2 wire prox problem

[LOWRAD777]

I just automated a pipe transfer system using pepperl fuchs 2 wire pancake type prox sensors and icecube relays. the proxs sometimes leak a steady 110v. they also surge when they are energized prematurely making my relays and isolating the process preventing the system from operating correctly.has any one seen this problem. and what course of action was taken to correct it.

 

[realolman]

I'm not the sharpest knife in the drawer, but I'm not understanding what you're describing ... could you elaborate a little


what exactly do these things mean?

the proxs sometimes leak a steady 110v.

they also surge when they are energized prematurely making my relays and isolating the process

 

[keithkyll]

Leakage is typical with 2 wire prox's. They need to "steal" a little power to drive the internal electronics. Use bigger relays that draw more current, or add resistors across the relay coil. Try 10K, 2 Watt. The added load should be enough to cure the powerup spike too.

 

[LOWRAD777]

two wire proxs have a control circuit that completes itself useing the output lead. they leak a small amouont of current until sensing is acompished then they use a transistor to turn the sensing signal into a full blown output in series with the control current leakage. My proxs are leakinig to much current sometimes energizing the control relays. but they dont do it all the time. also sometimes when you turn the system on they surge enough to energize the relays but not all the time. sometimes the system will run perfectly but sometimes the proxes act up and the whole thing goes haywire. the operators told me that it was possessed

 

[LOWRAD777]

shouldnt I put the resistor in series with the output?

 

[James Mcquade]

No,
place the resistor in parallel with the relay coil.
you are wanting to create a pull down current to prevent the relays from operating.
regards,
james

 

[rootboy]

And the 2 watt rating matters. Don't go below 2 watts.

 

[LOWRAD777]

10k 2 watt got it Ill try that.
I generally dont get into sizing semi conductors so im in the dark hear Im just an industrial electrician. But I look forward to figuring this out for problems i might run into later. An instrument tech told me to use an mov. varistor. I try two types and they didnt work one allowed a little more out put voltage than the other but they didnt work. I dont know how to size them either.
Thnx for the advice

 

[sbaum]

Ah the snare of 2 wire AC proximity sensors.
I never trust them even on larger relays because inductance on the wires to the relay can cause them to energize at random as you are seeing in your application.

Being a service tech I like to keep several isolating relays with the leakage suppression cct built in to remedy this problem.

For example Allen Bradley 700-HLT1L1 would fix your problem.

 

[Sherlock4me]

Why the 2 watt matters as per Rootboy's good advice
We are using a resistor in parallel with the coil as effectively a load to drop the voltage so the relay will not flow ENOUGH current through it's coil to generate enough electromagnetism to pull the relay contacts in. ( A small amount will still fow through the relay using ohms law.)
But as to the wattage rating of the Resistor:
Ohms Law (I=E/R) so, 120v/ 10,000 ohms = .012 Amps so Power (P=I*E) so 120v *.012A= 1.44 Watts of heat the resistor will have to withstand...That may fry a lower wattage resistor

 

[rootboy]

An instrument tech told me to use an mov. varistor. I try two types and they didnt work one allowed a little more out put voltage than the other but they didnt work. I dont know how to size them either.
Thnx for the advice

Yikes! Don't do that! (just ask Trace Adkins).

When a MOV conducts, it's a dead short to whatever you have the other lead connected to (pretty much it's always connected to ground). This will destroy your prox, and possibly take the entire control circuit with it.

And (this is the Trace Adkins part of the story) each time a MOV conducts, its breakdown voltage is lowered. Meaning that the next time it will conduct that much more, that much sooner.

And if you have the MOV "protected" by a circuit breaker that is much larger than the capacity of the MOV, then the MOV catches on fire.

The newer style of MOVs have what looks like a hunchback on them. This is a thermal fuse that will open the MOV up should it get too hot (but there goes your protection...).

This is better than catching the MOV on fire, but not by much (something else will likely catch on fire).

And it's unlikely that you are seeing much of a surge coming into the coil. The coil on the relay is going to take care of that. However, once the power is removed there is going to be a spike that can cause noise and/or accelerated contact wear. This is where a "snubber" comes in. This is probably what he meant:

http://www.bpesolutions.com/bpemanuals/Snubber.pdf

But it's not going to help you in your leakage problem (but you should always have snubbers for your AC relays and diodes for DC relays).

And as an aside, if you have a time critical operation then you want DC relays (no zero crossing delays), *but* you will want to add a resistor in series with the diode to get the relay to turn off as quickly as it can.

I've been told that you want to size your resistor to match the coil resistance.

https://www.physicsforums.com/threads/flyback-diode-on-solenoid-slows-solenoid-retract-time.747525/

 

[Sherlock4me]

As to the snubber,(the resistor in parallel with the relay, often right there at the relay base neatly wired to the two coil connections) generally the highest ohmic value that will DO the job is a good general rule of thumb, with a wattage rating to handle the heat as mentioned above, once the math is done.