question om slc 150

I have a SLC 150 running a machine my dad built 14 years ago and it use proximity switch that are tied to the slc 150.One of the proximity switches has gone out can anyone tell me what kind I need .Some have two wires and some have three wires I know that they are dc they measure 19 volts while they are on they have a 18 mm threaded dia. and are approximately 3.250 inches long with a led light.

Welcome to the Forum !

As you've found, DC proximity sensors can be built as 2-wire devices (+DC and Signal) and as three-wire devices (+DC, -DC, and Signal). 18mm threaded barrel is the most common physical package.

But there are literally thousands of different sensor types, speeds, and sensitivities.

Are there any part numbers on the switch ? Maybe you could post a photograph and it might look familiar to a Forum user.

The good thing about inductive sensors with threaded bodies is that they are pretty standard across multiple manufacturers, and you can find something that works easily.

Personally, when ever I find a 2 wire DC inductive proximity switch that has failed, I replace it with a three wire. I just have a thing against 2 wire proximity switches.

You can find something from Balluff, Turck, IFM Efector, etc.

Take a look here to find one that meets your specifications. If you go to a 3 wire from a two wire, you'll just need to purchase a three wire cable and do a little bit of wiring (but in my opinion, it will save hassle down the road).

http://www.ifm.com/ifmus/web/inductive_sensors.htm

heres a link to try:
http://www.automationdirect.com/adc...tive_Proximity_Sensors_-z-_Proximity_Switches

If you can not find the part number I would suggest replacing it with a 3 wire DC prox. The problem with 2 wire is that they will vary on leak thru current and can give you a false reading. THis is handled alot of times by putting a shunt resistor across the sensor. Hince most peoples preference for the 3 wire sensor.


If you do not have an account with an industrial supply house and need to order the part by credit card the web site that BCE posted is a good place to go.

Why do the 2 wire leak more than the 3 wire?

Is there any other reason to choose 3 wire over 2 wire?

2 Wire switches need a little power to power up their circuitry, just like a 3 wire does. But the current for their circuitry must flow on the 2nd wire, being the wire to your input.

The only real advantage I've ever seen for 2 wire over 3 wire is the fact that you only have 2 wires, rather than 3. Quicker (Slightly) and cheaper to wire.

I've found that people understand 3 wire sensors easier than 2 wire. They find it easier to grasp that you supply 0VDC and 24VDC to power up the sensor, and on the third wire you get a signal coming back depending on the state of the sensor.

Tim Ganz said:

Why do the 2 wire leak more than the 3 wire?

Is there any other reason to choose 3 wire over 2 wire?

Click to expand...

In addition to the reasons stated by Tharon, I also consider 3 wire sensors to have a higher degree of noise immunity.

Tim Ganz said:

Why do the 2 wire leak more than the 3 wire?

Is there any other reason to choose 3 wire over 2 wire?

Click to expand...

The issue has to do with solid state devices. Whenver you use a 2 wire sensor a certain amount of current has to travel down the input wire to power the sensor. This will vary depending on the sensor. The sticky part is your input card. Each dicrete input has a certain current level (most people assume it is voltage) where it will switch its transistor from off to on. If the leakage current is greater than the switching current the input will show a false on state. You can get around this with a pull down resistor installed in the circuit but this adds to the complexity of the circuit so it is generally simplier to use a 3 wire sensor. Also the current leakage spec and the switching spec on PLC cards tends to be a range instead of a fix value so it can bite you in the rear when you least expect it.

just a note ,I prefer the Imf brand its just eaiser to order the AD stuff for alot of people. however you can buy from imf with a credit card I think, I have an account with them so Im not sure. also I never use the 2 wire stuff on new apps.