Capacitor value selection for filtering and coupling

hello_mcu

Capacitor value selection for filtering and coupling [Copy link]

 

I want to use a capacitor to filter out a 50 Hz AC signal while coupling through a carrier signal (wideband carrier signal). What capacitance value should I choose?

The withstand voltage is 1000V.

maychang

[I want to use a capacitor to filter out the 50 Hz AC signal and couple through the carrier signal (broadband carrier signal) at the same time]

Since you want to filter out the 50Hz AC signal and require the broadband signal to pass, you should obviously choose a band-stop filter.

Such a band-stop filter cannot usually be realized with only one capacitor.

First select the filter circuit, then consider how much capacitance to use.

zhaoyanhao

maychang posted on 2023-3-23 07:34 [I want to use a capacitor to filter out a 50hz AC signal and couple through a carrier signal (broadband carrier signal)] Since I want to filter out a 50Hz AC signal and at the same time...

Teacher, can't I use a high pass filter?

qwqwqw2088

The capacitor filters out the 50 Hz AC signal and couples through the carrier signal

The coupling through the carrier signal needs to consider the frequency, which cannot be achieved by a single capacitor, and it is difficult to determine what capacitance value to choose.

maychang

zhaoyanhao posted on 2023-3-23 08:40 Teacher, is it not possible to use a high-pass filter?

You only said [you want to use a capacitor to filter out 50Hz AC signals and couple through carrier signals (broadband carrier signals)], but did not specify whether you need to pass signals lower than 50Hz. So it is impossible to determine whether a high-pass filter can be used. If you need to pass a 20Hz signal, for example, then a high-pass filter is obviously not suitable. If you want to filter out all signals below 50Hz, a high-pass filter can certainly be used.

maychang

zhaoyanhao posted on 2023-3-23 08:40 Teacher, is it not possible to use a high-pass filter?

Even when using a high-pass filter, the circuit should not extend the bandwidth to very high frequencies, but should cut it off at the high end of the frequency band that you need to pass, namely the carrier signal (wideband carrier signal).

zhaoyanhao

maychang posted on 2023-3-23 08:50 You only said [you want to use a capacitor to filter out the 50hz AC signal and couple through the carrier signal (broadband carrier signal)] but did not specify whether it is necessary to pass a signal higher than 50 ...

OK, thank you for your answer. But I am a passerby, not the author.

Gen_X

Calculate the following yourself:

According to the frequency F and the estimated capacitance C, calculate the impedance of this capacitor. If it is close to 0.1-10 ohms, congratulations, this capacitor is suitable.

If the resistance value is cancelled, the 50Hz leakage will be greater, otherwise the effect will be better.

chunyang

The idea is neither feasible nor necessary.

The frequency of power line carrier communication is much higher than 50Hz, so it can be directly coupled with a small capacitor. The 50Hz signal will not have any adverse effect on the demodulation of the signal.

led2015

Try using a 0.0159μF capacitor, which has a 1000V voltage tolerance, to filter out the 50Hz AC signal and pass the broadband carrier signal.

hello_mcu

chunyang posted on 2023-3-23 18:04 The idea is not feasible and unnecessary. The frequency of power line carrier communication is much higher than 50Hz, so it can be directly coupled with a small capacitor. The 50Hz signal will not affect the signal...

Hello, thank you for your answer.

My requirement is that the carrier meter reading needs to cross the transformer (isolated), so I want to use a coupling capacitor to allow the carrier meter reading signal to cross the transformer while preventing the two ends of the transformer from being short-circuited (the input and output of the transformer are isolated, and if the 50hz signal cannot be filtered out, it will cause the input and output to be short-circuited). Now I want to find a suitable capacitor that can filter out 50hz and couple the carrier meter reading signal.

hello_mcu

led2015 Published on 2023-3-23 20:20 Try using a capacitor with a capacitance of 0.0159μF, which has a voltage tolerance of 1000V, to filter out the 50Hz AC signal and pass the broadband carrier signal...

Thank you for your answer. I will buy some 0.0159uF capacitors to try.

hello_mcu

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

Do you mean that the smaller the calculated resistance value, the more 50hz leakage will be?

hello_mcu

qwqwqw2088 posted on 2023-3-23 08:41 The capacitor filters out the 50hz AC signal and couples the carrier signal at the same time. The coupling through the carrier signal needs to consider the frequency. One capacitor cannot achieve this. The choice is more...

Thank you for your answer. I am still looking up information. Now I think it is relatively simple. I want to use the characteristics of capacitors to see if the function can be realized.

hello_mcu

maychang posted on 2023-3-23 08:50 You only said [you want to use a capacitor to filter out the 50hz AC signal and couple through the carrier signal (broadband carrier signal)] but did not specify whether it is necessary to pass a signal higher than 50 ...

Thank you for your answer. I filter out all 50hz and below. In this application, an AC 220V voltage signal is input, which enters the isolation transformer. The transformer outputs a 220V AC signal, and the carrier signal cannot pass through the isolation transformer. So I want to use a coupling capacitor to filter out 50hz and pass the carrier signal at the same time. I checked the high-pass filter and it seems that it will affect the amplitude of the output voltage. I don't know if my understanding is correct.

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...

Pay attention to the reply from chunyang on the 9th floor.

Judging from your reply to Chunyang on the 11th floor, you do not fully understand why the carrier signal attenuation across the transformer is very large.

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...

An ordinary single-phase power transformer has two windings, primary and secondary. The two windings have distributed capacitance to the ground (iron core), and there is also distributed capacitance between the two windings. The capacitive reactance of this distributed capacitance at a frequency of 50Hz is quite large, and has little effect on the power frequency load, but the capacitive reactance is quite small at the carrier frequency. The power line carrier signal is greatly attenuated when passing through these distributed capacitances, so it is generally believed that the carrier signal cannot pass through the power frequency transformer.

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...

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 live wire of the secondary and the live wire of the primary of the transformer. If the signal attenuation still cannot meet the signal transmission requirements, a small inductor is connected in series at the secondary output end of the transformer, and a small inductor is also connected in series at the primary output end of the transformer. The purpose of the series inductor is to increase the impedance of the secondary winding and the primary winding to the carrier signal (low impedance is caused by the distributed capacitance of the winding to the iron core).

But this capacitor and this small inductor are very particular.

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...

You said [isolation transformer]. For industrial frequency power transformer, of course, the primary and secondary are isolated (0.4kV autotransformer is rare, except for step-down starter), but the power transformer with secondary rating of 0.4kV is usually reduced from 10kV to 0.4kV, and the voltage of the primary output terminal to the earth is 6.6kV effective value. The capacitor connected across the primary output terminal and the secondary output terminal of the transformer cannot use 1kV withstand voltage. The AC voltage across the two ends of this capacitor is much higher than 1kV.

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...

If you need to connect a small inductor in series, you need to pay attention that the inductor must be able to pass the entire power frequency current of the transformer secondary. This is not a current measured in mA, but a current measured in 10A or even 100A. You can see how thick the secondary output terminal of the transformer is. The diameter of the wire used for this inductor should obviously be similar to the diameter of the secondary output wire.