Apologies in advanced as this is kind of a vague question and due to my lack knowledge in this specific sector. I'm designing a circuit using a HLK-PM01 as the power supply which is a 3W AC-DC converter (240VAC -> 5VDC). The 5V will power some relays and will also further step-down to 3.3VDC through a voltage regulator which will power a MCU. Based on the datasheet of the HLK-PM01, a varistor, capacitor and common-mode choke are recommended to add on the AC side. enter image description here I don't really have a problem with the varistor and capacitor as they have a relatively small form factor. However, the common mode choke is quite huge and takes up a large space on the PCB. My question is how important is the common-mode choke?

My second question is selecting the correct common-mode choke. I am sourcing majority of my components from lcsc.com and some from element14, RS-Online and AliExpress. As LCSC is a china store, some of their products don't have clear specifications and I'm not sure what to look for exactly. I found a few chokes but not sure if it is suitable for this application and for AC lines.

UU9.8-10mH, UU9.8Y-10mH, FL2D-Z5-103

How do you identify if the choke is AC capable? Are there any small chokes that can be used for this application? Any help is much appreciated. Thank you in advanced.

  • \$\begingroup\$ You don't technically need those for proper function, but they are there to reduce noise and to meet EMI requirements. But more importantly: No good datasheet = no buy. \$\endgroup\$
    – DKNguyen
    Oct 30, 2019 at 17:28
  • \$\begingroup\$ @DKNguyen As my MCU is powered through a DC-DC regulator, will the choke reduce noise to the power of MCU? And as this will be installed in a house, will the EMI affect other electrical appliances in a typical house? \$\endgroup\$
    – Max
    Oct 30, 2019 at 17:47
  • \$\begingroup\$ It will since a choke works in both directions, but generally, the intent is to stop noise from the switching devices on the load side of the converter (and the converter itself) from getting onto the main line where the wires will radiate more. The mains tends to be relatively clean after all (i.e. a low frequency sinusoid without harsh square edges unless something else puts them there...like your switching converter.). \$\endgroup\$
    – DKNguyen
    Oct 30, 2019 at 17:50
  • \$\begingroup\$ I see. Thank you very much that's very helpful. I guess I should try to implement it as this is a prototype to be eventually commercialized. \$\endgroup\$
    – Max
    Oct 30, 2019 at 18:24
  • \$\begingroup\$ Oh yeah. If it's for production then definitely add those things in, or at least space for them and jumpers to bypass them. You can bypass them and not populate it if it isn't required to cut down on costs but you don't want to have to respin the PCB to make space the PCB if it fails EMI testing (well you might for the choke since it's so big, but you don't want to do that for little components that wouldn't make a big difference) \$\endgroup\$
    – DKNguyen
    Oct 30, 2019 at 18:25

1 Answer 1


The limiting factor is if you need EMI suppression. There are two reasons for this:

  1. is to pass conducted emissions regulatory testing a choke is needed to prevent conducted emissions from SMPS's

  2. is to prevent AC mains from passing noise to your device through the line

I don't know about chokes specifically, but if you want to sell this in a product determining capability will be asking the question: will this product by subject to regulatory standards. More specifically, conducted emissions on AC mains.

For UL or IEC testing, all the components must be certified, if the components are not certified then you'll foot the bill for the testing. So the first step would be to establish where you want to sell the product, then find the regulatory standards that you need for the product.

After that look for chokes with those standards.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.