Non-isolated 'mains' voltage acquisition circuit

bigbat

Non-isolated 'mains' voltage acquisition circuit [Copy link]

Please see this schematic diagram for measuring 'mains' voltage found on the Internet.

There are four series resistors from Lin and Nin. R6, R7, R8, R9 and R11, R12, R13, R15, where:

1. Why do we use three resistors in series, R6+R7+R8, instead of a large resistor with similar resistance value?

2. It doesn't say what special materials these three resistors are made of? Do they require special matching solutions?

3. C8 and C12 are filter capacitors, and the selected value is 102. This is an integral filter circuit. Will it affect the measurement?

4. How does IC1 output the differential voltage of L and N?

maychang

"Why do we use three resistors in series for R6+R7+R8? Instead of using a large resistor with a similar resistance value?"

The maximum voltage that a resistor can withstand is limited not only by the power dissipated by the resistor, but also by the resistor structure. Usually, a quarter-watt metal film resistor can withstand a maximum voltage of 200V, and metal film resistors with lower power can withstand even lower voltages (different manufacturers have different regulations). This is because when the voltage across the resistor reaches a certain value, sparks may be generated inside the resistor, causing damage.

To this end, when it is necessary to withstand a larger voltage, multiple resistors are often connected in series so that the voltage borne by each resistor does not exceed the allowable value.

qwqwqw2088

The three resistors in series are mainly considered in the selection of resistors. According to the selection of unused short circuits on the PCB, the actual PCB design considers the selection of resistors.

人去搂空

bigbat

In differential circuits, I have seen seniors use multiple resistors (four) in series to form a precisely matched circuit, but I don’t think this circuit needs to be matched?

topwon

The purpose of connecting multiple resistors in series should be, as Maychang said, voltage division. This is also common in the design of power meter solutions, added between the hot and the zero terminals for sampling, because everyone will choose resistors such as 0603 packages to design.

The RC constant formed by the 1000pF capacitor and three series resistors should be in the ms range, which should be an order of magnitude different from the frequency of the AC power.

Double_salt

What are the locations of C8 and C12?

maychang

Double_salt posted on 2019-11-5 08:51 What is the land of C8 and C12?

"Excuse me, what are the lands of C8 and C12?"

It is the "ground" of the power supply for the op amp.

bigbat

Double_salt posted on 2019-11-5 08:51 What is the land of C8 and C12?

The circuit has only two grounds, an analog ground and a digital ground, GAND and DGND. GAND and DGND are isolated by resistors or something else.