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Can this design cause any problems? I'm planning on switching 75W (220V) loads with this relays. I'm using 10 such relays and terminals (HB9500M). Note that it's highly unlikely all of the relays to be active at the same, even if they do, it won't last more than 2 seconds. I'm also planning on soldering wires with the PCB traces to improve current capabilities. PCB is using 1oz traces. As you can see, small red trace is controlling the relay, which is controlled by ULN2803. Can this switching induce unnecessary noises in the MCU? BTW all relay loads are resistive.

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    \$\begingroup\$ I would just try it and see, BTW it is slightly safer if you swap line and neutral around, so that the switched-off loads do not get line voltage. \$\endgroup\$
    – user253751
    Mar 26, 2021 at 16:36
  • \$\begingroup\$ @user253751 Yep, I didn't realized it till after I've completed the upload. thanks anyway. \$\endgroup\$ Mar 26, 2021 at 16:46
  • \$\begingroup\$ What is the grey shape displayed on top of the input connector? \$\endgroup\$
    – user253751
    Mar 26, 2021 at 17:13
  • \$\begingroup\$ If you are going to sell that or use it in your home, better figure out if it is made according to your local electric code. I think the clearance distance between mains voltage and low voltage is too small. Especially since many countries have bipolar plugs so use can plug in the mains so do not expect live and neutral to be live and neutral as they can be either way. \$\endgroup\$
    – Justme
    Mar 26, 2021 at 17:49
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    \$\begingroup\$ @user253751 I have made a board cutout since live and neutral are extremely closer together \$\endgroup\$ Mar 26, 2021 at 18:19

2 Answers 2


When switching the mains (230 VAC) to a load your most important concern is:

How long will the relay's contact last?

Each time you switch the mains, the energized contact of the relay will create a small electric arc in the air that goes to the non-energized contact. That arc shortens contacts' life.

To prolong relay life you might want to buy relays with silver plated contacts:


There's another and I think more interesting idea to prolong contacts life. It's called "zero crossing relay switching"

Switch the mains when the 230 V voltage crosses the time axis. Take a look at this Microchip application note:


  • \$\begingroup\$ I haven't even consider the arcing scenario, btw the relay turn on for 2 second and turn off for 2 seconds for at least 5 minutes per day. \$\endgroup\$ Mar 27, 2021 at 6:44

Just general comments:

  1. Line up each relay with its terminals for aesthetics and ease of debugging.
  2. Make the blue traces come in vertically to the common pin to maximise clearance between them and the coil pins.
  3. Move the red traces onto the bottom of the board. You have not need to move them to the top. That will keep all the mains voltages on the bottom and avoid electric shock to probing fingers.
  4. As pointed out in the comments, you should switch the live wire, not the neutral.
  5. Consider the use of one or more fuses.
  6. 75 W at 220 V is about 1/3 A. You should find a decent trace width calculator on line to work out this for you.
  7. Your live and neutral terminals have an unnecessary dance around each other. This brings the Line trace close to the edge of the board and unsuspecting fingers. Connecting the common to the neutral input solves that problem.
  8. You have plenty of space so you can use much wider lines from the ULN2003 to the relays.

Regarding the actual question, it's unlikely that you'll see noise on the CPU from the switching.

Have fun, but keep it safe!

  • \$\begingroup\$ I don't expect any fingers tinkering around the edges coz this whole PCB's going into an enclosure. but I will take that into my consideration. better safe than sorry, right?. Thank you for your feedback. it has been really helpful \$\endgroup\$ Mar 26, 2021 at 18:27
  • \$\begingroup\$ I have calculated the trace width, it's about 12mm. but it's not possible to make a trace that big in my design. so I'm going to solder some wires with traces and finish it with an insulation spray. \$\endgroup\$ Mar 26, 2021 at 18:32
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    \$\begingroup\$ 12 mm seems way, way overboard for 300 mA. Try 4pcb.com/trace-width-calculator.html and set a sensible temperature rise of 10 to 20°C. Cross-check with another site. \$\endgroup\$
    – Transistor
    Mar 26, 2021 at 19:21
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    \$\begingroup\$ I have changed @Enrico 's answer as "accepted answer" since it has the potential to help more people in our community . I hope you don't mind and understand that I'm still grateful for your answer.... anyway, the trace width has returned as 6mm, I think it's acceptable \$\endgroup\$ Mar 27, 2021 at 6:37

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