I am designing a simple lamp dimmer circuit for use with a 450W bulb for the purpose of heat fusing various synthetic materials. I have the basic dimming functional but now I am attempting to measure the actual power used for finer control. To this end I have selected the Allegro ACS71020 (datasheet) to do all of the heavy lifting.
Unfortunately, I am having trouble calculating the proper value for
Rsense as seen on the first page of the datasheet and reproduced below:
1) In general, how should I calculate the voltage at either terminal of
Rsense and, by extension, the inputs to the IC?
I feel like this should fall straight out of Ohm's law however the high impedance input throws me for a loop. The functional block diagram on page 4 of the datasheet indicates that the two sense inputs (VINP and VINN) are buffered. As I understand it, this should mean that practically no current flows which means the voltage (E=IR) is practically zero. Now, in reality, there will be some small leakage current however the datasheet doesn't list it (or calls it by a name I don't understand) so I have difficulty believing it will be significant.
The datasheet does list the maximal input voltage as +/- 275mV. I know that I need to select
Rsense such that the maximal input voltage (115V but call in 130V for safety margin) reads as +275mV. I simply can't figure out what the voltage will be at the other end of the second 1M resistor.
I think that I need to look at all five resistors in a chain and figure the voltage drop across
Rsense for the maximal voltage input, acting as if the high impedance connections are nonexistent. I am not sure of this, however.
2) Why use two separate 1M resistors instead of a single 2M resistor?
I got nothing on this one. According to good old Ohm's law, each of these resistors should drop half of the total voltage in this system but I don't see how that is beneficial to the overall design. Better thermal dissipation? Minimization of skew due to tolerances? The guy who wrote the datasheet has stock in a resistor manufacturing company?