Reed Switch Wire Length

I am tring to count reed switch closures with AC input card on my SLC. I can count them fine with 1 ft wire lenght but if I extend the wire to 1000ft the input counts 1 time and stays closed. Is this a function of wire length or AC input card. The switch is a 10W 120V .5A hamlin.

Does the input LED on your PLC stay on or is it just the bit that is staying on? Is there a lot of electrical noise? Is your wire shielded? I have heard of situations when using extremely long wire runs of unshielded wire, with cards that have a low turn on voltage that this can happen due to an induced voltage.

Yes the LED input light is staying on. The wire is shielded and right now it is a 1000ft spool sitting on the table.

You do realize that a SPOOL of wire is in effect a coil? Apply a voltage thru the spool and you will create a magnetic field which in turn can create an EMF and capacitance..ie can maintain an emf for a small period of time.

If the LED is staying on then I would have to agree that the problem is related to emf as rsdoran has suggested. Even if you un-spool the wire you will probably still have noise problems. If you need to mount the switch that far away from the PLC I would suggest using some type of remote I/O and Ethernet or Devicenet or something along those lines.

I would also agree that the spool of cable is creating an EMF field.

(side question - is the reed switch connected to the free / payout end of the spool or the short end near the spool hub?, if at the hub, that's the "Soleniod coil" effect that is latching in the reed switch)

I know your moving towards the reed switch, though I might suggest a two wire prox. switch might be a better option, IMHO

There are devices available that will bleed off some of the stray pick-up. Most have a capacitor and resistor made together. Quench-Arc is one brand.Suppressors used for relay coils work sometimes. Sometimes a simple load resistor across the input will do the job. It all depends on the particular situation what will work for you.

rsdoran said:

You do realize that a SPOOL of wire is in effect a coil? Apply a voltage thru the spool and you will create a magnetic field....

Click to expand...

Since he's using shielded wire, isn't he also creating, in essence, a transformer?!?! IOW, I wouldn't be surprised to read a voltage and current (possibly lethal?!) across the shield ends!

5 hours since QEDENV's last post... Are you still there QEDENV? utoh

beerchug

-Eric

Thanks for all the advice. I have taken the test off line and I am going to try using a DC input card insted.

This test has been successful with 3000ft of wire using a Campbell Scientific Datalogger that uses 5VDC as it's sensing power.

Eric Nelson said:

Since he's using shielded wire, isn't he also creating, in essence, a transformer?!?! IOW, I wouldn't be surprised to read a voltage and current (possibly lethal?!) across the shield ends!

5 hours since QEDENV's last post... Are you still there QEDENV? utoh

Eric

Click to expand...

LOL, interesting point.

Well since the "primary" is 2000' (out & back) and the "secondary" is 1000', the ouput should be in the neighborhood of 60 volts.

Just a little Tingler maybe...

DC IS THE WAY

I have changed to a 24VDC input card. The test is running with 3000ft of wire at a cycle rate of 10Hz. The test is running fine with no problems. It seems that AC is not a good power source to use for long runs.

Good news!

AC or DC are both good for long runs, comes back to what type of sensor/switch and all other functions your process/application needs.

Though it's best not to have DC sensor/switches wiring mixed with AC control(especially power wiring), AC can "Bleed over" to the DC controls. Also mixing low potential AC with higher potential AC can cause problems, (Had a machine once with 12 VAC for the interlocks and 120 VAC control wiring for the operator switches in the same conduit, the 120 VAC was causing false "ON" states in the 12 VAC interlocks back to the PLC)

I prefer 24 volt DC for my input controls with 120 AC output for within the control panel(Contactors, relays etc), when I desige a control system, tends to be safer when someone breaks a conduit/flex at a remote operator console.

When Contact -on the end of twisted 2 wires- opened the capacitance between those wires gives still current to input -enough for led-light and sometimes for input state '1'-.
Capacytance typically is 150 nF/1000 meters.
With DC-voltage there is not harm with slow switching frequence.

I tend to agree with the capacitive coupling idea. A coil of wire without an iron core and very little current will not likely act as a transformer.

I never said a spool of wire would act as a transformer. Its a known fact that a coiled wire can act the same as a coil though and in many cases will have a capacitance inherent in the process. The effect is more of inductance and capacitance. Using a 3000 foot spool of wire still on the spool is asking for "improper" results in any test, using either AC or DC. I would concede that using DC would be less likely to create an issue BUT believe it could in some cases.

Spooled wire when dealing with AC can create situations that many electricians will never see. For many years I worked in the amusement industry..ie carnivals. I have seen things happen with travel trailer lead lines (power cords) that most wouldnt see in a lifetime because they dont see these situations.

I can tell you from experience that a 100 foot 10/4 or 12/4 SO or SJO cable when rolled up or coiled because it doesnt have to go more than 20 feet can do things you wouldnt believe.