Why need safety capacitor in electronic devices?
Because after the safety capacitor fails, it will not cause electric shock and does not endanger personal safety. In industrial, safety capacitors are capacitors that meet safety regulations and have passed the safety test.
The safety regulations have clear statements and guidance on the equipment and electronic components, to avoid electric shock, energy (fire/arc/explosion), fire, radiation, mechanical and thermal/high temperature, accidents and disasters such as chemical hazards, due to poor design or improper use. They require manufacturers to provide users with safe and high-quality products to protect the lives and property of users and operators
What is safety capacitors ?
In switching power supplies and electronic circuits, there is an electronic component called safety capacitor. The full name of the safety capacitor is the capacitor for suppressing the electromagnetic interference of the power supply. Safety capacitors will be rapidly discharged after the external power supply is disconnected, and there will be no inductance when touched by hand.
How many certifications for Safety capacitor ?
Not all capacitors are safety capacitors. Safety capacitors are capacitors that can only be sold after a series of safety certifications, and different countries have different standards for safety certification. Common safety certifications include CQC certification, KC certification, UL certification, VDE certification, and ENEC certification.
CQC certification: The full name is China Quality Certification Center. It is a certification body established with the approval of the competent department. It is an authoritative certification body that carried out certification work earlier in China and is a compulsory certification.
KC certification: KC certification is a unified mandatory certification in South Korea, and the KC certification mark represents that the product meets the product performance requirements of South Korea.
UL certification: short for Underwriter Laboratories Inc., it is a non-mandatory certification, which provides objective certification for the marketing publicity marked by the manufacturer for its products, such as product performance, quality, function and other claims.
VDE certification: The full name is the German Prufstelle Testing and Certification Institute, which directly participates in the formulation of German standards and is one of the certification bodies with a high reputation in the world.
ENEC certification: European standard electrical certification, is a certification program for electrical products initiated by the European Electrotechnical Standardization Committee and recognized by all parts of Europe.
What’s the function of Safety Capacitor ?
(1) Filtering
(2) Safer and more reliable
Based on the two functions, for safety and EMC considerations, safety capacitors are generally used for power supply inlets.
At the input end of the AC power supply, three safety capacitors are added to suppress EMI conduction interference. They are used in power filters to filter common mode and differential mode interference.
For example: when capacitors are used to eliminate noise in power cross-line circuits, normal voltages and abnormal pulse voltages (such as lightning) must be considered, which may cause smoke or fire. Therefore, cross-line capacitors must use safety capacitors.
The safety capacitor has a large margin for the breakdown voltage. If the nominal voltage of the safety capacitor is only 400V or lower, its withstand voltage is at least 1000V, so it is difficult to break down.
Even if the capacitor is broken down, because it’s filled with flame-resistant materials, which will explode, only causing a short circuit not a fire.
The capacitor safety discharge is different from ordinary capacitor discharges. The charge of ordinary capacitors will remain for a long time after the external power supply is disconnected.
You'll get a shock if you touch it. But safety capacitors do not have this problem.
How to Select Safety Capacitors
Safety capacitors are familiar to us, and we often see them in switching power supplies. But many beginners don't know how to choose one for use. The following tells you about the type selection.
First, you can find a formal safety capacitor manufacturer, and it'll provide you with a selection book.
If you don’t know how to choose, you can talk with the technical staff, and he/she will give the recommended safety capacitor model. Or you can apply for a sample test to find a suitable safety capacitor.
1. Selection of Safety Capacitor Parameters
Safety capacitors are generally used in power circuits for bypass, decoupling, filtering, and energy storage. It is very important to select the parameters of safety capacitors. If the selected capacitor does not meet the actual requirements, it is likely to cause serious problems.
(1) Rated Capacitance (CR)
First, we should consider the rated capacitance (CR), the capacitance value marked on the capacitor. The capacitance of mica and ceramic capacitors is low (generally below 5000pF); the capacitance of paper and plastic capacitors is between 10μF to 5000μF; usually, electrolytic capacitors have a larger capacitance.
(2) Rated Voltage
The rated voltage is another important parameter. It's the maximum DC voltage or AC voltage effective value, or the peak pulse voltage that can be continuously applied to the capacitor, at any temperature between the lower category temperature and the rated temperature.
When capacitors are used in high voltage applications, we must pay attention to the effects of the corona. Corona is caused by the gaps between the dielectric/electrode layers. In addition to generating parasitic signals that can damage the device, it can also cause a dielectric breakdown of the capacitor.
Corona often occurs under AC or pulse voltage. We should ensure the sum of DC voltage and AC peak voltage does not exceed the DC rated voltage during use.
(3) Temperature Range
In addition, it is necessary to consider the temperature range. It refers to the ambient temperature range for continuous operation determined by the capacitor design. It depends on the temperature limit value of its corresponding categories, such as the upper category temperature, the lower category temperature, and the rated temperature.
The working principle of the safety capacitor step-down is not complicated. It is to use the capacitive reactance generated by the capacitor at a certain AC signal frequency, to limit the maximum operating current. For example, under 50Hz power frequency, the capacitive reactance generated by a 1uF capacitor is about 3180 ohms.
When a 220V AC voltage is applied to both ends of the capacitor, the maximum current flowing through the capacitor is about 70mA. However, there is no power consumption on the capacitor. If the capacitor is an ideal capacitor, the current flowing through is a reactive current, and the work it does is reactive power.
According to this feature, if we connect a resistive element in series with a 1uF capacitor, the voltage obtained at both ends and the power consumption generated completely depend on the characteristics of the resistive element.
For example, we connect a 110V/8W light bulb with a 1uF capacitor in series. When it is connected to a 220V/50Hz AC voltage, the light bulb is lit and emits normal brightness without being burnt. Because the current required is 8W/110V=72mA, which is consistent with the current limiting characteristics of the 1uF capacitor.
In the same way, we can also connect a 5W/65V bulb and a 1uF capacitor in series to a 220V/50Hz AC. The bulb will also be lit without being burned because the working current of a 5W/65V bulb is about 70mA.
Therefore, the safety capacitor step-down is actually the use of capacitive reactance to limit current. The capacitor actually plays a role in limiting the current, and dynamically distributing the voltage across the capacitor and the load.
