Safety capacitors are passive electronic components designed to mitigate the effects of transient voltages and interference in electrical circuits. Safety capacitors are designed to help safeguard users and equipment from electrical hazards, even in the event of failure. Since these components are commonly used in AC line filtering and power supply circuits where failure could lead to electric shock, fire, or equipment damage, safety capacitors are subject to stringent testing and certification requirements.
The Role of the Safety Capacitor
In a circuit, safety capacitors used on AC lines filter high-frequency electromagnetic interference (EMI), or undesirable electromagnetic emissions or disturbances generated either by electronic devices or natural sources in the environment that can interfere with the proper functioning of other nearby devices or systems. This is known as EMI filtering, with the filter providing a safe path for noise while maintaining electrical isolation and safety.
When placed in combination with a series of inductors or resistors, safety capacitors form low-pass filters that attenuate the high-frequency signals (EMI) and allow the lower-frequency power signals to pass (Figure 1). This is the classic EMI filter setup—attenuating unwanted noise above a cutoff frequency. As more electronic devices enter the market, EMI filtering becomes even more crucial as there is an increased likelihood that EMI could cause a device to malfunction, crash, or fail.
In power electronics specifically, EMI filters take the form of power line filters that protect the line from upstream noise. While this list may differ depending on system requirements, the core components of a power line filter include:
- A common-mode filter circuit with two or more line-to-chassis capacitors and a common-mode inductor
- A differential mode (DM) filter circuit with at least one pair of series inductors and one line-to-line capacitor
- Compensation networks to adjust the filter’s quality (Q) factor and adapt the output impedance as needed
- Transient voltage suppression device(s) to defend against surges
Figure 2 shows an EMI filter structure that could be useful for single-phase AC applications or for DC power inputs.
Figure 2: EMI filter structure designed for single-phase AC applications or DC power inputs
Where Safety Capacitors are Used
To ensure safe operations and compliance with electrical safety standards, you’ll find safety capacitors in nearly every electronic device developed today – from industrial motor drives and HVAC systems to consumer appliances to medical devices, telecommunications equipment, and automotive electronics. For many of these applications, safety capacitors are crucial components that ensure user safety and reliable performance.
In our next post in our series on safety capacitors, we will dive more into the functionality of safety capacitors and discuss how to select the capacitor that will best meet your needs by exploring the different classes and subclasses for these components.
Learn more about Knowles Precision Devices extensive line of safety capacitors.