Help in 2-wire sourcing input

[LadderLogic]

++However if I just hook the 24VDC OUT to the brown wire and put the blue on the input, won't I also have to attach the input to the common also or is this done internally? Am I just way off base here?++

The COM terminal on your PLC must be connected to your -24VDC (or DC COMMON) point. Otherwise, the circuit is not complete and none of your inputs will ever work.

Think of a 2-wire sensor as a 3-wire one, with the black wire removed and the blue one used in its stead. Disregarding all that was said above regarding the LED indicator, it is just a normally open contact... Or, if you consider the LED, it is just a diode in series with an N.O. contact, that can be turned on or off...

 

[Eric Nelson]

However if I just hook the 24VDC OUT to the brown wire and put the blue on the input, won't I also have to attach the input to the common also or is this done internally?

Yes, you WILL have to attach it. It is NOT done internally.

It doesn't matter where you get your 24VDC from, as long as it can supply the current you require (as Tim mentioned). Look at the 'on-board' power supply as just that, a 24VDC power supply. It only has 200ma, but it's still just a power supply...

Just make believe that it's an 'external supply' and it might make sense. The 'DC OUT' has terminals labeled +24V and COM (or something like that). So would an external power supply, so think of it as an external power supply that happens to be firmly attached to the PLC...

beerchug

-Eric

P.S. You might want to try connecting a pushbutton in place of your reed switch to make sure you have the polarity correct. If a pushbutton won't work, then your reed switch won't either...

 

[Eric Nelson]

Aw, ya beat me Ladder!...

beerchug

-Eric

 

[LadderLogic]

Eric:

I am just having a sleepless night... Don't tell me you are doing time at night shift

 

[JesperMP]

Test the reed switch without the PLC is in the circuit:

24V on the brown wire.
Multimeter on "Volt" setting and one lead on the blue wire.
Connect other multimeter lead to ground.
(Alternatively put the multimeter on "resistance" and put a lead on both brown and blue wire).
WARNING: Multimeter on "A" or "mA" will fry the reed.
Now manipulate the sensor with a loose magnet. Yous should now be able to determine if the sensor is OK or not.

If OK, put the sensor on the cylinder and try to find where it is triggered by the internal magnet.

If OK, connect to the PLC circuit ... well you can probably continue yourself.

Also: Are you absolutely sure that it is just a "reed" switch ?
Some cylinder sensors I have seen have a lot of electronic circuitry inside that little brick.

 

[scandle]

WOW I spent a lot of time trying to word stuff so as not to make anyone upset. You guys are on top of it. I get up relly early before work. Maybe I need to sleep more. I did the multimeter test but the internal LED resistance is quite high and altered my confidence. Thanks Eric Nelson I will do that. You guys are posting before I can get my replies done. Did anyone check out the differences in the manuals. Link below is broke. Try this one page 5.3-7 and Page 145.

http://eul0600038.eu.verio.net/Mult...en2(UK)/Serial2(UK)/Related/DSeries_(UK_).pdf

Page 145 of this
http://www.ab.com/manuals/cp/1747621.pdf

steve out

 

[JesperMP]

scandle:

I did the multimeter test but the internal LED resistance is quite high and altered my confidence.

This confuses me quite a bit.
What value [volts] do you get ?
If there is only a LED diode in series with the reed, then the internal resistance when measuring between brown (+) and blue (-) should be very little.
Maybe there is a resistor in the circuit (to limit the current so that the reed is protected). But in that case the resistance must be so little, that you still get 24V out of the sensor, when connected to a PLC input.

1. Check the sensor as described in my previous mail.
2. Check the PLC input: Simply connect the 24VDC OUT from the PLC PSU to the input terminal that you need to test. If it goes ON, you have a working circuit.

 

[scandle]

The horse is dead!!!

I have connected the VDC OUT COM to the DC COM terminal. This is where I originally ran into trouble. I thought all commons were connected internally, not just the DC COM and inputs.

This confuses me quite a bit.
What value [volts] do you get ?
If there is only a LED diode in series with the reed, then the internal resistance when measuring between brown (+) and blue (-) should be very little.
Maybe there is a resistor in the circuit (to limit the current so that the reed is protected). But in that case the resistance must be so little, that you still get 24V out of the sensor, when connected to a PLC input.

All of this stuff canfuses me. Anyhow I checked it again and the resistance is 5.8M and drops to 2.6M when in proximity to the magnet. (I used a very strong magnet). 24VDC did leave on the -blue wire and when hooked up the input was correctly working. I verified the polarity by knocking all the dust off and the LED was illuminated. Don't be mad at me. I made third shift go pretty fast for someone.
Here is a link to the internals of the switch. Page 5.3-7
http://eul0600038.eu.verio.net/Mult...en2(UK)/Serial2(UK)/Related/DSeries_(UK_).pdf

steve out and thanks again for all the help

 

[stevez]

What a scandle. (sorry couldn't help myself).

 

[Eric Nelson]

Re: The horse is dead!!!

A dead horse never stopped us before...

beerchug

-Eric

P.S. I couldn't help myself either...

 

[JesperMP]

Congrats Steve

I didnt want to confuse you.
What I meant to say:
A simple reed switch should behave like any switch (very little contact resistance).
If there was some protective circuitry in series with the reed , then the resistance can be anything, but it should still give 24V out when activated.
The diagram in the manual shows that the latter is the case.

Its an example of how electronics are creeping in everywhere. Of course, the manufacturer simply wants to improve his product by making it less sensitive (a simple reed is quite unforgiving).