How to change Normally Open switch contacts to Normally Closed?

Thread Starter

km4hr

Joined Mar 7, 2018
2
I have a machine that has three limit switches that indicate the physical position of three moving parts of the machine. When the machine parts reach a certain position I need to activate a relay. If the limit switches all closed when the moving parts reached the desired position then activating the relay would be easy. The switches would simply complete the circuit to the relay coil. But unfortunately one of the switches is Normally Closed (NC). It opens when the proper position is reached instead of closing. Is there a way to reverse the action of the NC limit switch, perhaps using a transistor or something? All the other switches have both NC and NO contacts, but not this one. Replacing the switch would be difficult.
 

MrChips

Joined Oct 2, 2009
23,235
You can use a single transistor to invert the logic of the signal.
Whether or not this is a feasible solution will depend on the signal and voltages operating the equipment.
 

crutschow

Joined Mar 14, 2008
26,999
The easiest is probably Dodgydave's suggestion of an added SPDT relay with its coil controlled by the NC limit switch.
The relay is actuated when the limit switch is closed, and you use the NC contacts on the relay for your limit switch output.
Thus the NC relay contacts are closed when the NC limit switch opens, as you want.

The reason for a using a NC limit-switch is that it is intrinsically more fail-safe.
If the switch fails open, or there is a wiring or connection open-circuit failure, it stops the machine.
Such failures are more likely than a switch or wiring short failure.

If failure of these limit-switches can cause damage or a dangerous situation, then I suggest converting all the limit-switches to NC type.
 
Last edited:

MaxHeadRoom

Joined Jul 18, 2013
22,855
If failure of these limit-switches can cause damage or a dangerous situation, then I suggest converting all the limit-switches to NC type.
If it involves any form of E-stop circuit then it is mandatory to be fail safe.
This could also preclude any form of solid state device.
Max.
 
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