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Need help with long distance circuit

Tha fios agaibh

Aug 11, 2014
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Within a signal pair, wire-to-wire capacitance actually is greater with twisted pairs than with straight parallel pairs. Also, I assume that in your first sentence you meant to say resistive rather than reactive. Like inductance, capacitance is a reactance.

ak
Sorry AK, I meant to say Long straight parallel spans are more capacitive than inductive.

I know twisted pair wires are less likely to pick up AC interference. I assumed it is because capacitive coupling is less of an issue. How do you explain the higher capacitance with twisted pair? Is it because the twisted wires are about 1/3 shorter in length?
John
 

davenn

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I know twisted pair wires are less likely to pick up AC interference. I assumed it is because capacitive coupling is less of an issue.

in long distance open wire lines, the wires also go through a rotation ( twist) to negate most external interference

How do you explain the higher capacitance with twisted pair? Is it because the twisted wires are about 1/3 shorter in length?

why would they be shorter ?, they both have to be the same length if covering the same distance
 

Tha fios agaibh

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in long distance open wire lines, the wires also go through a rotation ( twist) to negate most external interference

why would they be shorter ?, they both have to be the same length if covering the same distance
Yes I know. How do you explain the claim: twisted wires have more capacitance?

They are shorter because they are twisted. If I have two pairs exactly 100ft long and I twist one pair, it will shrink to about 75ft long because of the spiralling.
 

AnalogKid

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As noted for any given external cable length, the internal wires of a twisted pair are longer because twisting the wire around something takes more wire. This extra length increases both the series inductance and resistance, and the parallel capacitance. The more tightly the pair is twisted, the more the parameters increase.

By itself, twisting a signal pair does nothing to reduce interference pickup. All other factors such as circuit impedance being equal, the wires are exposed to the same fields and pick up what they pick up. Twisting guarantees that both wires are exposed *identically*. Without twisting, whichever wire is closer to the interference source, even if it is by 1 mm, will pick up more junk. The noise cancellation comes in the design and quality of the receiver circuit.

ak
 
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Tha fios agaibh

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Ahh,
As noted for any given external cable length, the internal wires of a twisted pair are longer because twisting the wire around something takes more wire. This extra length increases both the series inductance and resistance, and the parallel capacitance. The more tightly the pair is twisted, the more the parameters increase.

By itself, twisting a signal pair dows nothing to reduce interference pickup. All other factors such as circuit impedance being equal, the wires are exposed to the same fields and pick up what they pick up. Twisting guarantees that both wires are exposed *identically*. Without twisting, whichever wire is closer to the interference source, even if it is by 1 mm, will pick up more junk. The noise cancellation comes in the design and quality of the receiver circuit.

ak
So this wire to wire capacitance is only higher due to increased surface area, not because of the spiralling geometry.
You lost me on the second part. Twisting a signal pair does reduce the interference picked up on them, because the induced noise current flowing on one conductor is opposed by the other conductor (twist) where the noise current is flowing in the opposite direction. This nulls the currents and drastically reduces noise. Right?
 

davenn

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By itself, twisting a signal pair dows nothing to reduce interference pickup.

yes it does
if it didn't, there would have been no point doing it


Twisting guarantees that both wires are exposed *identically*.

and there in lies the reason, the sections receive equal amount and opposite polarity of induced voltages which cancels the external interference out
 

AnalogKid

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To Tha and davenn, consider this. The direction the signal current is flowing does not affect the direction of the noise current. Induced noise is the same direction in both conductors; that's why it is called common mode noise. No cancelling occurs in the wires. The cancellation happens at the receiver. For a balanced signal pair, the signal phases are + and - (or 0 degrees and 180 degrees) while the noise is + and +. It is critical that the noise arrive with equal amplitude and phase on both conductors, and controlled twisting achieves this. When the receiver circuit does an invert-and-add, the signal goes to 2x and the noise goes to zero.

For an unbalanced signal, where the 2nd conductor in the pair is a return, the signal again arrives at the receiver as a difference, this time between the receiver input and its signal reference (gnd). The noise again arrives in phase on both conductors, so when the input goes up due to noise, the input reference goes up also. The receiver sees signal at 1x and noise at zero. This works only if there is no other connection between the transmitter GND and the receiver GND. If there is, the receiver input moves due to noise but the receiver GND input does not. This is why ground loops "cause" noise. They don't. They allow the existing noise *not* to be cancelled. Close enough.

"
So this wire to wire capacitance is only higher due to increased surface area, not because of the spiralling geometry.
"
Turn it around. The spiralling geometry causes increased length, which causes increased resistance, inductance, and surface area, and the increased surface area causes the increased capacitance.

ak
 
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Tha fios agaibh

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Thanks AK, A good explanation but in practical terms, induced noise currents are measured and impacted at the end of the wire pair, not a tiny section in the middle. So when talking about noise induced in a span of wire, the same is implied. (Interference at the end of the twisted pair)
This is my only contention to your quote "By itself, twisting a signal pair does nothing to reduce interference"

John
 

Tha fios agaibh

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slick-rides

Dec 23, 2015
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Just to see what would happen, I hooked up the power supply direct through the float to the relay. The positive lead hooked directly to the relay coil, and the negative through the float. With the float switch open, I get a reading of 15 vdc at the relay coil. This is enough voltage that eventually pulls in the relay.

So, do you guys think I should follow bluejets circuit diagram or can I try just a resistor somewhere?

I really appreciate the help. I obviously don't know anything about this stuff.
 

Tha fios agaibh

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Does not sound right to me. I would check for continuity and ohms across the float switch. (Do not touch both lead with your fingers as this will give false ohm reading.)
When open, it should not pass any current, and certainly not energize the relay coil.
Also take a voltage reading across the alarm contacts, so we know what reference voltage we're dealing with.
 

slick-rides

Dec 23, 2015
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I'm trying to avoid having to replace the long wire run, but maybe there's a bad spot somewhere that could even be in or around water. The wire runs through a tunnel system and then through conduit underground.
 

duke37

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This thread seems to be going nowhere.
Measure the resistance of the loop.
Measure the resistance of the loop to ground.

The resistance of the loop can be compensated by increased driving voltage or a more sensitive detector.
Any connection loop to ground indicates a failure of the loop insulation and it should then be renewed.
 

slick-rides

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Well, looks like I found the problem. I started taking apart all the junction boxes along the conduit(pvc) run. In one of them the wires were just connected by wire nuts. They were encased in ice at the bottom! Chipped the ice out, made new connections and heat shrinked them. The 24 vdc "circuit" now is working correctly. I don't have any voltage through the loop until I engage the float switch. All seems well.

Thanks to all that tried to help me. I appreciate it!
 

Tha fios agaibh

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Nice find.
Hopefully your conductors, boxes and splices are all rated for wet locations. (They do make wire nuts listed for wet locations.)
Keeping water out of a pvc underground conduit run is next to impossible.
 
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