Smoke unit thermistor - what ohm rating?

Good question, there is no information on what's in those, so I'll be interested to see if someone actually has some information on it.

why not just order the correct one from Lionel as John has said there is no information on the device ! just look up your engine model number and you fix your problem! then desolder the part and be done with your repairs! 

Alan 

@gunrunnerjohn posted:

Good question, there is no information on what's in those, so I'll be interested to see if someone actually has some information on it.

Lionel has told me that the thermistor should be 50,000 ohms +/- 2,500 ohms.

Since mine is measuring at 62,000 ohms, it's failed, and it's causing the electronics to think that the smoke unit is too hot and to shut it down.

As it happens, the thermistor is not available by itself. I need to buy the smoke unit board. But heck, I wasn't looking forward to soldering on the PCB, anyway. I prefer soldering jobs that require less finesse. 

Just curious, wondering why you can't order the smoke unit board through Lionel's website, but, instead, you have to call Lionel to order it?

It would be easy to replace if we knew what it was.  50K tells me something, but I still need more specifications.

There’s two type of thermistors,  one type the resistance goes down with heat, the other goes up with heat.  So which one does Lionel use?

Correct, NTC and PTC models.  Probably the only way to figure out which one they use is hit a thermistor with a heat gun and see which way the resistance goes.

I had a early Legacy Big Boy prototype that came in for repair from the Neil Young collection sell off.  It didn't have a thermistor installed, just a straight leads between the connectors where it would go which make me think it could be a thermistor that increases when getting hot.

Jim

Well, you have a 50-50 chance of being right Jim.   I think the circuit design would dictate which type of thermistor was used.  Either temperature coefficient model could be made to regulate the current, so I measure one to find out which it is.

It’s sad no one from Lionel would respond to this thread.  Help us out with such issues.  Isn’t that the point of this section of the forum anyway?

I don't have one of the later Legacy smoke units on hand right now or I'd fire up my meter and heat gun and find out what it was.

@superwarp1 posted:

It’s sad no one from Lionel would respond to this thread.  Help us out with such issues.  Isn’t that the point of this section of the forum anyway?

Lionel told me offline that the thermistor should measure 50K ohms +/- 2,500, which was enough to indicate that the problem with my smoke unit was the thermistor, which was measuring 62K ohms. I reported this above.

I already ordered another smoke unit board. Whether the thermistor is NTC or PTC is sort of taking it another step. I suppose that knowing the ohm rating and whether it’s NTC or PTC, someone else with a failed thermistor could get just a new thermistor from an electronics supply house. But my problem, as the OP, has been taken care of.

@Keith L posted:

Lionel has told me that the thermistor should be 50,000 ohms +/- 2,500 ohms.

Since mine is measuring at 62,000 ohms, it's failed, and it's causing the electronics to think that the smoke unit is too hot and to shut it down.

As it happens, the thermistor is not available by itself. I need to buy the smoke unit board. But heck, I wasn't looking forward to soldering on the PCB, anyway. I prefer soldering jobs that require less finesse. 

If you're saying 62K Ohms is causing the electronics to shut down from heat then doesn't that suggest it's a PTC (resistance rises with increasing temperature)?

50K is an unusually large value for a PTC.  For example, DigiKey has 243 choices for a 50K NTC, and 0 (zero) choices for a 50K PTC.  Can someone post a photo of the smoke unit board showing the thermistor along with whatever supporting electronics?  I realize it's a moot point since replacing the entire board is the only remedy.  It's just I find it curious that a circuit with a 50K +/- 5% trips at 62K which can be as little as 20% variation.

I believe it's likely to be an NTC, I ordered a couple from Digikey on my recent order.  I guessed at the specs and got a couple of these.  For 62 cents, it wasn't much of a risk.  https://www.digikey.com/produc...eywords=TH350J39GBNI

Right.  Without getting into the muck, a typical 50K NTC (i.e., 50K Ohms at 25 degrees C) that measure 62K Ohms only represents a change of about 5 degrees C....so 20C.  It makes no sense that a smoke unit protection circuit would shut down at 20C (about 68 F)!  I think there's something else amiss with the defective smoke unit... 

Not to mention that it's quite possible that the ambient temperature when you check the sensor could be that much different than the nominal 25C.  Suppose it's 68F, now we're checking at 20C.

Exactly!  It's a rainy day here and my indoor temp is 68 F (20 C).  A typical 50K NTC thermistor would read about 62K Ohms!  LOL.  Good thing the OP bought the entire smoke unit assembly. 

Hard to say what is really going on there, I'm betting the thermistor isn't it.

@gunrunnerjohn posted:

Hard to say what is really going on there, I'm betting the thermistor isn't it.

John, as my father used to say, "When you're right, you're right."

I installed the new smoke unit board. The thermistor on the old board measured 62K ohm; the thermistor on the new board measures 51K ohm.

However, the smoke unit still keeps shutting down. It runs 1-2 minutes, often less, before the cab flashes the dreaded two blinks. This is maybe a tad longer than before--but it sure ain't the solution.

I even tried putting in new batting and using different smoke fluid.

What other variables are there that could affect the smoke unit shutting down? It can't be that complicated, can it? I'm flummoxed!