), but it has 3 problems:it's expensive, it's slow (<10kHz BW) and it has accuracy problems as the large power resistors heat up.

So I thought about using a differential amplifier and detect the line voltage between L and N, set ground to half ADC reference, i.e. 1.25V and have the output voltage equals 1.25 + k Vac, where k is the differential amp gain

This circuit :



Is this circuit going to work properly? how can I add isolation? and for the ground for (V-) and 1.25V voltage reference, will it be the GND of the MCU or the totem pole? sorry for the silly questions.

Thank you very much.

This system works and is used lots but you need to add some things in your circuit. Firstly the breakdown voltage of the resistors, for standard 250mW wire ended its common to use 3 resistors in series, so that's 6 in total. If you are going to implement it in smt then a much bigger problem. Secondly the power dissipation (helped by multiple resistors) and finally the current injection into your reference node. This also means you want quite high values (typically at least 1000K per leg) and very high tolerance to achieve the maximum common mode rejection ratio.

[ploutone]

This system works and is used lots but you need to add some things in your circuit. Firstly the breakdown voltage of the resistors, for standard 250mW wire ended its common to use 3 resistors in series, so that's 6 in total. If you are going to implement it in smt then a much bigger problem. Secondly the power dissipation (helped by multiple resistors) and finally the current injection into your reference node. This also means you want quite high values (typically at least 1000K per leg) and very high tolerance to achieve the maximum common mode rejection ratio.

Thank you for your help,
for the isolation and the GND, I'm using two grounds, one for the MCU and one for the totem pole PFC, which ones I need to use for the op amp?

[fourtytwo42]

This system works and is used lots but you need to add some things in your circuit. Firstly the breakdown voltage of the resistors, for standard 250mW wire ended its common to use 3 resistors in series, so that's 6 in total. If you are going to implement it in smt then a much bigger problem. Secondly the power dissipation (helped by multiple resistors) and finally the current injection into your reference node. This also means you want quite high values (typically at least 1000K per leg) and very high tolerance to achieve the maximum common mode rejection ratio.

Thank you for your help,
for the isolation and the GND, I'm using two grounds, one for the MCU and one for the totem pole PFC, which ones I need to use for the op amp?

Thats a slightly more complicated question. Normally a circuit measuring voltage in this way would have the ground leg of its power supply capacitevly coupled to earth or the common of the grid supply, the latter having Y capacitors that capacitevly couple it to earth. Add a VDR or two normally found in the input to a power supply and you have a coupling system that will limit the common mode voltage seen by the op-amp. In other words you have to consider the whole system, not just your op-amp circuit in isolation.