What mistakes do you make while using the power filter? II

In many tests, we can pass the test by changing the way the power filter installed. Here are some examples of filter performance when filter misinstalled.

1.Long input line

Many equipment power line enters the chassis, it receives the input of the filter after a long wire, as shown in Figure 1. Due to the long wire to the input of the filter, the electromagnetic harassment is recoupled to the power cord through capacitive or inductive coupling, and the higher the frequency of the harassment signal, the stronger the coupling, resulting in experiment failure.

2.Parallel wire

Sometimes engineers tie the cable together just for a good look, which is not allowed for the power cord. If the input / output lines of the power filter are wired in parallel or tied together, the parallel transmission lines is equivalent to a parallel capacitor between the input / output lines of the filter, providing a path around the filter for the harassment signal, resulting in greatly decreased filter performance and even fails at the high frequency (as shown in Figure 2). The magnitude of the equivalent capacitance is inversely proportional to the wire distance and proportional to the length of the parallel wire. The greater the equivalent capacitance, the greater the impact on the filter performance.

3.Earthing and housing

Many engineers install the filter with poor overlap (with insulating paint) between the filter housing and the chassis, and with a long grounding wire used, which will damage the high frequency characteristics of the filter and reduce the filter performance.

With the long grounding wire, the distribution inductance of the conductor at high frequency cannot be ignored. If the filter is well engaged, the interference signal can be directly grounded directly through the housing. If the filter is connected between the housing and the chassis is poor, it is equivalent to a distribution capacitor between the filter housing (ground) and the chassis, which will cause a large ground impedance of the filter at high frequency, especially near the frequency of distribution inductance and distribution capacitance resonance, and the ground impedance tends to be infinite.

Effect of poor filter grounding on the filter performance: because of the filter due to poor grounding and large grounding impedance, some harassment signals can pass through the filter (as shown in Figure 3). To solve the poor overlap, scrape off the insulation paint on the chassis to ensure a good electrical connection between the filter housing and the chassis.

In this kind of installation mode, the shell and shell of the filter have good contact, can block the opening of the power line on the chassis, improve the enclosure, the box shielding isolation between the input and output lines of the filter, eliminate the harassment coupling between the input and output lines, and ensure the filter performance of the filter.