I am planing to design a controller for a universal washing machine motor based on the TDA1085c. While designing the schematic, i have some problems to understand the sizing of some resistors.

Here you can find the datasheet of the TDA1085C.

The resistors to which I refer, are R10 for Pin10 and R15 for Pin15. In my opinion it is not very good described how to size these resistors.

Does anyone has a clue?


From the datasheet one might conclude that R10 is tuned by measuring. (On page 3 you have the section for VOLTAGE REGULATOR which can give additional clues)

And R15 is involved in a calculation with the C14 capacitor to determine que frequency (albeit the formula is not given). On page 3 you have the specification for pulse duration in the TRIGGER PULSE GENERATOR SECTION: Trigger Pulse Duration (CPin14 = 47 nF, RPin15 = 270 k)

You can also start with the values used in some existing circuit like this one (which has also 47nF for C14):

Beware the given schematic is for 230V.

  • \$\begingroup\$ Thx for that answer, I was guess I have to do some measuring.One more question: Any idea why R14 and C10 are needed in parallel to the triac? \$\endgroup\$
    – S.G
    Oct 18 '18 at 15:58
  • 1
    \$\begingroup\$ R14 and C10 form what is called "snubber". When the load is inductive, current and voltage are not in phase, so it is designed to ensure that the triac turns off. See st.com/content/ccc/resource/technical/document/application_note/… \$\endgroup\$
    – Undertalk
    Oct 18 '18 at 16:15

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