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 75 for 150 KHz-10 MHz (includes AM broadcast band) – useful for switching power supplies, wall warts, DC to AC inverters
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.
AC POWERED DC SUPPLY FILTERS
Got a noisy switch mode power supply? Here is a set of filters for the AC power line and the DC output line which helps suppress common mode current thereby reducing RFI/Noise radiated or conducted to nearby sensitive electronic equipment. Simple to install and no modifications needed to equipment.
DC POWERED AC (INVERTER) SUPPLY FILTERS
Got a noisy DC powered, AC ouput inverter? Here is a set of filters for the AC power output line and the DC input line which helps suppress common mode current thereby reducing RFI/Noise radiated or conducted to nearby sensitive electronic equipment. Simple to install and no modifications needed to equipment.
Here are some additional 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 120/240 VAC electronics
Typical installations suitable for most AC powered appliances
 F240 Donut Toroid on flat screen TV AC power cord |
 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 20 amp cables
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 Common Mode 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 mobile radio transceiver DC connectors to reduce ignition noise and prevent RFI from radio back into car computers and vice-versa). Also excellent noise suppression for AC powered switching DC supplies (SMPS) – use one filter on DC output and another on AC input. Twist wires prior to winding for best results. Use mix 75 for best suppression at switching frequencies.
 F240 on DC cable – 4 turns shown |
#12 twisted dc wires on toroid F240
LARGE Ring Common Mode Noise Filter (4″ OD, 3″ ID) – fit typical DC power cords over 20 amps, also works for AC power cords
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. 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
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.