Output signal of a NPN limit switch

[jungle]

Hi everyone,
I am using this limit switch (LS): D-F9N (NPN - sink) output. I used an oscilloscope to monitor its output. The observed output is under two cases as follows:
1. Zero volt DC if the LED of the LS on.
2. A noising signal around zero if the LED off.
Is it a normal working condition for LSs? (The LS is powered at 24VDC)
Please give me some suggestions.
Thank you.

 

[KlausST]

Hi,

I don't know what a D-F9N switch is....

But any low side switch can (as it name says) only switch to GND (=LOW).
If not switched LOW, then it is open = high impedance, this causes a floating node.

The voltage of a floating node is undefined. It may be DC of any value, AC of any value or any random waveform.

It's like a piece of wire in your hand.

A voltmeter is no useful device to "measure" the high impedance state. An ohm-meter is more useful.

Klaus

 

[FvM]

A NPN switch has no useful output signal without the pull-up resistor of the connected load, e.g. PLC "NPN" input.

 

[jungle]

Thanks for the useful reply!

One more question, should I connect to a pullup- resistor (If yes, how can I calculate resistor value?) or just directly connect is okay?

 

[D.A.(Tony)Stewart]

Thanks for the useful reply!

One more question, should I connect to a pullup- resistor (If yes, how can I calculate resistor value?) or just directly connect is okay?

A direct connection to 24V would burn out the transistor switch.

Normal use is 10mA or 2.2k to 2.4k in series to 24V if desired. with < 0.8V when the NPN conducts.