This is a circuit for detecting the grid voltage. I would like to ask how the differential circuit works.

西里古1992

This is a circuit for detecting the grid voltage. I would like to ask how the differential circuit works. [Copy link]

 

Figure 1 shows some connected sampling resistors, and Figure 2 shows the detection circuit. I would like to ask how to determine the resistance values of C17 and R7 in the detection circuit? How to determine the resistance values of a series of sampling resistors in Figure 1? Is this detection circuit practical?

maychang

In the previous picture, it is impossible to see clearly what is in the right half.

In the latter figure, the op amp forms a differential amplifier (actually attenuation, that is, the gain is less than 1).

西里古1992

maychang posted on 2020-4-28 11:24 In the first picture, it is hard to see what is in the right half. In the second picture, the op amp forms a differential amplifier (actually attenuation, that is,...

The one on the right is mosaic processed by me. It is the PFC control chip company document . The red circle is the input end of the second picture, which is the AC input end of the PFC circuit. I want to know how to choose R7 and C17 in the second picture, and what factors are related to it.

maychang

Sirigu1992 posted on 2020-4-28 13:12 The right side is mosaic processed by me, which is the PFC control chip company file. The red circle is the input end of the second picture, which is the AC of the PFC circuit...

R7/R11=R17/R16, this ratio is the gain of the differential amplifier at very low frequencies. In the figure, this ratio is less than 1, so the differential amplifier is actually attenuated.

西里古1992

maychang published on 2020-4-28 13:18 R7/R11=R17/R16, this ratio is the gain of the differential amplifier at very low frequencies. In the figure, this ratio is less than 1, so the differential amplifier actually...

So, C17 and others are just simple capacitor filters, and this circuit is just to attenuate the grid voltage detection signal entering the DSP.

maychang

Sirigu1992 posted on 2020-4-28 13:58 So, C17 and others are just pure capacitor filtering, and this circuit is just to attenuate the grid voltage detection signal entering the DSP

C17 and C24 further reduce the gain of this differential amplifier at higher frequencies.

maychang

Sirigu1992 posted on 2020-4-28 13:58 So, C17 and others are just pure capacitor filtering, and this circuit is just to attenuate the grid voltage detection signal entering the DSP

I don't understand why such a circuit is used. If it is to detect the grid voltage, it may be better to use a voltage transformer, and it can achieve isolation.

西里古1992

maychang posted on 2020-4-28 14:09 I don't understand why such a circuit is used. If it is to detect the grid voltage, it may be better to use a voltage transformer, and it can achieve isolation.

Probably to reduce costs.

chunyang

The original poster's circuit is actually an attenuator. The resistor selection is based on the attenuation ratio. According to the AC peak value, the attenuation is to the maximum range of the ADC, that is, R7 divided by the series resistance of the previous string of resistors, and the lower arm is symmetrical with it. C17 is a low-pass filter, and the capacity requirement is not strict. The specifications in the figure are the most commonly used in the circuit to reduce the component specifications. The advantage of this circuit is that it is cheap and saves the voltage transformer. But the disadvantages are also obvious, and whether it meets the safety regulations must be considered.

西里古1992

chunyang posted on 2020-4-28 14:21 The original poster's circuit is actually an attenuator. The resistor selection is based on the attenuation ratio. According to the AC peak value, it is attenuated to the maximum range of the ADC, that is, R7 divided by the previous...

OK, thanks, I understand.

西里古1992

chunyang posted on 2020-4-28 14:21 The original poster's circuit is actually an attenuator. The resistor selection is based on the attenuation ratio. According to the AC peak value, it is attenuated to the maximum range of the ADC, that is, R7 divided by the previous...

It is the resistance value of a bridge arm of the differential circuit composed of R11 and a group of resistors in the first picture.

西里古1992

chunyang posted on 2020-4-28 14:21 The original poster's circuit is actually an attenuator. The resistor selection is based on the attenuation ratio. According to the AC peak value, it is attenuated to the maximum range of the ADC, that is, R7 divided by the previous...

It means there is a high voltage risk, right?

maychang

Sirigu1992 posted on 2020-4-28 14:11 It should be to reduce costs

Using a small voltage transformer does not necessarily mean high cost. In the previous figure, two levels of common-mode inductors, so many capacitors, and so many resistors are used, and the cost is not low.

In addition, the voltage transformer itself has a low-pass filtering function, which may be better than the low-pass filtering formed by the two capacitors C17 and C24 in the latter figure.

chunyang

Sirigu1992 posted on 2020-4-28 14:43 It is the resistance value of a bridge arm of the differential circuit composed of R11 and a group of resistors in the first picture

Yes, that's the resistance of a bunch of resistors connected in series.

chunyang

Sirigu 1992 posted on 2020-4-28 14:49 It means there is a high pressure risk, right?

This depends on the specific standards of the product.

埋土书生

R7 and C17 have the function of filtering and frequency selection, forming a zero point at the position of f=1/(2pi*R*C). The frequency characteristic is similar to a low-pass filter, which should be used to filter out high-frequency interference and high-order harmonics.