I have PCB from broken dishwasher and I would like to learn something from it. There are 6 smaller - STMicroelectronics Z7S, 1A triacs and one bigger STM T4 3570. My guess is (cannot verify it now) that big one is used for Wash pump motor and smaller ones Lower wash arm valve, Drain pump, Salt valve, Water intake valve, Detergent door release.

At the bottom of PCB there are some LEDs connected to ULN2004AG - Darlington Array IC by their cathodes ( - ) . Other LEDs pines ( anodes(+) ) are short-circuited with small smd transistor . Base of that transistor -> MCU and to make sense for me Collector of that transistor is connected to 5V.

But I have checked by multimetr and all of that 7 triac A1 (MT1) pin is short-circuited with this smd transistor to Collector. MT2s are board pinouts.

I have seen some schematic of triac connected to MCU but always Netural of ACmains was connected with ground of MCU

enter image description here

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How that schematic ( 5V and Phase connected together ) can work without killing MCU and other DC parts?


I have prepared some circuits : enter image description here

"In this case, we choose to tie the MCU Vdd pin to the AC Mains HOT conductor." Do we have to tie Vdd(5v) to ACMains HOT? We it can't be like that: enter image description here

  • \$\begingroup\$ Can you clarify what you are asking a bit? Those last couple of paragraphs are hard to follow. \$\endgroup\$
    – justing
    May 11, 2015 at 23:55
  • \$\begingroup\$ Your "small transistors" MAY be opto couplers or similar. Some part numbers and/o sharp well lit photos would help immensely. It is possible to design what you are asking about without isolatio but we need to know the real question. \$\endgroup\$
    – Russell McMahon
    May 12, 2015 at 5:49

1 Answer 1


What you are describing is one of the methods I use to drive triacs. The Triacs MT1 terminals are connected to the AC Mains HOT conductor and the load common leads go to the AC Mains Neutral conductor.

I create a negative DC supply voltage relative to the AC Mains HOT conductor. That means that the micro Vdd pin is also connected to the AC Mains HOT conductor; the micro Vss pin is 5.1 Vdc below that rail (-5V supply).

That means that the micro output pins are HI when the triac is supposed to be OFF. The micro output pins goes LO (-5V) when it's time to turn the triac ON.

Simple and effective.


It was mentioned that you are asking why this does not damage or kill the MCU. The answer is that the microcontroller is floating with respect to Earth Ground. Because it is floating, you can tie either the Vdd or Vss pins to whatever voltage level you want.

In this case, we choose to tie the MCU Vdd pin to the AC Mains HOT conductor.

The only thing that matters is: Keep the voltage between Vss & Vdd (and all the I/O pins) to the value that the manufacturer specifies -and- Stay within the current rating of the MCU pins.

  • \$\begingroup\$ The OP is asking why this circuit does not kill MCU. \$\endgroup\$
    – ilkhd
    May 12, 2015 at 0:53
  • 1
    \$\begingroup\$ His "small transistors" MAY be optos or similar. \$\endgroup\$
    – Russell McMahon
    May 12, 2015 at 5:48
  • \$\begingroup\$ Do we have to tie Vdd(5v) to ACMains HOT, what's it purpose? I've made EDIT in first post. \$\endgroup\$
    – user76291
    May 13, 2015 at 21:42

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