Original question: * I wonder what would be the R27 value for this Power Supply (non-isolated, LNK IC-based, 5V dc 80mA nominal output in CC mode), given that original SMD resistor got off and lost, and I’m not much used to this kind of PS. Please feel free to comment on my scratched schematic and also to visit my other thread in which I post about a problem related to insufficient “fuel” from this in a ESP01+AtMega Tiny85 coffee machine control project...

Thanks So much in advance folks! *

PS1- Please notice I had an errata for the R2 value, see my comments below. PS2- Adding datasheet for the AC-DC chip. PS3- Adding corrected schematic.

LNK IC-based offline, non-isolated 5V power supply schematic



  • \$\begingroup\$ Highlight R23 please. \$\endgroup\$
    – Andy aka
    Oct 3 '20 at 15:21
  • \$\begingroup\$ Sorry I meant R27 lol. i’m editing original post and highlighting both R27 and R2 \$\endgroup\$
    – mosagepa
    Oct 3 '20 at 15:22
  • \$\begingroup\$ Please add a data sheet link to the chip while you are at it. And, have you left off a connection from 'phase'? Where did the circuit come from? \$\endgroup\$
    – Andy aka
    Oct 3 '20 at 15:23
  • \$\begingroup\$ @Andyaka thanks for your suggestion, I’ll do just that, since also the “example application” shown in such datasheet is very similar to this case, although for a !2v output case. The 390meg R is really a 39R, seems I got bad eye over the 0 in the ‘390’ marking, lol, it’s this “inside dotted 0” but I should have guessed this from the unreasonable value a ‘398’ marking would mean! This is the power supply section for a Kruder control pcb for an european Solac-brand coffee machine. I describe my Esp01 wifi control project for this in my other thread. \$\endgroup\$
    – mosagepa
    Oct 3 '20 at 15:35
  • \$\begingroup\$ Phase wire is as indicated, As you can see I had to wipe a false link between it and the Neutral (go figure!). Excuse the untidiness of the whole schematic but I was too lazy to redraw it, since it’s just a missing R value that I’m really focusing in right now... In any case, it appears a bit odd that the whole IC based circuitry is fed from the Neutral here but then the fuse Is really on the Neutral, too! (what’s the Sk05 marking meaning in this capacitor-like fuse btw?) So I guess this doesn’t matter to the PS operation. \$\endgroup\$
    – mosagepa
    Oct 3 '20 at 15:44

Solved! :)

I updated the question with a final, correct schematic and derived R27 value.

Also solved the (now funny) “enigma” for the SMD marked ‘390’:

  • First I thought it was ‘398’ (nonsense way huge R value)
  • Then thought it was 39R and I had a “dotted” zero as last digit
  • Turned out, final “digit” was not any digit but a ‘B’ !!!

Finally I noticed that this is an EIA-96-coded resistor (another nomenclature standard), see e.g.: EIA-96 SMD coding standard

So marking was really ‘39B’ which means a 2k resistor (measured it)!!! Why on earth -No pun intended! :)- does this not follow the usual marking convention, i.e. ‘202’ for a 2k value? I never faced this before (then I’m not totally used to SMD components in my ideas and designs). If someone can explain this marking’s logic, I’d appreciate.

Anyway, R27 must be a 2k, too, for the voltage divider to feed about 2V to the FB feedback input on the switcher IC. This “reference” FB value is mentioned in the Datasheet linked above, although there it’s shown the case for DC 12v output and quite arbitrary R values are given there.

The R values here for the 5V output case are quite convenient as you can see.

Here you can see the Diagram included in the "Application" mentioned in the Datasheet for the switcher IC, which inspired me to solve my own design's determining my R27 value (remember, this is for 12V output, so not entirely similar to my 5V case, and the labelling for the resistors and other components differ also from my schematic's):

Application of switching IC for 12V output case

OH! BY THE WAY! the right mode in which this design is operating is NOT the CCM (Continuous Conduction Mode) as I said initially, but the MDCM (Mostly Discontinuous Conduction Mode). Reason (according to the Datasheet) is that, D4 -in my schematic- being a "fast switching" UF4005 diode, 75ns Reverse Recovery (RR) time, is not "super fast", meaning that if you want the IC to work in the CCM "region" then this diode would have to be even faster than about 35ns (I don't know of any available replacement in this sense, and in any case this would only get us about 130mA maximum output current).

So, case closed I guess !!

Now it remains for me to figure out an appropriate "entry point" in which I should disconnect this design from the mains, in order to replace fully the Power Supply section with my own, "black usual brick" AC-DC adaptor. This is because, as I mentioned in another Question of mine in this website, I plan on fully controlling this Coffee Machine with ESP01+AtMega Tiny85 (i.e. project for a WiFi commandable machine), and the ESP01 module alone can draw as much as 750mA during WiFi-searching periods!!!

I guess that I could "cut" some connection in this schematic, to be able to add my own 5V supply with enough "beef" for this. BUT, I need to figure out:

a) which connection is going to be the GND common to the microcontroller's logic, since this also affects the gate voltages for the TRIACS in the original board, and

b) how to deal with the design being (oddly in my opinion) such that, it's the NEUTRAL which acts as positive "rail" in the AC-DC conversion here depicted, and how does this affect proper isolation for my newly (replacing) external 5V power supply.

This quirks I have yet to investigate deeper, no hurry.

Any further info/ advice/ remarks will be hopefully made in a separate thread, since this note closes more or less fully the original question I had.

Thanks to all you folks!


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