I am trying to build the following schematic. It is a AC/DC converter from Cui link

schematic picture

  1. I do not understand the function of L2. What is its purpose?
  2. I am having a very hard time finding L2 for sale. Most inductors with a value of 5 mH are out of stock, and the selection is very poor. Could someone recommend a more common value that would still work in this schematic?

Normally one would expect the purpose of L2 is to reduce conducted noise being transmitted back onto the source. However, I note that they include it even when a common-mode choke is used, so they're concerned with normal mode noise (power drawn in pulses across the line, as in charging C1 through some internal bridge rectifier, for example, or the switching pulses).

enter image description here

Since the product is a 'black box', it may be worth talking directly to the manufacturer if you want to explore using different configurations.

If you look on a distributor such as Digikey and use parametric search to select values between 4.7mH and 6mH, "in stock" and max current and saturation current >= 50mA (that's a guess, you need to figure out the actual requirements- maximum nominal RMS current must be 33mA at 100V for 3W, obviously) you'll find 3 pages of parts- shielded, unshielded, etc, with varying prices etc. Most likely 4.7mH will be what you'll want to use. Bourns has some nice shielded parts..

  • \$\begingroup\$ hey, thanks for a quick answer. My power requirements are 1.8W "at the wall", which translates to a current of 16.6 mA at 110V. Is this the current value I should use when choosing the inductor? It seems awfully small. Also I noticed that inductors don't specify operating voltage - is that ok? \$\endgroup\$ – gadget Mar 13 '15 at 4:35
  • \$\begingroup\$ @gadget I would err on the high side for the current- even consider what happens under accidental overload or short circuit and high temp. The Bourns ones are good for 280mA and are the cheapest shielded ones. No voltage rating is normal- make sure clearances and creepage is adequate in your design. \$\endgroup\$ – Spehro Pefhany Mar 13 '15 at 4:43
  • \$\begingroup\$ Is it possible that L2 is for inrush current limiting? \$\endgroup\$ – Li-aung Yip Mar 13 '15 at 8:25
  • \$\begingroup\$ @Li-aungYip It's possible they're counting on it, but there's no minimum resistance spec given and it would be redundant with the NTC current limiter in the OP's schematic. \$\endgroup\$ – Spehro Pefhany Mar 13 '15 at 12:11

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