8 Replies Latest reply on Sep 4, 2020 10:09 PM by jw0752

    How to sense mains AC?

    johninjax

      Hey,

      I need to know if a latching SPDT relay is on the A or B side. The relay has a 5V control, but it will have 120VAC on it. I was planning to use a half wave rectifier and RC filter with large R to tap off ~5V into a gate. Am I crazy? Is there a better/simpler way?

       

      Thanks,

       

      John

        • Re: How to sense mains AC?
          colporteur

          Well labelling you as crazy is a conjecture at this point. Reducing AC mains to a lower voltage by using a dropping resistor might put you in the running.

           

          I'm not confident I have a full understanding of your circuit. You have a 5VDC powered SPDT latching relay you are going to use/test. You want to tap the AC mains using a dropping resistor, rectify and filter the output to power the relay?

           

          Why not just test the relay contacts for continuity? That can tell you what side is engaged.

            • Re: How to sense mains AC?
              johninjax

              Added a pic like what I am talking about.

              1 of 1 people found this helpful
                • Re: How to sense mains AC?
                  shabaz

                  Hi John,

                   

                  Could you explain your actual goal / use-case?

                  The trouble is, with the small bit of information, any answer may not be appropriate or may not help you, because the answer can be different based on context.

                  Some example things that can usually help (not all may apply):

                  1. My Goal in detail

                          (e.g. climb a mountain next year)

                  2. What things I'm trying to solve

                           (e.g. establish how high I am)

                  3. What constraints I have

                           (e.g. cost, software experience, hardware experience)

                  4. What hardware, tools and software I'm using

                           (e.g. part number, link to the tools and software)

                  5. Diagram or sketch to show the scenario, and what I've attempted

                  6. Where I believe the problem lies, or which part I need help on

                            (e.g. unexpected response from the part I'm using)

                  7. Non-blurry photographs of the set-up (zoomed out for context, and detail), and video if needed

                  Non-fuzzy, focussed Overview Photo (for context)

                  3 of 3 people found this helpful
                  • Re: How to sense mains AC?
                    jw0752

                    Hi John,

                    The picture didn't post. Make sure you use the Icon in the tool bar that looks like  to insert pictures with your text.

                     

                    John

                • Re: How to sense mains AC?
                  jw0752

                  Hi John,

                  I recently had a circuit that sensed AC mains using an AC optocoupler. This would allow you to sense the mains without a direct connection to the rest of your circuit. On my board they used a 120K resistor in series with the input to the optocoupler and the output of the optocouple was tied into a 5 volt circuit that interpreted it. Here is a link to my blog.

                   

                  https://www.element14.com/community/people/jw0752/blog/2020/02/22/ac-optocoupler-ltv-814

                   

                  John

                  5 of 5 people found this helpful
                  • Re: How to sense mains AC?
                    dougw

                    Looks like a pretty hairy way to try to reduce voltage. - not sure why the switch is wired like that.

                    I've seen some motor controllers that use a sort of capacitor divider (C-R-C) to supply low voltage to the control circuitry, but it was all self-contained. You wouldn't want to poke around in there expecting to find just low voltage.