Re: Sensing switch closure with & without power applied?

From: Fred Bloggs (nospam_at_nospam.com)
Date: 01/08/05 

Date: Sat, 08 Jan 2005 16:17:50 GMT

Fred Bloggs wrote:
>
>
>
>> Switch closure needs to be monitored. On one side of the switch is
>> mains voltage (120vac); on the other, the load.
>>
>> Simple enough to monitor the switch's status: when mains voltage
>> appears on the load side, it's closed.
>>
>> But how to monitor this switch during a power failure? I'd like to be
>> able to report to an alarm sensor's input (simple current loop) when
>> this switch closes, whether mains power is applied to the switch, or
>> power has failed.
>
>
> Simple enough to do in practice- but the environment can make things
> "sticky"- be sure your sensor input is isolated or else fuse it:
>
> View in a fixed-width font such as Courier.
>
>
>
>
> +-------------------------------+
> | |
> +----DPDT---+ +-----------------+ |
> | | | | | |
> |NC| | | | |
> | o COM | | level sw | |
> | \ o-----|--------o o------+ | |
> LINE>-+-------|--o | | \ | | |
> | |NO | | ^ | | |
> | | | | | | | |
> | | | | --- | | |
> | | +--------|--+ | | | | |
> | | | | | | | | |
> | |NC| | - - | | |
> | | o COM | | | |
> | | \ o-----|-----------------+ | |
> | +-|--o | | |
> | | |NO | | |
> | | | | | |
> | +-|-----------|-----------------+ | |
> | | | | | |
> | | ====== | | | |
> +-------|---UUUUUU--|--+ | | |
> +-----------+ | | | |
> +----------------------+ | | |
> | | | |
> | +----+ | | |
> NEUT>-+------------+LOAD+-------------------+ | |
> | +----+ | | |
> | | | |
> | | | |
> | | | |
> | lvl sw monitor relay | | |
> | +--------+ | | |
> | | ====== | | | |
> +----------|-UUUUUU-|-----------------+ | |
> | | | |
> | / | | |
> +--|--o o---|--------------------+------->
> | | NO | |
> | +--------+ | to sensor
> | |
> +----------------------------------+----->
>
>

You can do a better job of preserving the level sw contacts like so:
              View in a fixed-width font such as Courier.

  .
  .
  . +---------------------------------------+
  . | +-----------------------------------+ |
  . | | | |
  . | | +----DPDT---+ | |
  . | | | | level sw | |
  . | | |NC | +---------+ | |
  . | +--|--o COM | | | | |
  . | | \ o-----|----|---o o---|--+ | |
  . LINE>-----+-------|--o | | \ | | | |
  . | | |NO | | ^ | | | |
  . | | | | | | | | | |
  . | | | | | --- | | | |
  . | | | | | | | | | | |
  . | | | | | - - | | | |
  . | | |NC | +---------+ | | |
  . | +----|--o COM | | | |
  . | | \ o-----|-----------------+ | |
  . | +-|--o | | |
  . | | |NO | | |
  . | | | | | |
  . | | | | | |
  . | | | | | |
  . | | | ====== | | |
  . +-------|---UUUUUU--|---+ | |
  . | | +-----------+ | | |
  . | | | | |
  . | | | | |
  . | +-----------------|------+ | |
  . | | | | |
  . | | | | |
  . +---------------------------+ | | |
  . | | | | |
  . | | +----+ | | |
  . NEUT>-+----------------+LOAD+-----+ | | |
  . | | +----+ | | | |
  . | | | | | |
  . | | | | | |
  . | | lvl sw | | | |
  . | | monitor relay | | | |
  . | | +--------+ | | | |
  . | | | ====== | | | | |
  . +--------------|-UUUUUU-|----------+ | |
  . | | | | | |
  . | | / | | | |
  . +----------|--o o---|---+ | |
  . | NO | | |
  . | | | |
  . | / | | |
  . +--|--o o---|--------------------+------->
  . | | NO | |
  . | +--------+ | to sensor
  . | |
  . +----------------------------------+----->
  .
  .

       DPDT is low power, lvl sw monitor relay coil is only load

                              -or-
              View in a fixed-width font such as Courier.

  .
  .
  . +---------------------------------------+
  . | +-----------------------------------+ |
  . | | | |
  . | | +----DPDT---+ | |
  . | | | | level sw | |
  . | | |NC | +---------+ | |
  . | +--|--o COM | | | | |
  . | | \ o-----|----|---o o---|--+ | |
  . LINE>-----+-------|--o | | \ | | | |
  . | | |NO | | ^ | | | |
  . | | | | | | | | | |
  . | | | | | --- | | | |
  . | | | | | | | | | | |
  . | | | | | - - | | | |
  . | | |NC | +---------+ | | |
  . | +----|--o COM | | | |
  . | | \ o-----|-----------------+ | |
  . | +-|--o | | |
  . | | |NO | | |
  . | | | | | |
  . | | | | | |
  . | | | | | |
  . | | | ====== | | |
  . +-------|---UUUUUU--|---+ | |
  . | | +-----------+ | | |
  . | | | | |
  . | +-----------------|------+ | |
  . | | | | |
  . +---------------------------+ | | |
  . | | | | |
  . | +-----------------------+ | | |
  . | | | | |
  . | |<|TRIAC +----+ | | | |
  . NEUT>-+------| |---+---|LOAD|-----+ | | |
  . | |>| | +----+ | | |
  . | | | | | |
  . +--[1K]-+ +----[1K]------+ | | |
  . | | | | | |
  . | | | | | |
  . | | | | | |
  . | | lvl sw | | | |
  . | | monitor relay | | | |
  . | | +--------+ | | | |
  . | | | ====== | | | | |
  . +--------------|-UUUUUU-|----------+ | |
  . | | | | | |
  . | | / | | | |
  . +------|--o o---|---+ | |
  . | NO | | |
  . | | | |
  . | / | | |
  . +------|--o o---|--------------------+------->
  . | | NO | |
  . | +--------+ | to sensor
  . | |
  . +--------------------------------------+----->
  .
  .

       DPDT is low power, lvl sw monitor relay coil is only load

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