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I'm studying the attached washing machine partial schematic at the point in which the main microcontroller activates 2 water elecrovalves. I would ask:

  1. It seems the microcontroller pins 36 and 40 are connected by the 1k resistor to the main AC "ground" (which means phase or neutral). How is it possible that the 230V phase, and not the neutral, could be connected to the pin? From my experience an optoisolator is the correct choice.
  2. The triac is not provided by a snubber. It is correctly designed?
  3. Why does one of the triacs have a voltage divider with 560 Ohm resistor and not the other?

Thanks a lot

Washing machine scheme (partial)

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    \$\begingroup\$ "main AC "ground" (which mean phase or neutral)" does it? Or does it mean PE? Around here washing machines are required to have PE. This thing seems to be russian, if its something older it can also mean "we don't care about safety" \$\endgroup\$
    – PlasmaHH
    Commented Sep 28, 2018 at 14:17
  • \$\begingroup\$ If all inputs and outputs of the microprocessor are referred to the same ground potential it is no problem if this ground potential is actually the phase. But all keys and LEDs connected to the microprocessor should be isolated the same way as a wire conected to the phase should be to guarantee users safety. \$\endgroup\$
    – Uwe
    Commented Sep 28, 2018 at 14:24
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    \$\begingroup\$ From my experience an optoisolator is the correct choice. In general opto isolators are needed in devices that have a mains connection and a low voltage, safe to touch connector. Example: a phone charger. Without the opto isolator there would be a direct mains connection so the output would be unsafe to touch. But if there aren't any connections to the outside and the metal case is grounded, then there is no need for an opto isolator. \$\endgroup\$
    – Bimpelrekkie
    Commented Sep 28, 2018 at 14:25
  • \$\begingroup\$ Hi plasma, this machine is not old and not only dedicated to the russian market. \$\endgroup\$
    – daigs
    Commented Sep 28, 2018 at 14:40

2 Answers 2

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The two pins you mention drive the gates of triacs. Obviously the ground that the cathodes of the triacs are connected to is the same ground that the micro is connected to. One way or another, a low voltage supply is created somewhere to run the micro from, and the negative output of that supply is tied to the system ground.

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  • \$\begingroup\$ Thanks Olin, but the pin 36 "sense" about 220 V from the 1K resistor, regardless it is in high state (220-5v in this case) or low. Don't this voltage destroy the pin circuitry? Thanks. \$\endgroup\$
    – daigs
    Commented Sep 28, 2018 at 14:37
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    \$\begingroup\$ @daigs: Your comment makes no sense. Pin 36 is going directly to the gate of a triac, with the cathode of the triac grounded. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 28, 2018 at 14:45
  • \$\begingroup\$ Olin, the "ground" can be either 220V or neutral, depending how you plug causally your Washing machine electrical socket. It is the first time I see a line starting from a micro pin that through a resistor is connected to 220V. \$\endgroup\$
    – daigs
    Commented Sep 28, 2018 at 19:44
  • \$\begingroup\$ @daig: But if the micro's ground is also at the same 220 V potential, then it doesn't matter. It's ground from the micro's point of view. \$\endgroup\$
    – Olin Lathrop
    Commented Sep 28, 2018 at 21:59
  • \$\begingroup\$ Thanks Olin, now it is more clear. I need to drive some water 230 AC elecrovalve and I would avoid the optocoupler for cost and PCB semplification reasons. As in the posted schema, the matter is too complicated to be made by me and not so safe. From the scheme I see posted around the web and in application notes, If I would avoid the optocoupler (with the triac working in I and IV quadrant) I would ever need to connect the gate loop circuit coming from the micro pin (5V) with the triac pin, which is connected to the 230AC... \$\endgroup\$
    – daigs
    Commented Oct 5, 2018 at 11:10
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The schematic is only partial but it looks to me like they are using the mains "neutral" as "system ground". To add to the confusion it looks like they are using the "earth ground" symbol to represent system ground.

The microcontroller only cares about voltages between it's pins. As long as the voltage between the pins is acceptable the microcontroller will be fine. The fact that in some cases the whole circuit will be moving up and down at 50Hz relative to ground doesn't bother it**

In designing an electronic system with logic controlling mains there are essentially two options. One option is to refer the electronics to the mains supply (either live or neutral) and put safety isolation between the logic and the user. The other option is to have the logic either floating or reffered to mains earth and put safety isolation between the logic and the mains.

In large volume the former approach is often more economical as it avoids the need for a bunch of isolation components. Yes it means your buttons and displays need to have safety isolation but that can often be done at very little cost through careful design of the front panel.

* As you point out in your comments on Olin's answer the "Neutral" connection on the machine may end up connected to either Live or Neutral as some countries use unpolarised plugs.

** If the frequencies were much higher then moving up and down relative to earth ground could be an issue as stray capacitance is everywhere.

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  • \$\begingroup\$ In Russia you have Europlug and Schuko, so neutral and line are not distinct (unlike Switzerland, for example) but I think they implicitly assume a TN earthing system, so the "ground" connection is really ground and not "system ground", and they show it inside. \$\endgroup\$
    – Astrinus
    Commented Sep 28, 2018 at 20:29
  • \$\begingroup\$ I am 99% sure the "earth ground" symbol on this schematic is being used for system ground not mains ground (the only alternative explanation would be that the appliance has mains ground shorted to neutral which is clearly absurd). The use of the symbol on the schematic does not nessacerally reflect how that symbol will be used for labeling stuff in the appliance itself. \$\endgroup\$
    – Peter Green
    Commented Sep 28, 2018 at 20:32

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