# I have a system that I need to help to figure out what max length line is

Discussion in 'General Electronics Chat' started by Benengineer, Aug 19, 2016.

1. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
I have light control system connection and try to figure out what max lengh is from switch to LMIO sensor control. Here is the connection graph.
View attachment 110643
You can see the LMRC(light management romm control) provide 24V throgh IRB cable. The LMIO control the switch on/off. LMIO is parallel.
View attachment 110644
Output is 24VDC and I connected pin 8 and 9 together. The limits is that LMIO can't be lower than 18V. What is max lenght for these system?
Part of LMIO circuit as follows.

Here is the test data I got.

Avage difference each LMIO unit = [(21.48-21.33)+(21.33-20.79)+(20.79-20.71)+(20.71-20.53)+(20.53-20.27)]/6 = 0.202V drop per unit added
I assume that the LMIO drops down 0.202 with each unit added until it stops at 18V. The total units I got (24-18)/0.202 = 30
6 units I got about 24.99mA total. So, my total current is about (29/6)*24.99 = 120.75mA

Based on the above 18V limit drop for LMIO, I got equation Lmax = (V2-18)*1000/(2*0.12075*6.4) = 2251.55ft
The other question is why I1 current is greater than 5mA or it doesn't matter? Please let me know if you have any question.

Thanks

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2. ### hp1729 Well-Known Member

Nov 23, 2015
2,084
232
I don't think wire resistance is so much a factor. How much noise in the area? Twisted pair cable? Shielded?

3. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
They never thought about this noise? What is relation between noise and wire length? What about Twisted pair? Yes, it is shielded. It is like enthernet cable, but it is different.

Thanks

4. ### hp1729 Well-Known Member

Nov 23, 2015
2,084
232
Sounds good. Try it and see if it works, all theory aside. If it works, it works.
Telephone wire is twisted pair, unshielded and works for miles.
For an industry or a residential applications? Industries can generate a lot of electrical noise. No so much with residential stuff.
Hundreds of feet? Twisted pair "phone wire".
Thousands of feet? Shielded twisted pair, which sounds like what you already have.

5. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
For commercial building. The IRB cable has to be connected the Light management room controller and IO device. Do you think it will generate noise? As I showed you, the current and voltage change are measured. How do I calculate length based on the parameters I measured?
Thanks

6. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,259
222
What is an IRB cable?

Did you take into account the series resistance of the ammeter or did you use a zero resistance ammeter?

Did you actually measure the voltage burden of the ammeter? Did you switch ranges of the ammeter?

It MAY expalin I1.

7. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
It is like enthernet. Yes, that is how we simulate the wire resistance. I don't know zero resistance ammeter. But as you can see the circuit connection picute, i also simulate switch with the first unit.
Please let me know if you have questions.
Thanks

8. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,259
222
Do the following:

Measure the current with an ammeter, and measure the voltage ACROSS the ammeter with another voltmeter.
Compute R=V/I for the likely ranges used.

The specs that you need could be in the meter's manual.

My point is, the meter resistance could swamp out your measurement.

A Zero Resistance Ammeter is also known as a Feedback Ammeter. It's a rare instrument at 10 mA.
It's not too difficult to build one. I've built several including one that had an effective cost of about \$5,000 USD.
\$3K of that was two system voltmeters and a system D/A converter and an 8 bit port. I needed the AC performance. The offset was about 40 pA when measuring DC in 4 decade ranges (+-10V out) at 100, 10, 1, 0.1 FS. It had bias ability too.

9. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
Thank you for your instruction. But I don't undertand what relation between line max length and R=V/I.

Thanks

10. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,259
222
Here are specs for two meters:

PDF page 46: http://assets.fluke.com/manuals/87______umeng0800.pdf

PDF page 43: http://isurplus.com.au/manuals/Tektronix DMM916 User Manual.pdf (No specs for voltage burden)

PDF page 13 http://assets.fluke.com/manuals/8024b___imeng0200.pdf (0.3 to 0.9V)

Section 8 here https://en.wikipedia.org/wiki/Multimeter#Burden_voltage has a blurb on burden voltage.

Here http://www.raeservices.com/Datasheets/Keithley_480_DS.pdf is a blurb about feedback ammeters where the burden voltage is less than 200 uV.

Whatever you do, make sure the voltage burden of the ammeter does not figure significantly into your calculations. As was stated, if you were measuring V and I in your wire with a meter and the I range changes the reading significantly you have a real reason to be concerned.

11. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
I used the resister 10 as my simulation value. So, the circuit shows that I got each circuit current in and out. My final goal is that through this circuit simulation to figure out how long wire can connect to the switch. Which means how much devices I can get. Please read the first post and find all data I got. I want to make sure I got correct calculation.

12. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
By the way, I can not find any pages in the link.

13. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,259
222
Try something like a select, left-click "Open in new tab. Some browsers need CNTRL to open a link/

If that doesn't work "save the link as" and open the pdf file/

14. ### Benengineer Thread Starter Member

Feb 6, 2016
101
2
For example, I can open the link PDF page 46: http://assets.fluke.com/manuals/87______umeng0800.pdf. But there is not any page 46.

15. ### KeepItSimpleStupid Well-Known Member

Mar 4, 2014
1,259
222
"PDF page 46" is equivalent to page 38,otherwise I would have used "Page 38".

Using "PDF page" allows one to go DIRECTLY to the page of interest by typing the value in. e.g Page x/52

I feel the "PDF page #" is more convenient. At one time, I used to supply both. Sometimes you get cover pages and prefaces etc, so numbering never corresponds to physical pages.

e.g "page 38, PDF page 46"

Sorry for the confusion.

Here is a "direct link to the actual page": [url[http://assets.fluke.com/manuals/87______umeng0800.pdf#page=46[/URL]