Switch works, NOT!

Narlin

Lifetime Supporting Member
Join Date
Mar 2010
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phoenix
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[FONT=&quot]I have a float switch from the bottom of a still that, according to the ohmmeter works fine. . The controller tries to pass 24V through the switch and feed it into a PLC input to determine if there is water or not. [/FONT]
[FONT=&quot]OPEN CONDITION: When the float is at the bottom, there is infinite resistance – the meter makes no noise and reads 0L, which is the same as when the leads are separated and not touching. [/FONT]
[FONT=&quot]CLOSED CONDITION: When the float is up the stem a bit, I get 0.5 ohms resistance and the meter indicates that the switch is closed.[/FONT]
[FONT=&quot]PROBLEM: When 24V is passed through the switch, it is always ON=CLOSED CONDITION regardless of the float position Why? I have confirmed the problem condition with 2 different voltage sources.[/FONT]
[FONT=&quot]OBSERVATION: If I use the 24V through the switch and an ammeter and an LED I see that the CLOSED switch position is passing 10.6mA and lighting the LED. The OPEN position is passing 12.3 mA and also lighting the LED.[/FONT]
[FONT=&quot]I have attached a photograph of the switch. The brand is not known and it has no markings. I have taken it apart more than is shown in the photo and where the wires go into the switch housing, the housing is rusty.[/FONT]

IMG_0793[1].gif
 
The photo is too low-res to be of any help and your information is a bit light, however I would bet a cup of coffee that if you put a larger load on the switch it will indicate properly.
First clean out all the rust and seal it from any more distillate and/or vapors, whatever that is.
 
By a larger load, do you mean putting a resistor, for example, in line with the switch? I'll try that in about an hour and let your know. The mA explanation is making sense to me.
 
If you gave the PLC input module catalog number, somebody could look it up for you and figure out what size resistor you need to increase the ON state current high enough so that the PLC could see it.
 
It is a micrologix 1400.. Where do i look to find the info.
The PLC is new. The original equipment was a Logix 5555.
 
A different take on your problem in case the above suggestions don't pan out

I am wondering if the switch built into the tube of the float switch is a magnetic reed switch ,
You mentioned rust being in the tube which to me indicates that the rust can be magnetised
and hold the reed switch closed after the magnet in the float has moved away from the switch

The tube is normally stainless steel so how did you get the rust in the tube in the first place ?
 
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It is a micrologix 1400.. Where do i look to find the info.
You can find the digital innput impedence on page 152 in the Specifications Appendix A in the "MicroLogix 1400 Programmable Controllers User Manual" (which you can get from rockwellautomation.com
http://search.rockwellautomation.co...ure&ip=205.175.239.7&access=p&entqr=3&start=0

The digital input impedance is 2 kilo-ohms (2,000 ohms) for Inputs 0 through 11, and 5.5 kOhms for Inputs 12 and higher.

The on-state minimum current is 7.0 mA at 4.5 DC for inputs 0 through 11, and 3.0 mA at 10 vDC for Inputs 12 and higher.

For 24 VDC the preferred On-state current is 9.5 mA for Inputs 0 through 11, and 5.0 mA for Inputs 12 and higher.
 
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Your switch doesn't work. The N.O. contact is shorted out, but your meter doesn't induce enough voltage to overcome whatever gunk has built-up inside the float, so sure at just a few volts it's going to work fine. But 24 Volts is evidently enough to overcome the resistance of the gunk and read shorted.

I use multimeters to test resistance, but I never take the result as gospel. Whenever I test motor windings, for example, I always use a megger (though you may not want to induce thousands of volts onto your float). Sometimes what reads open at lower voltages will short at higher ones.
 
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I appreciate the help. I have also concluded that i can't use the switch. I did get it to work with the PLC by putting a load of exactly 6630 ohms in series, but . . it was way to sensitive. At 6800 ohms it was always off and at 6110 ohms it was always ON. That's pretty close to useless. I wouldn't be messing with this except that the switch has a 3/8" BSPT imperial thread on it and I its looking like a machining job to replace it. Oh well, I know a guy.
 
Or... its not a discrete switch, it's an LVDT or a magnetostrictive analog level transducer and your meter's battery voltage was so low that it only showed up as a change at the extreme end of travel, but on 24VDC it was over excited.
 

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