Sensors that monitor resistance.

[kal.a]

Hello everyone. My first post though I have read quite a few.

I'm learning industrial automaton which invariably leads me to electronics which I know little (OK nothing) about. But I do like to learn and hence this question and the many to come. Hopefully you won’t think I'm spamming the forum.

I've encountered sensors that are "monitored" and non-monitored. The simple version of these is the two wire edge non-monitored that has in it a N/O contacts. The one I don't understand is the monitored one as the controller, according to the manual, measures a resistance of 68Kohm to make sure the sensor is not damaged and at the same time the sensor is a N/O contact that when impacted closes the circuit and activating the controller in some way.
How does that work? Is there a resistor installed in series with the NO contacts? What it the basic circuitry that measures resistance in a device of this nature?

I encountered another on a security panel where the inputs are "supervised with or without end-of-line resistors" see page 7 of the following manual:
KT300
What's meant by end of line resistor and is the presence of the resistor monitored in much the same way as the safety edge sensor?

Thanks a lot.
NB: Next question is on conductivity sensors

 

[MaxHeadRoom78]

I've encountered sensors that are "monitored" and non-monitored. measures a resistance of 68Kohm to make sure the sensor is not damaged and at the same time the sensor is a N/O contact that when impacted closes the circuit and activating the controller in some way.
How does that work? Is there a resistor installed in series with the NO contacts? What it the basic circuitry that measures resistance in a device of this nature?

I would suspect that the 5.6k resistor is to monitor the validity of the circuit conductors, when the sensor is open, say for conductor continuity check, if they were shorted, it may be a fail safe condition. and does not matter so much.
Max.

 

[KeepItSimpleStupid]

1You have to watch PDF page and page #. Sometimes they are different. PDF page is easier to "Goto".

Monitored contacts:

I'll use a very simple example, ignoring the specifics.

Suppose the sensors are fed with a 5V supply in series with a 10K resistor. (Panel might be 12 or 24 V, but I'm using 5 here.

Now, the circuit looks like a 5V supply 10K resistor and a 10K (EOL) resistor with a switch across it to common.

So, with no EOL resistor, we would read 5V

With an EOL resistor, we would read 2.5 V

With the contacts closed (Alarm), we would read 0 V.

That's the ideal world, The resistors and power supply have tolerances which have to be taken into account. Matched resistors (10K) probably would not be used,
The 0V level would depend on wire resistance.

We could have (Above 3.5V) - supervisory alarm
Between 1.5 and 3.5) V; - Supervised - not in alarm
Between 0 and 2.5 V ; Alarm

All measured at the panel.

If the contacts were unsupervised, then you would have 0 and 5 V levels to deal with. 0V alarm, 5 V not in alarm. Again tolerances apply, so say under 100 mV alarm, greater than 4 V alarm or use the same definitions above and re-define their meanings.

The levels are basically determined with wire length, wire gauge, power supply tolerances and resistor tolerances. Higher R values mean less power consumption.

EDIT: If I were doing the design, I'd use 10 mA with the contact closed. This provides sufficient wetting current.

==

I had to interface to an FAP panel once. My box used the alarm contacts from the FAP. Instead of a monitored contact, I used 3 conductors (An SPDT relay from the FAP). When not in alarm, I would show a LED indication and when in alarm, I would show the LED off. It's not a monitored contact, but if the entire cable was cut, the panel would know about it, We also knew when we could reset the panel. The hydrogen and toxic gas alarm fed the FAP panel and they had EOL resistors.

 

[MikeMl]

For example. An intrinsic sensor is a switch; either open or closed. The addition of R1 and R2 makes it possible to differentiate beween a shorted line, an open line, and a proper indication of the sensor being open or closed.

Note line voltage V(line), the line current I(R3), or the line resistance V(line)/I(R3) on a good line. What would be the indications if the line was open or shorted?

 

[KeepItSimpleStupid]

This "supervised with or without end-of-line resistors" is probably read as "supervised with" or "without end-of-line resistors"

It's not the best sentence.

Expect mistakes in manuals.

 

[kal.a]

Thanks for all the replies. I have to read it a few times to understand them lol

 

[KeepItSimpleStupid]

The feeling is mutual. I had trouble understanding your post too. I had to read it a few times. Hence, the second response.

Now, anyone can write documentation. It used to be proof read and checked thoroughly. Chinglish (Chinese to English) manuals are worse yet.

Welcome to the forum!

 

[kal.a]

Thanks KeepItSimple. Though the reason it took me a while to understand your post was my lack of understanding of electronics not for lack of writing skills. I do understand it now though.

Thanks a lot.

Kal

 

[kal.a]

For example. An intrinsic sensor is a switch; either open or closed. The addition of R1 and R2 makes it possible to differentiate beweent a shorted line, an open line, and a proper indication of the sensor being open or closed.

Note line voltage V(line), the line current I(R3), or the line resistance V(line)/I(R3) on a good line. What would be the indications if the line was open or shorted?

View attachment 96395

Ok let's see. If the switch is open we'd have 22.8 volts at the switch (positive side) and if it is shorted we'd get zero volts and if the R2 resistor is damaged we'd get 24 volts.
Now here's a question. Assuming my answers were correct does that mean R3 and R1 would have to be more than 1.44 watt to account for the case of a shorted line?

 

[KeepItSimpleStupid]

You can base R3 on short circuit current. I=24/100 and P>(I^2)*100

and base R2 on I= (24/3.9K+100); assume the 100 ohm in the unit is there and the other 100 is missing. P > (I^2)*3900

==

BTW KISS works, Short for KeepItSimpleStupid

 

[kal.a]

KISS, I like that.

What did you mean if you were designing it you would use 10ma closed circuit, what would the circuit look like?
And Mike, what software did you use to draw that circuit?

Kal

 

[MikeMl]

...Now here's a question. Assuming my answers were correct does that mean R3 and R1 would have to be more than 1.44 watt to account for the case of a shorted line?

Let's ask LTSpice: The sensor switches on at 10s, and the line is shorted to ground by the new switch I added at 25s. LTSpice knows how to compute and plot the power in any component, including R1, R2 and R3.



Note that I moved the power(R1) trace downward by subtracting 30mW from it just to make it visible.

 

[kal.a]

LTSpice, nice.
Thanks Mike. You know now I'm probably never gonna leave home