why there is a difference connection between these two proximty switches ??

the proximty switch 24 VDC has three wires :

two of them which are the power supply 24 VDC which the postive terminal is the brown and the ground is the blue , the third terminal is the signal which is black

while the proximty switch 220 VAC has two wires only ( brown and blue ) and the connection of it similar to the limit switch

so i want to ask why there is a difference connection between the proximty switch 24 VDC and between the proximty switch 220 VAC ??

why the proximty switch 24 VDC has not two wires as the proximty switch 220 VAC and as the limit switch ??

thanks

Why has a diode only two wires and a transistor has three? Why has an English car the steering wheel on the right hand side of the car, while continental cars have the steering wheel on the left side of the car?

Because these are not the same. There are various kinds of proximity switches: you have two-wire and three-wire system, PNP and NPN, AC, DC and even combined. You'll have to Google the internet for some kind of proximity switch 101 to get more info on the internals of proximity switches.

Kind regards,

thanks , but do not speak with me again by cynically !!


i only ask to learn

Jean Pierre was right -- they are different because they're built that way. Because the technology that's inside them requires them to be built that way. Don't worry about it -- just find the best prox that works for your situation and hook it up according to the manual.

thanks

magdyfayad said:

thanks , but do not speak with me again by cynically !!

Click to expand...

Sorry if you feel that way. It was not my intention to be cynical. I was only trying to help.

Kind regards,

thanks and i have already intered to google and search about my question

they are different animals and may not behave the same way in som situations. (speed, leakage current, voltage drop etc.). for example two wire solid state devices have greater leakage current and voltage drop (less than perfect switch). some controllers have very sensitive inputs when it comes to leakage. on the other hand, two wire is usually more practical for those who have to deal with both sinking or sourcing. more over some can handle AC and DC and quite wide voltage range. you need to know what you want from sensor when selecting one.

A 2 wire proximity must draw enough current for its electronics to be working even when it is not producing an ON signal. Thus it always draws a small amount of current. You must pay attention to this when designing a system. Very sensitive systems may turn on just from the small amount of current needed even in the OFF state. When in the ON state there is the necessity of a voltage drop accross the device to keep the internal electronic circuit running so it cannot deliver the full circuit voltage.

A 3 wire prox draws all operating current through 2 wires and provides the signal on the third wire. Thus the signal wire can be in a near true OFF state. Of course a solid state output will have a very slight current. A relay contact output will have very low leakage (there may be a protective circuit accross the internal contacts). So while there are more wires there is less to be concerned about in their application. Because the output is seperate from the power leads it can provide an ouput closer to the full circuit voltage than the 2 wire device.

Thanks All

Which mean the three wire is more safe and is the better ?

but if we need source or sink device the two wire is then the better , is it right ??

thanks

It's hard to denote one as 'better' than another. You must research your application thoroughly. Leakage current is always a concern when driving PLC inputs and other low curret loads.

While many 3 wire sensors are dedicated NPN or PNP outputs I'v seen a couple that were switchable.

most my applications are the proximty connected to realys or contactors or sometimes PLC , I found 99 % of my projects use three wires proximty switch , but only at one case i found my project use two wires proximty switch which the proximty is connected to a selonid valve

please i want to know what do you mean exactly by this staement " Leakage current is always a concern when driving PLC inputs and other low curret loads "


thanks

If the leakage current (which as I explained is much higher on the 2 wire devices) is more than the Maximum OFF Current for a PLC input (or any other device for that matter) then the input/device will fail to turn off. The leakage current applied to these high impedence devicesd creates a voltage sufficient to turn on the device (or at least once turned on fail to turn off).

thanks again

bernie_carlton said:

If the leakage current (which as I explained is much higher on the 2 wire devices) is more than the Maximum OFF Current for a PLC input (or any other device for that matter) then the input/device will fail to turn off. The leakage current applied to these high impedence devicesd creates a voltage sufficient to turn on the device (or at least once turned on fail to turn off).

Click to expand...

I have also seen where the leakage current could damage the input card, I learned this the hard way when using a 2 wire capactive prox in place of a 3 wire when the correct switch was not available. While switches can be interchanged in most applications, there are times when the OEM design criteria must be followed. :site: