An AC line filter is a crucial tool in dealing with unwanted EMI (electromagnetic interference). In most countries, law demands that every electronic device be resistant to EMI to a degree, and also not emit too much of it as well. Though most devices will have some kind of EMI filter already built in, there are situations that require the use of external filter as well, since the built-in one might not suffice. In this article, we’re going to take a look at how such a filter works and the considerations when you’re getting one.

How does an AC Line Filter Work?

While the operational details might vary slightly, the principle is the same. The filter should attenuate the higher frequencies (which form the majority of EMI), allowing only the lower ones to pass through. This is basically achieved with the help of two components, namely the capacitors and inductors. The capacitors serve to impede the direct current, which is the most common carrier of interferences, while allowing the passage of a clearer, non-interfering alternating current.

As for inductors, these devices are basically small electromagnets that can hold electric energy in the magnetic field as the current passes through it, resulting in a decreased voltage.

In the case of line filters, the capacitors fulfil an important function – they serve to redirect the high-frequency current away from the parts of the circuit that need to be protected, instead feeding them into a series of inductors, each of which reduces the interference until there is almost nothing left. This is called shorting to the ground.

Check this video that deals with the inner workings of the filter in more depth.

What to Keep in Mind when Selecting a Line Filter?

Is Your Filter Sufficient to Meet Emission Standards?

There are varying standards placed on the particular equipment, as well as on whole markets and industries. Sometimes multiple standards have to be met. EMI measurements of your equipment and its ability to handle interference should always precede the purchase of a filter.

In some devices, getting a high cutoff frequency filter should be sufficient, while others will require a low cutoff filter, which tends to be bigger and more costly.

How Susceptible Is Your Device to Conduct RFI?

This is really hard to determine due to the unpredictable nature of RFI, but certain tests can yield telling results. One might attempt to simulate the malfunction by isolating the device from the power line and then artificially simulate RF by signal generators. Based on the offending RFI noise mode, as well as the power supply type, a more informed choice of a filter can be made afterwards.

Which Noise Mode Needs to be Attenuated?

There are two kinds of noise a power line filter can deal with.

•         Common mode. This noise leaks through the ground (usually through a stray capacitor) and returns to the power supply line, potentially creating problems.
•         Differential mode noise. This occurs between the lines of power. Such a noise flows in the same direction as the power supply current does.

Some filters are able to attenuate both noises, while others can do so with only one. Make your choice according to the type of noise you’re dealing with.

What About Leakage Current?

This is more straightforward and depends on the safety agency requiring a limit to the device’s leakage current. Usually, the lower the leakage current, the bigger the size of the device.

Insertion Loss

Insertion loss is basically the loss of signal that results from adding a device (in this case, a filter), to the transmission line. Generally, you want to reduce the unwanted signal, so choose a device with sufficient insertion loss, depending on the kind of signal you’re dealing with. This article deal with the question of insertion loss more in depth.

Hopefully you’ve learned something new with regards to these peculiar devices! Such a short space is not enough to cover everything, so feel free to add your insights to the comment section!