When selecting an X2 safety capacitor what must be taken into consideration. There are only a few types of capacitance ranges and seem to be chosen based on voltage rather than capacitance.

Take mains AC, the cut off frequency is difficult to determine because we don't know the impedance so which capacitance do we pick? Can we assume a low resistance. Also, if the source is an alternator it will change the resistance to.

Is it common to put multiple of them in parallel to get the required range??

Also, will current draw in the circuit affect the value of capacitor? If we have more current flow do we need a bigger cap?


Edit 1:

The Kemet R463R422050M2K is a possible device that could be used.


  • \$\begingroup\$ X (and Y) capacitor values are selected to attenuate specific frequency ranges when paired with a specific common-mode inductor as part of an EMI filter, usually for frequencies in excess of 150kHz. Each capacitor will have its own frequency vs. attenuation curve and the precise selection is usually a mix of experience and empirical experimentation (measure, tweak, measure, tweak) \$\endgroup\$ Oct 11 '18 at 16:13
  • \$\begingroup\$ @Adam Lawrence if the source is inductive is it best to use that as inductance or should more be added? \$\endgroup\$
    – MXG123
    Oct 12 '18 at 7:20
  • \$\begingroup\$ A possible part has been added however I am more concerned in picking a suitable capacitance at the moment rather than a specific device. \$\endgroup\$
    – MXG123
    Oct 12 '18 at 13:15
  • \$\begingroup\$ I wouldn't say current "drawn" will change the capacitance, however if you go by safety MLCC capacitors which usually is voltage load dependent it will alter with voltage load. \$\endgroup\$ Jan 9 '19 at 14:39
  • 1
    \$\begingroup\$ This is a EMI suppression capacitor. The amount of capacitance/inductance required to suppress EMI depends on the power/current in the EMI, but not the power/current in the load. The impedance of the capacitor at the suppression frequency only has to be less than that of the device: it doesn't matter as much what the line impedance is. \$\endgroup\$
    – david
    Jan 18 '19 at 0:05

The X2 rating is mainly its ability to withstand high voltage surges. It is somewhat lower than the X1 cap and quite a bit lower than the Y1 and Y2 caps. Otherwise, you select voltage rating and value in much the same way as any other place. X1, X2 caps are for across-the-line applications and Y1, Y2 are for line to earth connecting. Although these caps are intended for AC applications as RF (EMI) filters, you will find them unsuitable for applications such as snubbers, etc. They do not have the greatest dissipation factors.


All X, X1 or X2 capacitors are rated for safe across the line usage.

X2 caps are somewhat like 2 X caps in series and are more durable against damage from high voltage spikes. Failure mode (if it ever happens) is meant to be open-circuit.

So, the only thing to be considered is the capacitance value. Talk of tuned circuits et al is blue sky thinking. In practice a suitable value will be found that passes EMI compliance regulations. When visiting the lab, always have plenty of caps to hand !

Yes, a circuit drawing higher power ( therefore current) will typically require a larger value.


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