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Using coils for occupance detection

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PED
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Using coils for occupance detection
Posted by PED on Friday, January 04, 2019 11:18 AM

Currently adding coils on track feeder wires for determiation of track occupancy. The track feeder wire passes through the hole in the coil so the coil can detect current to the track.

My question is....can adjacent wires (not passing through the coil) induce current into the coil? Lots of 12VDC and DCC wiring under my layout. If that is possible, I need to be cautious about coil placement so that it has no extraneous wire next to it. 

Paul D

N scale Washita and Santa Fe Railroad
Southern Oklahoma circa late 70's

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Posted by rrinker on Friday, January 04, 2019 12:14 PM

 Yes, a wire assing by on the outside can induce current, if it's bundled in close. Most coil detectors need one loop through rather than just sticking the wire through the hole on the way from the bus to the detected section, so outside of a high current (short) situation, a wire passing by on the outside wouldn't induce enough current to exceed the detection threshold. Just don't bundle the wire to the detection section with those other wise, and don't bundle all the wires up around the coil and it should be fine. You don't have to go nuts and put the coil a foot away from every other wire or anything.  It's just as important to not bundle the feeder, or twist it with the feeder for the common rail, or have anythign attached past the transformer except the track that is to be detected                                                                                                                                                                           --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

PED
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Posted by PED on Friday, January 04, 2019 4:08 PM

Thanks Randy. That is what I expected but I have no past experience with coils to base my actions on. I am adding these coils to existing feeders and I think I have room on all to keep the coil away from other wires. I will need to do some testing on the double wrap angle. I am using coils from RR-CirKits and they make no mention of double wraping them. I suppose that double wraping them will not cause any undue reaction.

Paul D

N scale Washita and Santa Fe Railroad
Southern Oklahoma circa late 70's

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Posted by rrinker on Friday, January 04, 2019 5:44 PM

 I forget how they are set up on the club layout, but i THINK the wire to the block goes through the donut hole, then loops around and goes through again, and then heads to the track. Interesting but I did check, there seems to be no recommendation in any of the manuals for the devices that use the coils.

                            --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by gregc on Saturday, January 05, 2019 4:54 AM

i found a significant difference between just running the wire thru the transformer and looping it one time.   Each additional loop improved sensitivity.

Detector Output Using 5V Reg Supply
LED/1k  5.0     5.0     4.6     3.7     2.2     0.6     Volts
Detector Output Using 5V & ~50T Transformer LED/1k 4.2 2.7 1.0 0.49 0.44 0.43 Volts

   

greg - Philadelphia & Reading / Reading

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Posted by richhotrain on Saturday, January 05, 2019 5:02 AM

On my layout, I use NCE BD20s as occupancy detectors for my grade crossing signals. I wrap the wire (22 gauge copper stranded, plastic coated) once through the donut hole, and I experience zero interference by neighboring wires (DC and DCC) under the layout.

While I suppose that electrical interference by neighboring wires could be a problem, I simply have never had an issue, and I have a lot of wiring under the layout that is close enough to the occupancy detector wire to be a problem.

Rich

Alton Junction

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Posted by betamax on Saturday, January 05, 2019 6:04 AM

gregc

i found a significant difference between just running the wire thru the transformer and looping it one time.   Each additional loop improved sensitivity.

Detector Output Using 5V Reg Supply
LED/1k  5.0     5.0     4.6     3.7     2.2     0.6     Volts
Detector Output Using 5V & ~50T Transformer LED/1k 4.2 2.7 1.0 0.49 0.44 0.43 Volts

   

 

Yes, the more loops through the CT the smaller the current it can detect.

When you do this, spread them out so the flux is more distributed.  Sometimes there are inconsistencies in the winding, this helps get better results. Also prevents saturation at one point in the winding.

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Posted by gregc on Saturday, January 05, 2019 6:06 AM

since transformer detectors are sensing a current, not a voltage, i don't see how interference from neighboring wires, a false detection, is even possible without there being a circuit thru which a current can flow.   In other words, if there is no resistive path on the track being monitored completing the circuit through the transformer, not current can flow.   If there is a resistive path, any interference insignificantly adds to the current already being detected.

greg - Philadelphia & Reading / Reading

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Posted by richhotrain on Saturday, January 05, 2019 6:12 AM

gregc

since transformer detectors are sensing a current, not a voltage, i don't see how interference from neighboring wires, a false detection, is even possible without there being a circuit thru which a current can flow.   

