Common Mode Noise Filters – AC/DC Power

Common mode current often enters the victim device via the AC or DC power lines to the device.  Likewise, the RFI source device often conducts common mode current through the AC power line where it is picked up by the victim device.  The AC or DC path must be choked to suppress the interference.  Common mode chokes are used for this purpose.

Choose a choke that physically fits the power cord as a “SLIP ON” or as a “SNAP ON” around the cord.  SNAP ONs are very convenient when large power plugs are present or thick cables such as 240 volt lines used to power linear amplifiers.  As with all chokes, the more turns around or through the choke the better choking effect it will have.  In some difficult cases it may be necessary to cascade or put several chokes in series to get enough choking impedance to reduce the common mode current.  The choke impedances are additive in a series connection.  A higher impedance gives a lower common mode current (I=E/R).

Mix Selection

Remember to choose the proper “MIX” for the interfering frequency:

Mix 77 for .01-50 MHz (includes AM broadcast band)

Mix 31 for 1-300 MHz (suitable for most HF amateur and commercial transmitters)

Mix 61 for 200-2000 MHz

Mix 43 for 20-250 MHz (includes FM broadcast, aircraft, commercial radio)

Not sure of what mix to get for your RFI/EMI needs?  Try the Ferrite Ring Combo Pack which covers RFI suppression from .1 – 2000 Mhz and solves most AC/DC cable and many coax antenna feed line common mode issues.

Here are some choices for various AC and DC power line chokes (click on product link for pricing info and quantity discounts):

AC POWER LINE CHOKES

STANDARD Ring Noise Filter (2.4″ OD, 1.4″ ID) – fit typical AC power cords of 120 VAC electronics

 

 

Typical installations suitable for most AC powered appliances

20141222_100827

F240 Donut Toroid on flat screen TV AC power cord

AC Cable on F-240 Toroid - 6 turns

AC Cable on F-240 Toroid – 6 turns on Desktop computer AC power cord

Application Note: Cordless Electric Drill Battery Charger RFI suppression – use F240-31 on AC power cord with 4-6 turns similar to picture above.  Symptom is RFI noise on 20-60 MHz. Just another example of a Switch Mode Power Supply (SMPS) RFI issue which is usually very broadband in nature.

LARGE Ring Noise Filter (4″ OD, 3″ ID) – fit typical AC power cords of 240 VAC electronics

Excellent for large cables or multiple cables  – use multiple cores for higher choking.  Also useful when you need high turns count for high choking

 

 

 

Big Clamp On Noise Filter (1″ ID) – for large power cables or multiple turns of standard 120 VAC 15 amp cables

 

Typical installations on power cables (good for 240V and thicker cables – try to get at least 2 turns of cable otherwise use multiple clamp FSB-1 in series on cable

FSB-1 x 1 on 240V AC power cord of linear - 3 turns through center

FSB-1 x 1 on 240V AC power cord of linear – 3 turns through center

FSB-1 x 2 in series for additional choking on 240V linear AC cord

FSB-1 x 2 in series for additional choking on 240V linear AC cord – 3 turns each core

DC POWER LINE CHOKES

Wall Wart Power Line RFI Kit

Wall wart dc power supplies can be a source of significant RFI noise from common mode current using the DC power lead as the “antenna”.  To suppress the RFi noise and to decouple the antenna use a small ferrite ring with the DC cord wound through it.

Since most users have many wall wart power supplies for various electronic devices, we have a special bulk supply RFI kit for 10 wall warts.

 

STANDARD Ring Noise Filter  (2.4″ OD, 1.4″ ID) – fit typical DC power cords up to 20 amps

Typical installations suitable for most DC powered appliances (excellent for ham mobile DC connectors to reduce ignition noise and prevent RFI from radio back into car computers)

20150125_103738F240 on DC cable – 4 turns shown

#12 twisted dc wires on toroid F240

 

LARGE Ring Noise Filter (4″ OD, 3″ ID) – fit typical DC power cords over 20 amps

Excellent for large cables or multiple cables  – use multiple cores for higher choking.  Also useful when you need high turns count for high choking

 

Typical application: Very good for high turn count/high choking on large cables or cables with big connectors.  Higher turns count possible than FSB-1 with 3″ hole size.

F400 on AWG 4 battery cable - 8 turns very effective on Solar, DC Powered Inverters

F400 on AWG 4 battery cable – 8 turns very effective on Solar, DC Powered Inverters.  8 turns is approximately 6 feet of cable in this example

Big Clamp On Noise Filter (1″ ID) – for large power cables or cables with connectors

Typical installations (very good for high current DC cables, AC and DC inverter cables, solar panel DC cables, marine/boat DC battery cables, boat radar DC cables, ham/marine SSB radio DC cables, etc)

FSB31-1 DC Noise Filter

DC Common Mode Noise Filter for marine, mobile, portable and fixed station HF operations

The RFI-DC-30 is a common mode noise filter for DC powered equipment up to 30 amps and 600 VDC at 105 degrees C and provides effective dual suppression of common mode current with individual chokes on the positive and negative cables in the HF frequency range from 1-61 MHz (beneficial up to 160 MHz). Very useful for suppressing noise from generators, inverters, battery chargers, solar panels, wind generators, and other noise producing devices sharing the same DC power bus.  Simple to install between noise source and noise victim without any modifications to either device.

 

The enclosure box is a PVC 4″ x4″ x 2″ weather resistant enclosure with 1/4″ stainless hardware for ease of connection to DC voltage circuits and (optionally) to ground for greater attenuation of the common mode current present on the DC positive and negative lines.