I have an old electric knife (220V 50Hz) which seems to produce a lot of EMI. Some clues on that were a "bzzzz" noise from PC speaker I noticed sometimes when someone use it.

Recently I bought a device which has touch-buttons and I noticed when the knife is in use the device detect false touch. So at this point I think that knife produce a lot of EMI (maybe due to it is an old model?)

So, I would ask: is there any way to reduce them (capacitor on AC motor,..)?

Or can I make a sort of shield? (eg. whith alluminium sheets glued inside the plastic cover).

Please don't suggest to replace it :) the question is just to know something more about EMI, shielding, etc..

  • 3
    \$\begingroup\$ A straight X-capacitor over the motor like you suggest would be a good start if there isn't one already. Remeber to place an R in parallel if not built in. In the EU you would be required to bleed down to some voltage (~50 V) within one second or you will get nasty shocks from the plug if you toutch it once unplugged. How eager/knowledgable are you in this feild? Aluminium foil won't do crap here. \$\endgroup\$
    – winny
    Jan 30, 2019 at 12:00
  • 4
    \$\begingroup\$ The motors in most counter-top and hand-hand, AC appliances and tools are universal motors. A universal motor is a DC motor with a commutator that has been adapted to operate on either AC or DC. It is the sparking from the commutator that causes the EMI. \$\endgroup\$
    – user80875
    Jan 30, 2019 at 12:20
  • 1
    \$\begingroup\$ @CharlesCowie yep, I also supposed the sparks generated by brushed motors may be the cause... in that case there's no filtering circuit which can avoid that, right? \$\endgroup\$
    – Noisemaker
    Jan 30, 2019 at 12:37
  • \$\begingroup\$ electronics.stackexchange.com/questions/274991/… electronics.stackexchange.com/questions/237209/… \$\endgroup\$
    – user80875
    Jan 30, 2019 at 12:53
  • \$\begingroup\$ Given the motor's age, the amount of EMI could be a symptom of severely worn brushes. I would check them ASAP and replace them if needed. Severely worn, lose, or depleted, brushes can permanently damage the motor. \$\endgroup\$ Jan 31, 2019 at 6:34

3 Answers 3


Sounds like you have low frequency conducted EMI. An X-capacitor straight over the motor would be the first order of business, but I've included the steps I would have taken down the line (litterally).


simulate this circuit – Schematic created using CircuitLab

Couldn't find a symbol for you motor, nor lines to draw a correct CM choke.

More luck with LTspice: enter image description here

All values are estimations, but at least not several orders of magnitude off.

To be compliant, your capacitor needs to be at least X2-rated. The R in parallel would be required in the EU to bleed down to 60 V (thanks Martin Bonner!) within one second or you will get nasty shocks from the plug if you toutch it once unplugged. There are special X-rated capacitors with this resistor built-in. This is called a bleeder resistor. There is a tradeoff between time and power dissipation when plugged in so make sure you have sufficient voltage and power rating on it.

Your suggestion of aluminium foil wrapping won't do crap here due to too low frequency and conducted emission as opposed to radiated emission, where alumimium foil might have worked.

If you don't want to dissassemble your device, at low frequencies there is a fairly low penalty in placing the filer down the cable instead. Probably measureable, but managable.

  • 2
    \$\begingroup\$ If you live inside the EU and don't fit the bleed resistor, nobody will ever know unless someone gets badly hurt. If you live outside the EU, adding the bleed resistor is still an excellent idea. \$\endgroup\$ Jan 30, 2019 at 16:50
  • \$\begingroup\$ @MartinBonner Indeed! Do you remember the exact voltage? It was something uneven like 47 or 52 V IIRC. \$\endgroup\$
    – winny
    Jan 30, 2019 at 16:58
  • \$\begingroup\$ I found: "IEC 60204 and EN 50178 require that active parts must be discharged within a period of 5 s to a voltage below 60 V (or 50 µC). If this requirement cannot be observed in view of the operating mode, the hazardous site must be permanently marked in a clearly visible way. This must be done by means of an appropriate text as well as by graphical symbols, such as "Hazardous electrical voltage" (417-IEC-5036) and "Caution" (7000-ISO-0434). Exposed cables of connectors must have a discharge time of 1 s, or else degree of protection IP2X or IPXXB (IEC 60529) must be ensured." \$\endgroup\$ Jan 31, 2019 at 8:11
  • \$\begingroup\$ That was from: tdk-electronics.tdk.com/download/528632/… \$\endgroup\$ Jan 31, 2019 at 8:12
  • \$\begingroup\$ @MartinBonner Thank you very much! I'll update my answer. \$\endgroup\$
    – winny
    Jan 31, 2019 at 8:24

One easy way would be to put a toroid or ferrite on the cable, they increase the inductance on the cable for high frequency signals and prevent conducted emissions.

I've had success with a radio and blocking emissions from a PC with the mains cable from the radio wrapped around a ferrite toroid. You can also get clamp on ferrites almost everywhere.

This will work if it is conducted emissions from the knife, if it's radiated emissions (radio waves) then your best bet would be to shield the speakers. It might be good to put a ferrite on the speakers AC mains cable too.

enter image description here
Source: https://palomar-engineers.com/ferrite-products/ferrite-cores/ferrite-ring-toroid-combo-pack

This is what a clamp on ferrite looks like:

enter image description here


You can also tear down the machine and make sure the commutator and brushes are tip-top. The source of the EMI may not be the machine itself, but defects or wear in the machine.


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