That would be my thinking as well, but, hey, I'm no electrical engineer.

Rich

Alton Junction

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Posted by betamax on Saturday, January 05, 2019 6:43 AM

gregc

since transformer detectors are sensing a current, not a voltage, i don't see how interference from neighboring wires, a false detection, is even possible without there being a circuit thru which a current can flow.   In other words, if there is no resistive path on the track being monitored completing the circuit through the transformer, not current can flow.   If there is a resistive path, any interference insignificantly adds to the current already being detected.

 

Considering that the current in a wire next to the CT would be probably be 180 degrees out of phase with what is happening in the the CT's coil, it would subtract from the secondary current if it did anything (assuming the currents are flowing in the same direction in both wires). The resulting current would be very small anyway. CTs are designed for the current conductor passing though the middle of the coil, not on the outside.

DC wouldn't bother a CT because it needs a changing flux, which DC doesn't really provide, to induce a current.  (Yes, there are transformers to measure DC current, but they are very expensive and have limited applications).

An issue with DCC is parasitic capacitance on the power bus.  Get enough caused by twisting the bus wires, and a small current will flow.  It can be enough to trigger dectection.

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Posted by rrinker on Saturday, January 05, 2019 10:39 AM

 A wire for a different block running close by on the outside of the sense transformer can still induce a slight current - probably not enough to actually cause detection, but just don;t bundle the wires all together. Might even be more likely that two wires run closely in parallel, current flowing in one wire could induce a current in the second wire, which would be detected by the secoond wire goign through the sense coil. That in fact may be a more likely way to get a flase detection than the wire running along the outside of a different block's sense coil. 

 I think just about anything outside of bundling all the wires with a wire tie will be OK.

 Getting the right number of turns - that's always the fine point. Too many turns and while it all works when just a loco or a car with resistor wheels runs throught he block, if there's a short and your circuit breaker is set for 3 amps, so 3 amps is flowing, the output from the coil can exceed the input of the next component. Some designes for coil detectors have a clamping diode to prevent this and the limit then becomes more or less the PIV rating for the diode. Problem is, it also then is a viscious circle - with the diode then the minimum detection threshold needs to just exceed the forward voltage drop of the diode, which may require more turns of the feeder through the coil, which raises the peak voltage at maximum current....

                                   --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by gregc on Saturday, January 05, 2019 3:23 PM

rrinker
 A wire for a different block running close by on the outside of the sense transformer can still induce a slight current

how can it induce a current in an open circuit?

rrinker
Problem is, it also then is a viscious circle - with the diode then the minimum detection threshold needs to just exceed the forward voltage drop of the diode, which may require more turns of the feeder through the coil, which raises the peak voltage at maximum current....

a schematic for the BD20 shows a pair of diodes in series to clamp the trigger voltage.   They will limit the voltage across the transistor to 2 diode drops, ~1.4V.  While more turns will increase the output from the transformer, the clamp voltage remains the same.

The protection diode needs to be able to handle the maximum current induced thru the transformer due to the highest short circuit current which may be 10A booster w/o a circuit breaker.   w/ 5 turns and a 50 turn transformer, there is a 10:1 ratio; a 10A short may induce 1A for the duration of the pulse.    If the pulse were as high a 5 usec, the average current over 100 usec would be 50 ma.   Looks like the 1n4150 diodes used in the bd20 are rated for 4A

i believe the threshold is in terms of time and current, not voltage.   The transformer generates a current pulse.   That pulse causes the transistor to conduct for a fraction of the duration of the pulse and during that time, discharges the capacitor proportional to the current in the pulse, if the transistor doesn't saturate.

the capacitor is recharged thru the 10k during the absense of the pulse.   If the capacitor is discharged more than it is re-charged between each pulse, its voltage will drop with each pulse every 100 usec DCC cycle.   It may take a while, but should eventually drop below some devices (LS, CMOS, 555, Atmel, ...) threshold for recognizing a low input.

of course it's desirable that it not take more than a second or so.   While a car with only a 10k resistor would have taken a while to discharge the capacitor, it may be able keep it discharged.

greg - Philadelphia & Reading / Reading

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Posted by rrinker on Saturday, January 05, 2019 9:18 PM

 Who says it's an open circuit? I was talking about the feed for an adjacent block passing close to the block being discussed. Of course there is zero chance of false detection if the block whose wire runs too close to the other block's is empty.

