On Fri, 7 Jan 2005 16:02:26 -0800, the renowned DaveC
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.

When power fails, I could put a simple battery and coil of a l.v. relay
across the switch. But when mains voltage reappears, removing the battery
circuit in time becomes a bit risky.

How best to accomplish this task? Ideas?

Thanks,

Maybe something like this:
20K 20K
___ ___
+---|___|-------+--+----|___|-Vdd
| | |
| _/ | |
o------+--+ +----o/ o-+-------------- A
| | | | | |
V - | | | |
- ^ | | | +------ B
| | | | | |
+--+---------------+ .-. | |
120VAC | | | |
| | V -
Load '-' - ^
| | |
o------------------------------------+----+--+-------


If there is 120VAC on A, then the switch is closed

If there is not 120VAC on A, but (say) 300mVDC on B, then
the switch is open. You might have to shunt the load with a
resistor and perhaps reduce the mV comparison level if the load
doesn't look like a relatively low resistance.

It reduces the load voltage by one diode drop and the diodes on the
left have to be rated for the load current. The 20K on the left has to
be rated for any reasonable line voltage transients.



Best regards,
Spehro Pefhany
--
"it's the network..." "The Journey is the reward"
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