Looking for a known resistance on a cable run

[Wobbles]

Hi , I'm looking for a simple method of checking for a resistance at the end of a cable run based on the comparison to a given similar resistance. I need the comparison to have a tolerance of e.g. 5-10% to allow for cable length and the initial tolerance variability of the component..
Im guessing this somewhat ressembles a wheatstone bridge however I'm looking for an LED to be illuminated either in no match or in match whichever is easier.
Does anyone have any ideas.

 

[MikeMl]

If you were using an Ohmmeter, would you short two conductors at the far end, and then measure the loop resistance at the near end?

Do you need to light a LED if the resistance is >XΩ?

What is X (a range will do)?

 

[Wobbles]

If you were using an Ohmmeter, would you short two conductors at the far end, and then measure the loop resistance at the near end?

Do you need to light a LED if the resistance is >XΩ?

What is X (a range will do)?

Hi ,
Imagine this was a fire alarm with eol termination , my device although not a fire alarm needs to see the presence of the termination or otherwise. - does that help

Im not fussy if i light an LED upon absence (red) or Presence (green) or both (Bi Colour)

 

[MikeMl]

Hi ,
Imagine this was a fire alarm with eol termination , my device although not a fire alarm needs to see the presence of the termination or otherwise. - does that help

Im not fussy if i light an LED upon absence (red) or Presence (green) or both (Bi Colour)

I still need to know the loop resistance of a good circuit, and the loop resistance of a bad circuit. Also, when testing, you will disconnect whatever normally senses the loop?

 

[Wobbles]

I still need to know the loop resistance of a good circuit, and the loop resistance of a bad circuit. Also, when testing, you will disconnect whatever normally senses the loop?

The loop resistance would vary, i intend to switch in a given value resistor (via dip switch or jumpers) to compare. Typically the EOL would be 6K8 or 4K7 .
The original monitoring equipment will still be connected but only to serve as a power source (Typically 24 Volt)
in summary when fault finding a cable fault on a conventional fire alarm its usual to break the system down , my device will break the connection but have a look to see if it can see the termination further down the line OR the source voltage . if it sees both there is no fault - does my waffle make any sense ?

 

[ronsimpson]

Is this thing a "test machine"?
Help me understand!
There is a "monitoring machine", then a long wire, then a resistor. (?)
You want to break in some where and see if (there is 24V) and (if there is about 5k of resistance). (left, right)
Is this right?

 

[MikeMl]

Here is my idea for a line tester:



Red LED turns on when Res is < X, where X is adjustable from 0 to 10K by setting the Ten-turn 10K Pot. If you get a 10K 1% precision pot with a turns counting dial, you can dial in the threshold resistance that causes the LED to switch..., e.g. 635 on the dial would be 6.35K

Circuit has a small amount of hysteresis to prevent the LED flickering.

 

[gophert]

Wobbles
Are you looking for a security system-like circuit with End Of Line resistor?

Here is an example with an op amp that will switch red to green as the remote switch is activated. Swap the LEDs to revers color as needed. The 150 ohm can be increased if you use higher voltage.

A 10k potentiometer will let you compensate for long runs of cable so not a big deal.

 

[strantor]

Sorry I'm not getting this.
Maybe you are oversimplifying?
From what I've read so far, it seems we are dealing with only 2 wires. I cannot understand why you don't simply probe them with a DMM.
What kind of "faults" are you expecting to find? From what I've read, it seems like a pass/fail with only 3 possible outcomes:
PASS: continuity, resistance as expected
FAIL: continuity, resistance too high or low
FAIL: open, no continuity

A DMM could tell you all of that, and also would adapt for conductor resistance (well, YOU would adapt for it, given conductor resistance is known).

So I'm left to assume there is a lot more to this scenario that you haven't divulged, that makes this make more sense. Am I right? Maybe there are several conductors, not just 2? If this is confidential, maybe you could just throw me a bone in a PM? or not, but some clue is missing here and I'm not sure this will be solved without it.

 

[KeepItSimpleStupid]

The original monitoring equipment will still be connected but only to serve as a power source (Typically 24 Volt)

The "system connected" bothers me.

On the "expensive side", there is this: https://www.tequipment.net/Fluke773.html?Source=googleshopping&gclid=CPzF8Kyr0soCFdCPHwodgs4H5w

So, you can measure the current and the voltage without breaking the cable and your good to go.