 That may be so based on the BD20 schematic but others show a resistor across the coil - as do many of the data sheets for the current sense coils. Induced current through the resistor results in a detectable voltage proportional to the current, by a factor of the value of the resistor.

 Anyway we are drifting off into a discussion of the design of the detector circuits, which was not the OP's question. For the OP's question - bundling the feeders from the coils out to the isolated detection sections = bad, but thee's no need to make sure they are all a foot apart or anything. I think we had another thread on designing detectors for those of us interested in circuit theory. 

                            --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by richhotrain on Sunday, January 06, 2019 3:41 AM

rrinker

Anyway we are drifting off into a discussion of the design of the detector circuits, which was not the OP's question. For the OP's question - bundling the feeders from the coils out to the isolated detection sections = bad, but thee's no need to make sure they are all a foot apart or anything.  

More precisely, the OP's question was...

PED

My question is....can adjacent wires (not passing through the coil) induce current into the coil? Lots of 12VDC and DCC wiring under my layout. If that is possible, I need to be cautious about coil placement so that it has no extraneous wire next to it.  

While there is general agreement here that adjacent wires will not induce current into the coil, if the OP remains concerned, he could always run the coil wire thru a piece of plastic tubing under the layout.

Rich

Alton Junction

PED
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    April, 2016
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Posted by PED on Sunday, January 06, 2019 3:56 PM

As the OP, I am satisfied with the info in this thread. Although it took some side trips, my basic questionn has been answered. I am no longer concerned with nearby wires causing me a problem. The only coils I have installed so far is on a piece of test track for testing my components. Those coils have a single wire (no double loop) and they are performing fine with a loco present. Have not tested with a resister axle yet.

Paul D

N scale Washita and Santa Fe Railroad
Southern Oklahoma circa late 70's

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Posted by rrinker on Sunday, January 06, 2019 4:23 PM

 If you have one, take a regular leaded resistor the same value as the ones that you use in the resistor wheelset and test with that.

 In making my resistor wheels, I found that a single car was sometimes hard to detect - and by detect I mean registor on a multimeter, not activate any sort of detection circuit - unless the wheels were SUPER clean - and that's with an NMRA weight HO car, some even slightly over the RP. N scale will be just that much lighter. What I had to do to make the cars reliably detected (and will need to redo after many hours of running) was to polish the wheel treads with a Dremel - there is a nylon version of the wire wheel attachment - do NOT use a metal wire wheel. Holding the car upside down with my thumb on one wheel of a wheelset, and held the Dremel in my other hand and lightly touched the 'wire' wheel to the tread of the other wheel - you don;t want to just allow the wheeselt to spin at Dremel level ROM, so you brake the opposite wheel with your thumb. I use 2 resistor wheelsets per car, one on each truck, but I ended up polishing them all. Since I weathered the wheel faces on all the wheels, resistor or plain, the contrast of the oily rusty wheel face and the bright shiny tread looks good, like a real car, at least one that's been moving in a train and not parked on a siding for a few weeks. After doing this, I had 100% success in setting the car on a piece of track connected to my meter, and getting the expected resistance - in my case, about 5K, because I had a pair of 10K resistor wheelsets on each car. A pair of 10K in parallel like that is 5K. No pressing down, no wiggling of the car to get that. After a few hours of running (and now some 6 years packed away in their carry cases) I'm sure they need the wheels cleaned again - but in a full train, enough made contact that my train was detected in each block from the moment the loco crossed the gaps until, if not the caboose itself, then no more than 2 or 3 cars from the end, left the block. With the slow release of most block detection circuits, this was more than good enough. 

                                          --Randy

 


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

PED
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    April, 2016
  • 569 posts
Posted by PED on Sunday, January 06, 2019 4:30 PM

Randy...sounds like a solution for you but I think my MOW crew (me) would go on strike if tried to do that.

Paul D

N scale Washita and Santa Fe Railroad
Southern Oklahoma circa late 70's

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