I have an application which uses a PCB to control a big AC fan (approx 1300VA) and a cartridge heating element. Since the application will be operated on regular mains, there needs to be some form of Line filtering, considering the motor is a big inductive load. Note that it is designed for 230v 50Hz european mains.

Searching the internet i found a few methods of filtering:

  • A common mode choke filter
  • A balanced LC filter

The common mode choke: Farnell To be used directly at the mains. And possibly a smaller one only before the control PCB to prevent the motor from releasing its voltage spikes in the control circuit. Though what worries me is that the Farnell page only lists DC current and not AC.

The balanced LC filter, to be used in the same locations, for the same reasons.

As for specifications: it doesn't need to be perfect, it should just prevent the device from misbehaving and from terrorizing other devices in the vicinity

The question is: will these work/ which one is better?

EDIT: i should have mentions that the motor is being switched by a TRIAC + RC snubber circuit. The heating element is switched using a relay.


1 Answer 1


The motor won't cause any harm when it's just running- spikes only happen when it is being switched. The power factor will be on the inductive side most likely. You might be able to get away with good design of your control board and an R-C snubber or a MOV across the relay contacts, but perhaps there is some regulatory requirement to do more in your jurisdiction.

Electrically, the common mode choke you link to is rated for 12A DC, meaning it would be suitable for 12A RMS AC from a heating point of view. I don't see safety agency approvals on it though. The X and Y capacitors and the choke should be festooned with safety agency approval markings appropriate for that application (you must not use X capacitors or general-purpose capacitors for Y capacitors) and have file numbers available or it could be much more expensive to perform testing.

One common method is to use a power entry filter, which would typically carry all the safety agency approvals and would be built into a metal casing. Two Y capacitors, one X capacitor and a common-mode choke.

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There are also line filters, which are similar electrically but without the IEC plug.

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Again, they have one X capacitor, two Y capacitors and a common-mode choke.

  • \$\begingroup\$ I should have mentioned that the motor is actually being speed controlled. But i assume that doesn't make much of a difference for a pre made filter \$\endgroup\$
    – user43487
    Jul 15, 2014 at 20:39
  • \$\begingroup\$ @user43487 Yes, if you've got a speed control, the filters will likely be useful unless there is some filtering built into the speed control circuit. \$\endgroup\$ Jul 15, 2014 at 20:43

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