I am building an automated fermentation controller for a brewery which integrates 110V and runs an Arduino with respective sensors and other valves.

Can you offer any feedback on my overall schematic below? New to working with high voltage (left side)

enter image description here

  • \$\begingroup\$ You'd need to post the schematic in the question, not on an external link which may die. Your question may be closed as "opinion based" unless you have a particular answerable design question. You can edit to improve it. The Tour may be useful to you. \$\endgroup\$
    – Transistor
    Sep 26, 2020 at 12:57
  • \$\begingroup\$ @Transistor- Can I not ask for general design feedback on here? e.g. electronics.stackexchange.com/questions/290558/… \$\endgroup\$
    – sensei247
    Sep 26, 2020 at 13:40
  • \$\begingroup\$ You can ask, but you might not be successful. You've got two close votes already. Just edit to ask a specific question on a section where you have doubts. \$\endgroup\$
    – Transistor
    Sep 26, 2020 at 13:45
  • \$\begingroup\$ A GFCI that's only connected to one side of the AC mains? How does it work? \$\endgroup\$ Sep 26, 2020 at 18:55
  • \$\begingroup\$ @BruceAbbott- Sorry uploaded wrong diagram. Updated above. Thanks! Any other thoughts? \$\endgroup\$
    – sensei247
    Sep 26, 2020 at 20:03

1 Answer 1


GND shouldn't connected to earth ground (main ground). But you can protect low voltage circuits with varistors connected to earth. In case 110V would appear on a 5V or 12V line, the varistor will clamp and trigger the circuit breaker. Use varistor with working voltage +- 2 volts above the voltage of the line.

The amperage of the main circuit breaker should be as low as possible. If you installation will never use, in any circumstance, more than 5A, then choose a 5A breaker. it will be safer.

It's much safer to use a differential breaker, for use in bathrooms, since you are working with liquids.

I would use 12V relays and valves. Less curent needed. Using a separate power supply for the relays and valves is a good idea.

I would also use as many 12V, 15V or 24V devices as possible instead of 110V ones if you can find them.

Don't use thin wires with low voltage where significant current must be drawn (>1A).

Working with "high" voltages is more a matter of care in the montage, the quality and robustness of the connectors and the isolation. The high voltage part should be distinct and physicaly separate from the low voltage part. Don't put high voltage wires near or above the low voltage section. Put the high voltage section in a separate enclosure.

Use only monolithic wires except with moving parts if any and the main cord if the installation has to be plugged in a wall outlet.

Every metallic part should be grounded to earth ground.

  • \$\begingroup\$ @Fredled- thanks so much for your help here. Greatly appreciate the time you put in to write this. I updated the schematic per your rec of removing GND/Ground connection. 3 follow up questions: 1. Varistors: Where is an example in my schematic where you would place the varistor? Outbound on the voltage line from the 12v and 5v terminal bus? If helpful, the 5/12v supplies will be dedicated switched supply modules. 2. See below \$\endgroup\$
    – sensei247
    Sep 26, 2020 at 22:11
  • \$\begingroup\$ @Fredled- continued: 2. As for differential CB, isn’t a GFCI doing this exactly? If a difference in current is identified, the CB trips on GFCI breaker therefore this seems to operate the same as a differential CB…correct? 3. Relays: I need to run dedicated brewing pumps/heaters which are 110v unfortunately. In this case, is it correct to assume the main advantage of moving to 12v active low would be to reduce current draw? \$\endgroup\$
    – sensei247
    Sep 26, 2020 at 22:12
  • 1
    \$\begingroup\$ @Fredled what is the explanation why the GND should not be connected to earth ground? If the power supply must be earthed, it might connect the GND to earth internally already, like many power supplies do. \$\endgroup\$
    – Justme
    Sep 26, 2020 at 22:50
  • \$\begingroup\$ @Justme Because you want a clean circuit out of your power supply. If you add the planet Earth to your circuit you don't know what can happen. Often, nothing will happen. But there is a risk of interference. Another reason is, that if the neutral is disconnected, the phase could arc inside the transformer. \$\endgroup\$
    – Fredled
    Sep 27, 2020 at 22:01
  • \$\begingroup\$ @sensei247 You can place varistors at the outputs of the power supplies. And at the outputs of the Arduino if they are not used with frequencies (serial data). You can place a varistor between + and - and between + and Earth, and between - and Earth. \$\endgroup\$
    – Fredled
    Sep 27, 2020 at 22:08

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