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schematic

simulate this circuit – Schematic created using CircuitLab

I am evolving an existing circuit which has a 230VAC input. The 230VAC has multiple loads:

  1. Transformer followed by rectifing circuit;
  2. Small DC motor 3.6W.

There is an X2 security capacitor of 0.22uF across the 230 VAC input.

I am not sure of all the reasons to have this capacitor. Q1: What are the reasons to have an X2 security capacitor?

I suspect that it is required for the DC motor. However, the specification of the DC motor says that it requires a 0.1uF/10% capacitor @230V . Therefore the existing 0.22uF X2 CAP seems to be historical and out of the required range. If this corrects the power factor, then a 0.22uF could overcorrect this.

Q2: How to determine the value of the X2 security capacitor. (Should I change the capacitor to 0.1uF).

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Class-X and Class-Y capacitors are safety-certified capacitors generally designed and used in AC line filtering in many electronic device applications. These safety capacitors are also known by other names, including EMI/RFI suppression capacitors and AC line filter safety capacitors. (EMI stands for electromagnetic interference and RFI stands for radio-frequency interference; RFI is simply higher-frequency EMI.)

So, Class-X and Class-Y capacitors help to minimize the generation of EMI/RFI and the negative effects associated with received EMI/RFI. They are classified according to their peak voltage/rated voltage and the peak impulse voltage that they can safely withstand.

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  • \$\begingroup\$ Further investigations showed that this capacitor is not for the small motor, so it must be for EMI/RFI suppression. I am not sure though how the value is chosen. \$\endgroup\$ – le_top Aug 13 '19 at 11:12
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I don't know what power factor requirements there are for such small appliances, but the cap is almost certainly there for EMI suppression, not for power factor. There's a good chance that it's 220nF because the company that designed it in has a standard input circuit, and that's it.

If it were there to correct power factor, it would be there to counteract the inductive load of the transformer -- but the 20mA that the capacitor draws at 240V would be consistent with a transformer that's supplying at least 20W to the point of load, and possibly more if I'm misremembering my rules of thumb.

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  • \$\begingroup\$ Further investigation showed that the capacitor requested by the motor's specification is soldered on the motor terminals itself. Therefore the capacitor is not used for the motor, but the analysis is interesting. \$\endgroup\$ – le_top Aug 13 '19 at 11:15

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