You can look at the EOL unit (voltage and begin voltage) and current anywhere to characterize the system. The voltage across the source and the current would characterize everything inclusive.[/quote]

 

[dr pepper]

Fire alarms and networks use a termination resistor for a diffrent reason.
If I get this right if you compensate for the cable runs resistance its straightforward, just a case of a voltage diivder and a simple comparator.
If you wanted to get complicated then you could use an impulse at the sending end and basically measure the swr, this would mean using coax or twisted pair.

 

[Wobbles]

I'm sorry guys I'm trying to give the info you need. When fault finding a fire alarm say for example with 20 devices on a zone, typically you would break the line down halfway and check if their is voltage, if there is then move further along the line and test again. At each point I would look for the 24volts and look for the end of line termination. By unplugging ( usually bayonet style) the existing device and plugging in the "test device" is a lot quicker than disconnecting the cabling from the mounting base and taking the readings particularly on a large system. The idea being that the resistance I'm looking for would be compared with a sample resistor of approximately the same value ( selectable at the " test device " ) additionally the same device via removing a jumper could insert a terminator for the panel to see whilst breaking the connection beyond the point of " test " - Does that make anymore sense ?

 

[KeepItSimpleStupid]

I'm sorry guys I'm trying to give the info you need. When fault finding a fire alarm say for example with 20 devices on a zone, typically you would break the line down halfway and check if their is voltage, if there is then move further along the line and test again

At least refine that to the binary search method.

break into two groups: 20/2 =10 and the nearest power of 2 is 8, so 8+10.

test at the 8th unit. Is fault before or after. If before test at 4th unit.

Is fault before or after. test at 6th unit if after.

etc.

it's called binary searching and it's easier done when the split is a power of 2.

 

[Wobbles]

At least refine that to the binary search method.

break into two groups: 20/2 =10 and the nearest power of 2 is 8, so 8+10.

test at the 8th unit. Is fault before or after. If before test at 4th unit.

Is fault before or after. test at 6th unit if after.

etc.

it's called binary searching and it's easier done when the split is a power of 2.

Yes your correct but your called " keep it simple " so I did ( no offense intended )

 

[Wobbles]

Wobbles
Are you looking for a security system-like circuit with End Of Line resistor?

Here is an example with an op amp that will switch red to green as the remote switch is activated. Swap the LEDs to revers color as needed. The 150 ohm can be increased if you use higher voltage.

A 10k potentiometer will let you compensate for long runs of cable so not a big deal.

View attachment 97126 View attachment 97127

Hi Gophert,

Thanks for your input , I assume if I change the 4k7 in your schematic will allow the comparison to the end of line to vary correspondingly ?
I would supply the circuit from the 24 volt given from the fire alarm panel but in the absence of the connection from the panel would switch in a 9v pp3 or similar. If I get really smart I might get it to auto switch to battery in the absence of 24 volt

 

[dr pepper]

I dont get it, never mind.

I have though worked on fire alarm systems, the addressable kind, a cable tracer is a handy way you dont need to disconnect anything just hold it near the cable, you can hear the datastream.

 

[Wobbles]

I dont get it, never mind.

I have though worked on fire alarm systems, the addressable kind, a cable tracer is a handy way you dont need to disconnect anything just hold it near the cable, you can hear the datastream.

Yep, no data stream on conventional

 

[gophert]

Yep, no data stream on conventional

It would help to have a schematic of a standard system (complete) and when you break the connection to do your test in the field. Then we can suggest a circuit to test one of the two sections when you break the connection.

 

[KeepItSimpleStupid]

Can't you just remove the EOL resistor first.

This should then tell you if your looking for a short or open.

MikeML did some SIMS on how an EOL resistor should work. You basically supply the loop with a power and a resistor which acts as a voltage divider.

An open should be easy to chase with or without the EOL.

The short requires removal of a section (Again, a binary search).

 

[strantor]

Can you confirm that the 2-wire system in this link is what we are talking about?

https://www.home-security-systems-answers.com/smoke-detector-circuit.html

If so, I think I understand why you don't just use a DMM; it's because the signal and the power for the detector devices are on the same two wires. So if you disconnect the power source and test with a meter, your meter does not source enough voltage to power the detector devices. Is that correct?