What is the difference between resonant capacitor and filter capacitor?
1. The bypass of the signal generally refers to the bypass of high frequency and spike interference, so the capacitance is generally not large. The bypass capacitor generally has a few nF-even hundreds of nF according to the main frequency of the signal, and the bypassed high frequency signal is dozens of M to hundreds of M, of course, the peak is also reflected in the tr along the edge, so after the bypass capacitor, the peak is weakened, high-frequency components are basically bypassed, and the main signal (low-frequency component) is not filtered out.
2. Therefore, the choice of capacitor should make the signal pass (low-pass filter) and high frequency (bypass) filter, so the higher the frequency, the smaller the capacitor capacity.
3. Whether it is used for rectification or bypass, in fact, the principle can be regarded as capacitor charging and discharging, such as bypass, high-frequency spikes are short-circuited to the capacitor instantaneously (the voltage across the capacitor cannot be abrupt), and then the voltage slowly rises ( Charging) This will slow down or even basically remove the high frequency).
4. In fact, each capacitor has a resonance point. Before the resonance point, the capacitor can be used as a capacitor. After that, the capacitor characteristic is more like an inductance, so the application is to try to be before the resonance point. The larger the capacitor, the lower the resonance frequency, and the lower the use. The frequency, such as the resonance point of ordinary aluminum electrolytic capacitors is several hundred Hz to several KHz, so it is only suitable for low-frequency power supply rectification and filtering.
Inquire
If I want to filter out the frequency of 50MHz, what capacitor should I use?
Reply
It depends on the specific situation,
The approximate range is only a few nF to 100nF. The center frequency is not good. What are the uncertainties? There are many factors such as power line impedance, selected capacitor type, etc. You can first Try how the effect of 10nF becomes larger or smaller.
Inquire
I really want to know how you got this result. .
Reply
In fact, there is also a simple estimate,
For example, high-frequency filtering on the power supply, if the impedance of the power line is 2 ohms, and then the capacitor C is connected in parallel to the ground, the cut-off frequency f can be simply estimated according to the formula of RC low-pass filtering. Of course, it is actually very complicated and many conditions exist. It may be difficult to know the frequency value accurately, so as long as we know the general range, we can judge whether it is possible according to the actual test results.
Of course, the simplest estimate is the reciprocal of the frequency, such as the reciprocal of 1MHz takes the value of 1uF, but this is when the impedance of the power line is very small.
The capacitor itself is no different. The difference lies in the circuit. After the capacitor and the inductor are in series resonance, the same phase of power, voltage, and current appears in the series circuit of resistance, capacitance, and inductance, which is called series resonance. Its characteristics are: the circuit is purely resistive. The power supply, voltage and current are in the same phase, the reactance X is equal to 0, and the impedance Z is equal to the resistance R. At this time, the impedance of the circuit is the smallest and the current is the largest. Inductance and capacitance may produce a high voltage that is many times greater than the power supply voltage. Called voltage resonance.
Parallel resonance: In the parallel circuit of resistance, capacitance and inductance, the phenomenon that the voltage and total current of the circuit end are in the same phase is called parallel resonance. Its characteristic is: parallel resonance is a complete compensation. The power supply does not need to provide reactive power. To provide the active power required by the resistor, the total current of the circuit is the smallest at resonance, and the branch current is often greater than the total current in the circuit. Therefore, parallel resonance is also called current resonance.
The single-phase conduction performance of the flow diode, although blocking the negative (positive) half-cycle current, allowing the forward current to flow, but its reverse impedance also has hundreds of thousands of ohms, and a small part of the negative current flows. Yes, it forms an AC signal voltage with a small part of the forward current. If this voltage is input to the audio amplifier, it will generate a large AC hum and enter other circuits, which will seriously interfere with the normal operation of other circuits. The capacitor is used to short-circuit it directly to the ground, and the rest is smooth DC. The purpose of the parallel small capacitor is: in the power input circuit, it is mixed with high-frequency interference, such as lightning waves, electrical sparks Interference, as well as multiple harmonics generated by the circuit itself, their frequency is higher and the speed is faster. Due to the limitation of the low-frequency characteristics of the rectifier and the reaction speed of the larger filter capacitor, it will also reach through the rectifier and filter capacitor The output circuit damages the DC power equipment, so it must be filtered with a capacitor (0.1-0.01μf) suitable for this frequency band.
Resonance capacitors require higher accuracy of parameters, and stability including thermal stability is better, while filter capacitors only require reliable pressure resistance and a relatively small leakage rate. At present, in the demanding oscillation circuits, capacitors are often not used as resonant devices, but resonant crystals with higher performance are used.
If you want to know more, our website has product specifications for capacitor, you can go to ALLICDATA ELECTRONICS LIMITED to get more information