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normally open switch - monitoring ?

normally open switch - monitoring ?

normally open switch - monitoring ?

I have a normally open switch that temporarily shorts out part of a control circuit for machine to continue operation,

from a safety perspective I wish to monitor this switch in case it 'welds shut'

volts = 24v dc
current = 1A max

one switch is reed type, other is plunger type Link

apart from bespoke electronics to do this, does anyone know of safety type normally open switches

or some off the shelf solution for this problem


RE: normally open switch - monitoring ?

One could use the old trick of a lite bulb across the contacts. These days it would be a high brightness (sensitive) LED. The obvious problem is that it'll be dark when the contacts are welded. Opposite of good design sense.

So the next evolution is a sensitive relay. Etc. ... Obvious.

There will be a few mA leakage with this sort of approach.

RE: normally open switch - monitoring ?

don't quite understand you use of "safety" as the application seems to be a normal operating mode

having said that, where you have a critical operation and are concerned about a single device failure, simple redundancy is commonly used.

without a complete description of your equipment (wiring/mech drawings) you might use two swtiches wired in series and both activated by the same mechanism. That way it is necessary for two switches to weld shut to cause problems.

RE: normally open switch - monitoring ?

Quote (hacksaw)

...you might use two switches wired in series and both activated by the same mechanism. That way it is necessary for two switches to weld shut to cause problems..

Given the architecture (series, NO, and you propose used together), combined with welded contacts being the concern, I'm not convinced that a second switch gets one very far. If something happens to weld the first set of contacts closed, and lacking any indication, the second switch's contacts will be welded closed shortly thereafter.

A second switch in series is a good idea, but could perhaps be better employed as a manually operated switch used as a emergency back up to open the circuit. It should be much heavier duty, and can be since it would be a manual switch.

Perhaps a fuse could eliminate the over currents that are welding the contacts.

RE: normally open switch - monitoring ?

to help this discussion i have attached snapshot of the circuit, you can see the x2 switches marked as 'override'.


the dc current flows from left to right in pic

the ovveride normally open is only actuated (whether plunger or reed), for small amount of time, say around 3s.

the two overrides are part of separate circuits in effect. One for one direction of machine, and one for other direction of machine,

if one welds shut, the normally closed 'safety edge' switches (N/C) will be permanently shorted out.

RE: normally open switch - monitoring ?

24V 1A is not rocket science. The OTS solution is a switch designed to handle more than 1A so it won't weld itself closed.

Is there some other design constraint missing?

Also, why are you bypassing your safety switches as part of the normal operation? Safety switches are there for when something goes wrong. Are you sure you understand this architecture correctly? It might be time for a redesign if this is a new circuit.


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