maychang

hello_mcu posted on 2023-3-24 23:26 Thank you for your answer. I filter out all 50hz and below. In this application, the input AC 220V voltage signal enters the isolation transformer, and the transformer outputs...

Finally, you plan to use a capacitor, but this capacitor is installed on the power transformer. Whether the power department will allow you to touch the power transformer, especially the primary wire of the power transformer, is a question.

maychang

hello_mcu posted on 2023-3-24 23:26 Thank you for your answer. I filter out all 50hz and below. In this application, the input AC 220V voltage signal enters the isolation transformer, and the transformer outputs...

As for [I filter out all frequencies below 50hz], it is not necessary at all. Please refer to Chunyang's reply on the 9th floor.

Gen_X

Gen_X published on 2023-3-23 16:37 Calculate the following yourself: Based on the frequency F and the estimated capacitance C, it is correct to calculate the impedance of this capacitor. If it is close to 0.1-10 ohms, congratulations, this...

Generally, the carrier frequency is above 100kHz. For the same capacitance, the higher the frequency, the smaller the impedance:

The capacitance corresponding to 1 ohm impedance is calculated below: C = 1/(2ΠFRc) = 1.59uF

Therefore, it is recommended to use 1-1.5uF/1000AC capacitor.

If 1uF is used, how much will the resistance to 50Hz be: Rc = 1/(2ΠFC) = 3.18K

3.18k ohms is indeed large enough for the leakage of a 220V loop, so it is recommended that your carrier frequency should be higher and the capacitor should be smaller.

If the frequency is indeed 100K, it is recommended to use a capacitor between 0.01-0.022uF, that is:

The high-frequency impedance is around 100 ohms, and the 50Hz impedance is above 318k, which is basically sufficient for use.

Please analyze and select according to the above principles based on actual conditions and frequency range.

In order to prevent LN reverse connection, it is generally recommended to use two capacitors for the carrier, one on each incoming line, to increase safety and isolation.

吾妻思萌

The carrier bandpass filter, if there are other harmonics, just filter them out together

chunyang

hello_mcu posted on 2023-3-24 23:16 Hello, thank you for your answer. My requirement is that the carrier meter reading needs to be across the transformer (isolated), so I want to use the coupling capacitor to make the carrier...

Transformers are originally coupled with small capacitors, so there is no "short circuit" here, unless the capacitance is large enough, which will be a value you can't imagine! It seems that you are still very vague about the concept of capacitance. Think carefully about the concept of "capacitive reactance" and think about the relevant content in the circuit principles learned in college courses (mechanical majors learn electrical engineering, and both have the same content in this part, but to different degrees). Even if you don't remember it, you can still draw the correct conclusion by using Ohm's law learned in junior high school.

hello_mcu

Gen_X posted on 2023-3-25 15:35 Generally, the frequency of the carrier wave is above 100kHz. For the same capacitance, the higher the frequency, the smaller the impedance: The following is the calculation of the capacitance corresponding to 1 ohm impedance: C=1 ...

Thanks for your answer. I tried it, using 104, 0.1uF capacitor, the broadband carrier frequency is 2M-12MHz, I tested it, and no problem was found, the carrier communication was normal, and no abnormal sound or abnormality was found in other circuits.

hello_mcu

chunyang posted on 2023-3-27 12:02 The transformer is originally coupled with a small capacitor, and there is no "short circuit" here, unless the capacitance is large enough, which will be what you imagine...

Thanks for your answer. I really don't understand the characteristics and principles of the device, or even know nothing about it.

I tested it with a 0.1uF capacitor and no problems were found so far.

hello_mcu

maychang posted on 2023-3-25 10:44 As for [I filter out all 50hz and below], it is not necessary at all. Please refer to the reply of chunyang on the 9th floor.

Thank you for your reply. I didn't express it clearly. The signal is not coupled through a power transformer, but a low-voltage 220V isolation transformer, with a primary of 220V and a secondary of 220V. There is no load and very little current. The carrier signal frequency is also very high, 2M-12Mhz signal.

After reading everyone's replies and suggestions, I tried a 0.1uF capacitor. Now it seems that the function is satisfactory and no other problems have been found.

hello_mcu

Azuma Simeng posted on 2023-3-27 11:52 The carrier bandpass filter, if there are other harmonics, just filter them out together

Thanks for your reply. I have tried it with a 0.1uF capacitor and have not found any problems. If I make a board later and have higher requirements, I will consider a bandpass filter.

hello_mcu

maychang posted on 2023-3-25 10:21 In order to solve the problem that the carrier signal cannot pass through the power frequency transformer, it is usually necessary to connect a capacitor between the secondary live wire and the primary live wire of the transformer. If...

In your reply, the series inductor increases the impedance of the secondary winding and the primary winding to the carrier signal. My understanding is that the greater the impedance, the more difficult it is for the signal to pass through.

maychang

hello_mcu posted on 2023-4-1 14:05 In your reply, the series inductor increases the impedance of the secondary winding and the primary winding to the carrier signal. I understand that the greater the impedance, the more difficult it is for the signal to pass. ...

[I understand that the greater the impedance, the more difficult it is for the signal to pass through. 】

That's right. The purpose of the series inductor is to prevent the signal from short-circuiting to the ground.