A Line Impedance Stabilization Network (LISN) is an active device that injects a known impedance into a circuit to measure its characteristics. LISNs are used in electromagnetic compatibility (EMC) measurements to determine the amount of interference a device emits.
They connect the power line and the device under test (DUT). A LISN typically consists of an impedance-stabilization network (a constant impedance over a wide frequency range), a filter (to remove conducted emissions below 30 MHz), and a resistor (to provide a low-impedance path to ground for conducted emissions above 30 MHz).
The working principle of a LISN is based on the fact that the impedance of a conductor is proportional to its length. By connecting a LISN in series with the test device, the impedance of the test device can increase, which allows for more accurate EMC testing. Here is a quick overview of what a LISN is used for and how it works:
What is a LISN Used For?
There are several ways LISN is used. Here are a few :
Conducted Emissions Testing
This type of testing is required by the FCC in the United States and other regulatory agencies worldwide. Emissions testing determines the amount of electromagnetic interference (EMI) a device emits. The results of this testing ensure that the device meets the required emission levels.
Electromagnetic Compatibility (EMC) Testing
Electromagnetic compatibility (EMC) testing ensures that electronic devices do not emit too much electromagnetic interference (EMI). This is common for line impedance stabilization networks, used to couple the circuit under test to the measuring equipment. EMC testing is also used to ensure that electronic devices can tolerate the amount of electromagnetic interference (EMI) they are likely to encounter in their intended environment. LISNs are used in EMC testing to determine the amount of interference a device emits.
EMC testing is vital because it helps ensure electronic devices’ safety and that electronic devices will not interfere with each other.
LISN devices are used in immunity testing to provide a stable impedance of 50 ohms. This allows for more accurate measurements, as it eliminates the effects of impedance mismatches. LISN devices also help to isolate the device under test from power line transients. These transients can corrupt immunity measurements and make it difficult to get accurate results.
It Prevents External Noise Coupling in the Device
External noise can come from various sources, such as power lines, radio transmitters, and other electronic devices. Connecting a LISN in series with the test reduces the amount of noise that can be coupled to the device. Additionally, the filter in the LISN removes conducted emissions below 30 MHz, reducing the amount of interference the device can receive.
Reduces the Risk of Electrical Shock
Another benefit of using a LISN is that it reduces the risk of electrical shock. When a LISN is used, the voltage on the device under test (DUT) is effectively isolated from the power line. This isolation will reduce the risk of electrocution if the DUT malfunctions and comes into contact with the power line.
Simulate the Impedance of an Electrical Grid.
LISNs can also be used to simulate the impedance of an electrical grid essential for testing devices connected to the grid, such as solar inverters and wind turbines. Connecting a LISN in series with the test device increases the device’s impedance, allowing for more accurate testing.
Harmonics and Flicker Testing
Harmonics and flicker testing determine the amount of distortion that a device produces. The results of this testing ensure that the device meets the required harmonic levels. Additionally, the testing results can help improve the device’s design.
Harmonic testing is essential because harmonic distortion can cause problems with other equipment on the same power line, such as voltage fluctuation and surges. Flicker testing measures how much a device’s light output varies when it is powered by AC power.
Stable Line Impedance
Impedance is a measure of the opposition to current flow in an alternating current circuit and is represented by the letter Z. The impedance of a circuit is made up of two components: resistance and reactance.
Resistance is the opposition to current flow due to heat build-up, while reactance is the opposition to current flow due to inductive or capacitive effects. The LINS provides a stable impedance of 50 ohms. This allows for more accurate measurements and eliminates impedance mismatches’ effects.
The stability of a voltage source is essential for many applications, and thus it is crucial to understand how impedance can affect voltage stability. Generally, a voltage source with low impedance will be more stable than one with high impedance. This is because a low impedance voltage source will have less variation in its output voltage as load resistance changes. For this reason, it is often desirable to use voltage sources with low impedance to ensure stability.
A stable impedance is essential for accurate testing, as it ensures that the results are not affected by changes in the signal frequency. Without a LISN, the impedance would vary with frequency, making it difficult to obtain accurate results.
Eliminates the Effects of Power Line Transients
Power line transients are sudden changes in voltage that can occur on the power line. These transients can damage electronic equipment and cause data loss. To prevent this, LISNs help to protect your business’s electronic equipment against these transients by providing a low impedance path to the ground. This gives the transient current a path to follow, thus preventing it from damaging the equipment.
How Does a LISN Work?
A LISN consists of a capacitor and an inductor connected in series. The capacitor filters out conducted emissions below 30 MHz while the inductor stabilizes the impedance of the circuit.
The capacitor is connected between the line and the neutral conductor. The inductor is connected between the line conductor and the ground.
When a LISN is used, the voltage on the device under test (DUT) is effectively isolated from the power line. This isolation will reduce the risk of electrocution if the DUT malfunctions and comes into contact with the power line.
Additionally, the LISN filters out conducted emissions below 30 MHz, which reduces the amount of noise that couples into the device under test.
A LISN is a vital tool used in many industries. It aids with measurements and protects equipment from damage. It is crucial to understand how a LISN works to use one properly. Overall, the LISN is a versatile and essential tool for any engineer working with electronic equipment.