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AC Polarity when using a solid state power relay

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  • AC Polarity when using a solid state power relay

    Ok, something is gnawing at my brain lately.

    When using a solid state power relay to take the load off of the run switch, how critical is AC polarity?

    I have done this before with success, as well as carefully marking the polarity on the outlet box, but I'd like to make it more foolproof. I know at least one member has reported using an IEC socket for this purpose. I'd like to just keep the stock AC plug, wire it up correctly, and make sure whoever uses it to plug the cord in properly, but what are the actual consequences of it being backwards, if it is not earth grounded?

  • #2
    AC solid state relays are not polarity sensitive. You may connect "supply" and "load" to either terminal with wild abandon 8)
    Tom in Tulsa

    Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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    • #3
      That's what I figured. I guess the main concern would be fusing, and fairly cheap insurance is to fuse both incoming AC lines.

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      • #4
        You raise an interesting issue. Are you referring to the vintage non-polarized 2-pin male ac connector on the outlet box of a "B" style cabinet, for example?
        Fusing both sides presents an interesting possible failure mode. Say that a transformer primary winding overheated, causing a short from the winding the the chassis. The over-current condition could theoretically open the "cold side" fuse leaving the "hot side" fuse intact. This would leave the chassis "hot" even though the system appeared to shut down when the fuse blew 8) FWIW, I'm actually rewiring my BC with vintage three-pin Amphenol connectors to implement grounding for both organ and tone cabinet/Leslie hardware.
        Tom in Tulsa

        Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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        • #5
          Another possibility would be to install a polarized 2-pin connector and fuse the hot side...
          Tom in Tulsa

          Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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          • #6
            How can an AC current be considered polarized when it`s changing direction 120x/ sec?

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            • #7
              Originally posted by tpappano View Post
              You raise an interesting issue. Are you referring to the vintage non-polarized 2-pin male ac connector on the outlet box of a "B" style cabinet, for example?
              Yes I am.

              Originally posted by tpappano View Post
              Fusing both sides presents an interesting possible failure mode. Say that a transformer primary winding overheated, causing a short from the winding the the chassis.
              One would hope a fuse would catch that, but say it did not, then yes that's bad.

              Originally posted by tpappano View Post
              The over-current condition could theoretically open the "cold side" fuse leaving the "hot side" fuse intact. This would leave the chassis "hot" even though the system appeared to shut down when the fuse blew 8) FWIW, I'm actually rewiring my BC with vintage three-pin Amphenol connectors to implement grounding for both organ and tone cabinet/Leslie hardware.
              I think the safest bet to protect against that scenario would be to use a plug-in GFCI adapter.

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              • #8
                In this context, a "polarized" ac plug is a 2-pin plug that can only be inserted one way. In the US one blade (the neutral) is wider than the other (hot). These days most 2-pin ac plugs are made this way.
                Tom in Tulsa

                Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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                • #9
                  Yes, a gfi will trip if there is already a path from chassis to ground such as external equipment that is already grounded, or a human comes along later and creates a ground path with their body.
                  Tom in Tulsa

                  Fooling with: 1969 E100, 1955 M3, 1963 M100, Leslie 720

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                  • #10
                    Any time you make a change like this, it's best to switch to a grounded three pin plug, or IEC socket. Fuse just the hot, and ground the chassis. No extra fuses or gimmicks.

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