How should the “normally closed” function of the Namur proximity switch be understood?
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I used to think that the Namur signal limit switches used in control valves did not have the concept of \"normally open\" or \"normally closed,\" and instrument drawings never indicated whether a Namur switch was normally closed. But today I found that the manufacturer’s documentation indeed states that it is normally closed, and there is only the option of normally closed – no option for normally open or normally closed. How should this be understood?
This proximity switch has only one normally closed contact. If you choose the normally open or normally closed wiring method on-site, you need to select the right model; for the feedback signal of the control valve, it should be the normally open contact, right?
I’m not quite sure; isn’t a NAMUR signal simply two current values that are used to achieve a digital switching effect? Is it treating a small current as an open circuit and a large current as a closed circuit? It often carries large currents, which is why it’s called normally closed? Well, all of the above is nonsense.
Personally, I think it is normally closed when no signal is triggered, and it becomes open once a signal is triggered
Isn’t the principle of the NAMUR signal a current of 1.2MA to 1.55MA that is generated when a metal object approaches the sensor? Does current also have normally open and normally closed types?
The working principle of the NAMUR proximity switch is as follows: it requires a DC power supply of around 8V. Depending on the distance of a metal object from the sensor, different current signals are generated; a current range of 2.1 mA to 6 mA corresponds to state 1 or ON, while a current range of 0.35 mA to 1.2 mA corresponds to state 0 or OFF. A circuit open fault is considered when the current is less than 0.28 mA, while a short-circuit fault is considered when it is greater than 6.5 mA.
In fact, when conducting inspections, we rarely measure the current; instead, we directly measure the voltage, which is 4V and 8V. I’ve conducted actual tests here: when the metal is close, the voltage is 8.42V and the current is 0.56mA (normally open); when it is far away, the voltage is 4.18V and the current is 4.87mA (normally closed). Is that correct?
The instructions state that if the current on the measurement board is >3mA, it is a normally closed contact; otherwise, it is detected
Thank you very much to all the experts for their explanations
It is actually a change in current; it can also be understood as low-resistance and high-resistance states. This SJ3.5N can be used with the WE77 to create dry contacts.