Gents
How and why is a Thermistor in a smoke unit used?
And why so some Legacy engines have them and some do not?
Thanks
Dave
Original Post
Replies sorted oldest to newest
A thermistor is a negative coefficient resistor. That means as the surrounding temperature goes up, its resistance goes down.
It can be used with a sensing circuit to shut down your smoke unit heating element if the heating element gets too hot. Your heating element will get too hot when your run low on fluid and then you end up burning the wick.
Lou N
Lou,
How do you figure the size of the thermistor, if you wanted to install one in a smoke heating element unit, of some sort?
Steam Forever
John
It's the sensing circuit that will determine the trip point. You would have to determine the temperature setting desired for the position of the sensor and then build the circuit to handle that switch point. For the Legacy models that use them, the position of the thermistor is fairly critical. I've repositioned a couple of them slightly to get the proper smoke level. This was the problem with my VL-BB blowdown smoke, moving the sensor slightly really made a huge difference in operation. When I matched it to the position of the whistle smoke (which worked fine), I had good smoke again.
I suspect you'd have to do some measurements with the sensor installed to determine the proper trip point.
Is the sensor used just for temperature control or did they integrate some kind of fluid-out detection?
Stan, I'm almost certain that it's simply a temperature control, though when you get low on fluid, it may reduce the heat to the smoke resistor. Lionel's Jon Z. could more fully answer the total question I suspect.
Thanks for jumping in John....
As a note, I have seen temps as high as 750F on the heat element surface in a run dry situation.
Lou N
Lou, you probably know a lot more about this than I do as you worked on the TAS smoke control, right?
One issue here is spacing would be very critical I would imagine,
Just a heads up, there are both NTC and PTC thermistors.
True Steve, but it doesn't change the discussion, only how you'd use the information from the sensor.
Thank You for the replies.
If this is to be considered a "help" or assest to the operation of a Legacy smoke unit.....then I wonder why Lionel is not including them in all the Legacy offerings?
Lionel just testing out something new in the marketplace?
Dave
I think Lionel is just trying to improve the design of the smoke units. I suspect they'll be standard if they really do materially improve the smoke performance and longevity.
Thank You All who responded.
John G.
gunrunnerjohn posted:Lou, you probably know a lot more about this than I do as you worked on the TAS smoke control, right?
One issue here is spacing would be very critical I would imagine,
GRJ~
Yes I worked on the TAS smoke unit. And, yes, spacing is critical. The thermistor was part of a voltage divider with the midpoint feeding an analog in on a PIC. The Vin at the PIC was a specific value (in code) that gave a digital out to a logic triac to turn off the heating element. I think it was a 12C671.
Lou N
I'm guessing that's why the Lionel one needed a "tuneup" for my VL-BB, it was quite a bit different in spacing than the whistle steam which worked great. Once I adjusted it, things fell into place.
I don't see this as an "add-on", as it requires mechanical changes to the smoke unit as well as control electronics.
While adding a super chuffer board to my Kline Berkshire, I took the opportunity to change the wicking in the turbo smoke unit I installed well over ten years ago. When I pulled the unit apart I was amazed there was no charing of the wicking. Great design, and a great product.
Lou N posted:Thanks for jumping in John....
As a note, I have seen temps as high as 750F on the heat element surface in a run dry situation.
Lou N
Wow !
750F...... so in a normal smoke unit with fluid running normal under command......roughly what temps would be reached?
Thanks
Dave
Soo Line posted:Lou N posted:Thanks for jumping in John....
As a note, I have seen temps as high as 750F on the heat element surface in a run dry situation.
Lou N
Wow !
750F...... so in a normal smoke unit with fluid running normal under command......roughly what temps would be reached?
Thanks
Dave
Keep in mind that 750 is a run dry unit burning up you wick.
Depends on whose smoke unit you are looking at but 250~300F surface temp. Remember you are vaporizing some heavy fluids (its against Rich's rules to identify them exactly).
Lou N
Mineral oil? 
Mr Lou, working on an Atlas GP60 with the TAS smoke unit that had the thermistor almost touching the element; what should be the "ball park" gap between the element and the thermistor (problem was intermittent smoke); thanks for your help; Falcon70
I am in the process of fixing my VL BB dual stack smoke unit; does anyone know the gap between the thermistor and the heating element?
Rick, as you can see I asked the same question last year and got zero response; just did my VL BB as you are and found the thermistor and the element at about 1/8" or 3/32" apart; replaced the wicking and left the gap the same; worked out much better with the new wicking; good luck; Falcon70
Thanks Falcon70, when positioning my thermistors next to the heating element, per advice, I re-positioned them horizontally, parallel to the heating resistor and at the same height and found a gap measuring 1/16"; I know I saw somewhere on the forum where there was a spec for the gap. However, my smoke unit is now done for: see
Falcon70,
I found the following comments in my Lionel notes by Jon Zahornacky retired smoke unit engineer for Lionel:
Jon Zahornacky, aka SantaFeFan (smoke unit engineer for Lionel, retired) said that the smoke units were engineered for the factory block of wick to work with the heat resistor, a thermistor located 1.6mm from the heating resistor and with the wick block a certain distance and density to the heating resistor. You cannot buy the factory block of wick. So, it is critical that you locate the replacement wick and its density and thickness and depth as close as possible to the original factory block and not block the fan air holes.
1.6mm converts to 0.0629921 inches which converts to 1.009/16ths.
RickM46 posted:Falcon70,
I found the following comments in my Lionel notes by Jon Zahornacky retired smoke unit engineer for Lionel:
I think Jon was just a tiny bit more than a "smoke unit engineer" for Lionel. He was the CTO and was the big gun behind the Legacy system design as well as a host of other electronic projects for Lionel.
Should have worded that a bit better about Jon; he has a ton of patents.
More comments on Jon: in my searches on the forum for info on my problem (self induced) with the BB smoke unit, I did come across Jon; he was an engineer, inventor, patent holder, and technical force directing development at Lionel bringing modern technology into our hobby; happy retirement Jon!
