Interference from Switched Mode Power Supplies


Switched-mode power-supplies can be found in everything from home computers, to mobile phone chargers, to games consoles, and as power-supplies for lighting systems, and more. What may surprise some, is that Radio Amateurs and Citizens' Band radio users also use switched-mode power-supplies. For their size and weight, they can deliver a considerable amount of power (e.g. 13.8Vdc at 23 Amperes), they do not load your transmissions with a 50Hz buzz, and they do not waste energy as heat. It is for those reasons that you will find them powering just about every electronic gadget available. When correctly manufactured, they provide an extremely efficient way of converting mains electricity to lower direct-current voltages.

However, you are probably reading this page as you have the pleasure of a fake, or extremely poorly manufactured power supply, and you need an answer as to why your radio is buzzing, or has stopped working altogether.

Missing components

Fake power supply manufacturers are going to great lengths to pass off their products as the real thing. They will often submit a fully-compliant design for testing, only to remove a number of key filtering components once full production starts.

The image below is an example of a missing common-mode choke and filter capacitors. The job of the common-mode choke and capacitors is to prevent conducted emissions from radiating along the mains wiring from the power supply. With the choke missing, the power supply will generate considerable radio frequency interference (RFI).

missing choke


Savvy purchasers of computer power-supplies would often decide upon the suitability of a particular model based on its weight. The idea being the heavier the power supply, the better the quality of filtering used inside. With the high cost of copper, fake manufacturers found an alternative way of increasing the weight.

Fake PFC
External link Hantol PSU Fake PFC Made of Cement

Chinese website details an alarming range of fake computer power supplies (link runs through Google Translate).

Fake PFC
External link Fake Power Supplies

Affected services

The following are known to be affected by SMPS interference:

Service Frequencies affected
High frequency services, inc. HF marine, HF airband, Shortwave broadcasts, and some Amateur radio allocations 3 - ~10 MHz
Long wave & Medium wave broadcast, plus some Shortwave services. 100kHz - 3MHz

What if I have purchased a fake?

If you believe you have purchased a fake power supply, and your radio is suffering interference from it, remove it from operation and report it to External link Trading Standards with details of where you purchased it from (receipts are always useful to keep!). You may also like to use our EMC non-compliance PDF form when returning the faulty item.

Radio interference from fake power supplies

Page updated: 15th August 2021

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