How to process the weak AC signal collected by the voltage transformer and the amplified signal after being amplified and biased by the operational amplifier?

323cc2

How to process the weak AC signal collected by the voltage transformer and the amplified signal after being amplified and biased by the operational amplifier? [Copy link]

 

The purpose of this circuit is to convert AC power into analog quantities that can be collected by the microcontroller. So should I add an addition circuit to the amplified signal or use precision rectification for conversion?

It would be best to get a smoother DC signal. How should I do this? Is there any expert who can give me some advice?

323cc2

Attached circuit diagram

323cc2

Is there anyone who can save the child? I don't understand.

maychang

323cc2 posted on 2023-8-30 14:09 Attached circuit diagram

Your circuit does not have a bias applied, but uses a single power supply, so it does not work. This circuit needs to be modified.

323cc2

maychang posted on 2023-8-30 14:28 Your circuit does not have a bias, but uses a single power supply, so it does not work. This circuit needs to be modified.

Can't the bias effect be achieved by using two resistors to divide the voltage?

maychang

[The purpose of this circuit is to convert the mains electricity into analog quantities that can be collected by the microcontroller]

The microcontroller uses a single power supply, so the analog value that the microcontroller can collect must be between GND and the microcontroller power supply voltage. The output of the transformer is AC. If the output of the transformer is amplified and sent to the microcontroller, and a certain DC bias is added, then the complete AC waveform is sent to the microcontroller. If the output of the transformer is precisely rectified and then sent to the microcontroller, then the absolute value of the AC waveform is sent to the microcontroller. Both methods are acceptable, depending on what kind of waveform you need.

maychang

323cc2 posted on 2023-8-30 14:09 Attached circuit diagram

What's the frequency of the primary signal of your transformer?

The output of the op amp in this circuit is connected to ground with a capacitor, which is useless and very likely to cause parasitic oscillation.

A capacitor is connected in series on the secondary side of the transformer, but this capacitor is also useless.

maychang

323cc2 posted on 2023-8-30 14:09 Attached circuit diagram

To add bias to this circuit, the op amp's non-inverting input and inverting input should be swapped to form a non-inverting amplifier. The bias is added to the op amp's inverting input.

maychang

323cc2 posted on 2023-8-30 14:30 Can't the bias effect be achieved by using two resistors to divide the voltage?

To add bias, two resistors are usually used to divide the power supply voltage. However, the power supply voltage is often not stable enough, and if the requirements are high, a standard voltage source may be required.

maychang

The output amplitude of a voltage transformer is usually not very small. Since the amplitude is not small, there is no need to amplify it.

What is the output amplitude of your voltage transformer?

maychang

323cc2 posted on 2023-8-30 14:09 Attached circuit diagram

Sorry, I misread it. The schematic diagram already has a DC bias, which is formed by two resistors dividing the voltage. The reply on the 4th post is invalid.

323cc2

maychang posted on 2023-8-30 14:36 What is the high frequency signal of your transformer primary? The output end of the op amp in this circuit is connected to a capacitor to the ground. This capacitor is useless and extremely easy to...

OK, thank you for your guidance. The primary of the transformer inputs 220V, and the transformation ratio is 1000:1000, 2ma:2ma. I want to get a signal that can change with the primary voltage of the transformer, and the output end will also change. It is best to have a DC signal. The advantage of this is that it is easy to process in the software.

323cc2

maychang posted on 2023-8-30 14:36 What is the high frequency signal of your transformer primary? The output end of the op amp in this circuit is connected to a capacitor to the ground. This capacitor is useless and extremely easy to...

Because I have a poor foundation, I made the first version of the circuit after referring to the scattered information on the Internet. Please forgive me. I hope the teacher can give me some advice.

maychang

323cc2 posted on 2023-8-30 16:57 OK, thank you for your advice. The transformer primary input is 220V, the ratio is 1000:1000, 2ma:2ma, I want to get a transformer that can follow the transformer primary voltage change...

[I want to get a signal that can change with the primary voltage of the transformer, and the output will also change. It is best to have a DC signal. The advantage of this is that it is easy to process in the software. ]

The question is: when outputting a DC signal, should this DC signal be proportional to the peak value of the AC signal, or should it be proportional to the effective value of the AC signal?

If it is proportional to the peak value, it is more convenient to use precision rectification, and the microcontroller software almost does not need to consider processing the input signal. If it is proportional to the effective value, the more convenient way is to let the microcontroller process it (collect instantaneous value, square it, average it, and take the square root), and then output it through DA. Of course, it is more reasonable to use an amplifier with bias.

So I said, 'see what kind of waveform you need''.

maychang

323cc2 posted on 2023-8-30 16:57 OK, thank you for your advice. The transformer primary input is 220V, the ratio is 1000:1000, 2ma:2ma, I want to get a transformer that can follow the transformer primary voltage change...

[The primary input of the transformer is 220V, the ratio is 1000:1000, 2ma:2ma]

【ma】 should be 【mA】.

For such a transformer, the output amplitude can reach 2V without any problem. It does not need to be amplified, and the voltage can be followed by adding bias. With such a low frequency, precision rectification is also easy.

323cc2

maychang posted on 2023-8-30 18:49 [The transformer primary input is 220V, the ratio is 1000:1000, 2ma:2ma] [ma] should be [mA]. Such a transformer has an output amplitude of 2 ...

Is this okay, teacher?

maychang

323cc2 posted on 2023-8-31 10:21 Is this okay, teacher?

This circuit uses a single power supply and has no DC bias, so it cannot work properly.

323cc2

maychang posted on 2023-8-30 18:49 [The transformer primary input is 220V, the ratio is 1000:1000, 2ma:2ma] [ma] should be [mA]. Such a transformer has an output amplitude of 2 ...

It suddenly occurred to me that if the output amplitude reaches 2V, can we directly use a diode for rectification?

maychang

323cc2 posted on 2023-8-31 10:28 I suddenly thought that if the output amplitude reaches 2V, can we directly use diode rectification?

If the diode is used directly for rectification, it is not "precise" and the output voltage is one or two diode drops less than the input. As a signal measurement, if the error of the diode voltage drop can be tolerated, then it is OK, otherwise it is not OK.

The voltage drop of the diode is close to a fixed value, so the error caused by the voltage drop when inputting high voltage can be ignored. For example, the error of direct rectification of 220V AC mains is only one or two thousandths. However, the error of 2V AC voltage is very large.

323cc2

maychang posted on 2023-8-31 10:27 This circuit uses a single power supply and no DC bias is added, so it cannot work properly.

I modified it again, do you think it's ok? I'm confused about how to connect the other end of the transformer output, how to connect it, directly to the ground or to the non-inverting end