Emission and Immunity Tests(CE) - FAILED

Thread Starter


Joined Apr 22, 2019
Hi everyone,
We are developing a Smart Device that is classified for Class III Immunity and Emissions CLASS B (CE).
We begin testing a Chinese power supply called "wx dc12003" switch mode power supply. We did Conductive Emission tests and failed of course. Also, the surge test fried our cheap power supply.
Now we are thinking to add external circuits to increase the performance and maybe pass the tests.
What do you think, can it be solved with these external circuits that I've attached below, or what other proposal do you have. Thanks.



Joined Dec 13, 2021
Work out how many of these your going to sell,
then work out the cost of the second / third EMC test
then work out how much time it will cost
and multiply the time by 10 times your salary
come up with a "cost" for being months late to market

Add up costs, divide by number of units,
is the cost of the approved PSU worth more or less.


Joined Sep 22, 2009
Chinese PSUs are notorious. I would start by placing a varistor immediately at the mains power input to protect against surge/burst, followed by an AC common mode choke on the AC input side. On the DC output, you could either use another common mode choke or a pi low pass filter to get reduce output radiation. Its difficult if you haven't got a spectrum analyser to see the effect of these components. As always, short leads and loads of grounding copper and decoupling caps.


Joined Aug 7, 2020
Before you embark on correcting your Chinese power supply, make sure it passes the electrical safety tests, because, if it doesn't, there's nothing you can add to it to make it pass. Then you will have wasted your time making it pass the EMC tests.
I agree with @sagor about buying a properly CE marked power supply, but that doesn't exempt you from having the finished product tested including its already-tested power supply.
One further point - in some circumstances, for instance if you can make the PSU on the same board as the rest of the circuit, it can save you money to design it yourself. If you follow the instructions from a supplier like Power Integrations, for which you can buy ready-made flyback transformers, you can easily pass the relevant regulations.
Invest in a spectrum analyser - they are cheaper than they used to be. If you sell Chinese imports, then just blithely accepting a Chinese CE certificate as valid doesn't constitute "due diligence". I have tracked down so-called test-houses that are issuing CE certification and found them to be Chinese restaurants in the East end of London.


Joined Jan 23, 2018
I think that this is the same supply presented in a post a few days ago. The point about the safety test first is smart because if it fails that check no point in going farther.
Certainly by adding enough filtering ad shielding any supply can pass.. But the cost may be a lot more than a certified supply from a reputable supplier.

Thread Starter


Joined Apr 22, 2019
Thank you all for your kind replays.
I think we are going to design one of our own. We already have some schematics and materials for a design that already is implemented in an IoT product and has passed these tests.
I will make a post about it and how it went.
Thank you again.