What is it?

This is a “room sensing probe” from a pellet stove. It failed. The replacement was $60. I suspect the cost of the actual sensor was less than $1. I also suspect the new one will fail in less than 10 years like the last one. In the hopes of saving some money (and possibly replacing it with something a bit more intelligent) I want to identify the sensor.

For various reasons I believe it is not a thermocouple.

What is it?

I have seen "bare" NTCs and PTCs like that many years ago in TVs.

Can you measure the resistance at two known temps.
e.g. room temp, and while you're warming it with your hands.
Does the resistance go up or down.

It could also be a diode (because of the red dot), so measure resistance both ways.

Make a chart of the resistance at various temps, so you can find a cheap replacement later.
Leo..

Thermopile?

Where do they locate it in the heater?

If you have the replacement, what is the resistance, does the resistance vary with temp.
Does resistance change when ohm meter leads are reversed?
Does it exhibit a voltage when heat is applied?
Is that a ferrite bead on the red wire?
What is the resistance of the old one?

Thank you for the reply.

Wawa:
Can you measure the resistance at two known temps.

I can. In about two weeks. I'm traveling without my electronics kit.

However, it is defective so measuring it may be fruitless.

And, I will not be able to get the good one within 1000 miles (literally) of my meter. At least until next year when I return.

So eyeball may be the only way to identify it.

Educated guess is that it's just a big-sized NTC (thermistor).
Leo..

Thank you for the reply.

LarryD:
Thermopile?

Uh ... let's see ... it can't be that. The cable is just too crappy for millivolts. And, as far as I can tell, there is no differential to it.

Where do they locate it in the heater?

It's in the room. The cable is about 12 feet. It's essentially a room thermostat.

If you have the replacement, what is the resistance, does the resistance vary with temp.

See the reply above.

Is that a ferrite bead on the red wire?

It is not. It is a tiny chunk of the same gray insulation that wraps the wire pair. I suspect it was supposed to keep the bare sections apart. (I wonder if that was the failure. The bare sections touching.)

Wawa:
Educated guess is that it's just a big-sized NTC (thermistor).

Huh. I assumed, being essentially a chunk of metal, thermistors would seldom fail. According to the Navy (via Google) they do have an age-related failure.

Hi,

However, it is defective so measuring it may be fruitless.

What makes you think that?

Tom… :slight_smile:

Hi,
Here is what E14 have;
au.element14.com/amphenol-advanced-sensors/rl0503-17-56k-96-ms/ntc-thermistor-30kohm-1-c-radial/dp/2317215

NTC..PTC not sure..I think the red dot on Coding Badly means one or the other.

Tom... :slight_smile:

For tests , replace it with a variable resistor - say 50k. Set your controller to 25C. adjust the resistor from low to high till it just fires up.

Measure that resistance

Now you know what value NTC thermistor to buy..

regards

Allan

TomGeorge:
Hi,What makes you think that?

Replace it with a brand new then the room fan works correctly. Restore the old one the room fan never runs.

TomGeorge:
Here is what E14 have;

Sweet! That looks exactly like it! (When I return I will verify the resistance with the good one.) Thank you!

au.element14.com/amphenol-advanced-sensors/rl0503-17-56k-96-ms/ntc-thermistor-30kohm-1-c-radial/dp/2317215

Our website is
Under Scheduled Maintenance
...

Argh!!!

NTC..PTC not sure..I think the red dot on Coding Badly means one or the other.

The red dot may have been placed there by the stove company.

What makes you think a low quality cable can't handle millivolts?

The key point is a thermocouple wire has two different metals for the two wires
(between the hot and cold junctions, that is).

MarkT:
What makes you think a low quality cable can’t handle millivolts?

Electrical noise. The cable is placed by the consumer who could route it over AC cables, fluorescent lights, one of the motors contained within the stove, etcetera.

Our previous home had a pellet stove. When we bought new a new home, we rented the old home and replaced the old with a new pellet stove. Both ran from a standard wall thermostat, just a temp controlled switch. The pellet stove fan came on when ever the pellet stove got hot enough.

Sounds like your stove controller is made to sense the room temperature and start up a new fire if the temp is low, or turn off the stove if the temp gets above a set point. Very interesting.

If your device died, there may be something more seriously wrong.

Paul

Paul_KD7HB:
Sounds like your stove controller is made to sense the room temperature and start up a new fire if the temp is low, or turn off the stove if the temp gets above a set point. Very interesting.

Close. The fire gets very very small when the target is reached. The fire size is related to the difference between the setpoint and the process variable with some lag / hysteresis.

If your device died, there may be something more seriously wrong.

From some internet searching it is apparently a somewhat common failure. Given the margin ($60 - ~$3) I have an uneasy feeling it may have failed by design.

Maybe you can replace it with a pot. :wink: