How a voltage sensing circuit works

[ploutone]

Hello;

In a project I am working on (totem pole PFC), I need to sense the input AC voltage and for that I am thinking of using LEM LV 25-P, but I can't figure out how the circuit exacly works ( especially the highlighted red part of the circuit ) and how is the ratio equals to  2.5*RM/(R1+R2)



Thank you very much.

[Phoenix]

The module is a really a current -> current converter. You use resistors to convert from voltage to current and back to voltage.

The actual internals are a feedback compensated hall effect sensor. It detects the magnetic field in the transformer and injects an opposite current into the secondary with the aim of zero flux in the transformer. It's kind of like a current transformer, but able to operate at DC current too.

E.g. if you inject 10mA into the primary, it will output 25mA on the secondary.

[tatus1969]

googling for "closed loop hall current transducer" leads you to http://www.lem.com/hq/en/content/view/168/155/, among others.

For your application, the datasheet figures are more important. Can you live with 0.9% accuracy, 2.5kV isolation, 40µs step response (corresponding to roughly 9kHz bandwidth)? If yes, then for every mA you feed in, it produces 2.5mA at its output. Use resistors at input and output to convert voltage to current and vice versa.