Humidity-resistant X2 capacitors

X2 capacitors are used not only for EMI suppression. Thanks to their robustness and resistance to humidity and high temperatures, they are also suitable for voltage dividers in capacitive power supplies – for example, those of smart meters that are mounted outside buildings.

A new series of EPCOS X2 interference suppression capacitors offers stable capacitance values even under extremely humid conditions. The B3292*H/J* series for operating voltages of up to 305 V AC comprises types with rated capacitances ranging from 0.1 µF to 15 µF, lead spacings from 15 mm to 37.5 mm and operating temperature ranges from -40 °C to +110 °C.

In addition, the capacitors, which employ a metallized polypropylene dielectric (MKP), also feature excellent self-healing properties. These allow weak dielectric areas to be burned out in the event of overvoltages.

Particularly in the case of capacitive power supplies in which the capacitor - in contrast to the interference suppression function - is switched in series with the load (see figure), extremely high demands are made on these components.

Figure 1:

Wiring example using X2 capacitors: Whereas X2 capacitors for interference suppression are switched in parallel with the power cable (left), intelligent energy meters have a capacitive power supply with a capacitor switched in series.

The X2 capacitors must also offer considerable long-term stability, even under harsh operating conditions such as high temperature and air humidity. With standard capacitor types, this can sometimes result in a degradation of the metallization, causing a reduction of the capacitance. This in turn means that, in the capacitive voltage divider of a capacitive power supply, sufficient voltage is no longer available for supplying the load.

The new EPCOS X2 capacitors of the B3292*H/J* series, however, function reliably under even the most demanding ambient conditions and effortlessly meet the demands of the humid-heat tests. This means that the capacitance value during a test period of at least 1000 hours at a temperature of 85 °C, a relative humidity of 85 percent and a voltage of 240 V AC does not change by more than 10 percent. Consequently, the new capacitors are very suitable as voltage dividers in the capacitive power supplies of outdoor energy meters.

Main applications

• Capacitive power supplies of outdoor energy meters
• Across-the-line industrial applications

Main features and benefits

• Stable capacitance values even under extremely humid conditions
• Excellent self-healing properties
• High maximum operating temperature up to +110 °C

Key data

* IEC 60384-14