Difficult resistor to read values

[cybermaus]

So I have 40 years of electronics hobby in me.
And I can read resistors without needing to think about it. Usually.


Except these. These confuse me. Maybe there are some variations I am not aware of
Please some help.

3 different but identical devices (central heating thermostats, these power the underfloor actuators)
I read brown, white, brown, black
(I assume the blueish one is also that, but color has faded to orangy, disappeary, orangy)

I am assuming the resistors are OK, the problem is not the resistor, and they measure: 98 ohm, 100 ohm, 101 ohm
Pretty close together, and confirming they are probably OK

So the question is:
How do I make brown, white, brown, black or 1,9,1,0 into 100 Ohm ??


Black is never the first band (that would be like writing 0100 when you mean 100)
And it is never the last band for tolerance (as far as I can find)

Is it some sort of thermistor? But even then I have trouble reading the brown-white-brown into 100 Ohm
Please advise this fool who thought he knew resistors








 

[Ian.M]

I note that all three resistors are on their PCBs, not loose.  My first suspicion is that there is actually something in parallel with the resistor or otherwise shunting it, reducing the value measured in circuit.  If you haven't done so already, please desolder one end and re-measure.  If you've already done that, please confirm the measured values above are with the resistors out of circuit.  Also, sanity check your ohmmeter with a known resistor of similar value!

[inse]

Some of them seem to be quite toasty, I have come across those kind of resistors where color bands have discolored from heat.
E.g. a supposedly 100 Ohms resistor showed brown black grey gold (which doesn’t make sense).

[cybermaus]

I note that all three resistors are on their PCBs, not loose.  My first suspicion is that there is actually something in parallel with the resistor or otherwise shunting it, reducing the value measured in circuit.  If you haven't done so already, please desolder one end and re-measure.  If you've already done that, please confirm the measured values above are with the resistors out of circuit.  Also, sanity check your ohmmeter with a known resistor of similar value!

All 3 were de-soldered to measure the ~100 Ohm
Also note that even if they were broken, and al 3 broken in the same way, what is 1910 supposed to be? 190 Ohm with a 0 tolerance?

They are a bit toasty, that is true. But well, they are 1 or 2 watt models. Probably supposed to be a bit toasty.

[Ian.M]

Thank you.

Some of them seem to be quite toasty, I have come across those kind of resistors where color bands have discolored from heat.
E.g. a supposedly 100 Ohms resistor showed brown black grey gold (which doesn’t make sense).

Inse's explanation is probably correct though I am surprised the middle of the body was sufficiently hotter than the ends to burn off the black in a black band, (possible if it was carbon black) without affecting the black band on the end significantly.

[inse]

The center heats up the most as the ends are cooled by the metal cap and the leads

[Ian.M]

Certainly, but I'm still amazed it was enough to totally burn off the black pigment from the middle without any fading of the end band.

[cybermaus]

Not only evaporate the black band, but leave a white shadow?

I realize it must be something, but that would be the weirdest burned resistor I have ever seen.
And then all of them identically weird, even though one of them is clearly from a different manufacturer.
I cant make sense of it, but I feel the old "heat discoloration" is also not it.

----

Edit, well, I best let it be. Should not spend too much on it. At least it would make the two browns match:

So: Black evaporated (left a ghostly white imprint). And silver or gold burned to black.
Would make it a bog standard 101-5%

And that 3 times. On 3 devices from 2 different production runs ... it still does not sit wel.

[Ian.M]

When a resistor burns off the pigment what remains is usually grey or white so that doesn't surprise me.  I wouldn't expect a metallic pigment end band to burn to black, but a red 2% one might.  Alternatively a black last band was coding something manufacturer specific e.g. that the resistors were non-inductive or flameproof.   

[cybermaus]

Ok, so lets take your white ghost as true. Never seen it, but maybe I was not paying attention, and the brown-<something>-brown is too tempting to ignore for a 100 ohm resistor.
So I'll take that as lesson.

Lesson two would be black as non-inductive.

Do you have any ref for that? It would be interesting to know

But honestly, I can only find black as thermistor indicator in a band past the tolerance, so 5th or 6th. Non-induction would be a whole new thing.
Also, they just *look* like absolutely normal  resistors. Most normal (read cheapest) you could get. And also, used for resistive current limiting on a badly rectified 24Vac 50Hz, so no way anyone would buy expensive non-inductive rated stuff for that.

[Ian.M]

I've seen odd black final bands before on power resistors.  Some manufacturers seem to use it (+ body color) to code some sort of 'special' resistor range, presumably because it is*NOT* a tolerance color.  Without knowing the manufacturer - only possible if there's a service manual with a detailed BOM - we cant know which manufacturer specific coding applies.   I wouldn't be surprised from the application if for that manufacturer, it coded a 'flameproof' resistor.

[cybermaus]

it coded a 'flameproof' resistor.

LOL. that at least makes more contextual sense.
Especially considering the failure mode was: Got so hot the solder drained and one side of the resistor no longer made contact.
For all 3.


Edit: Yes, I am checking the heating knob actuators too. They are ruptured, and I think a cause for prolonged duration of the current. I mean, surely it is not normal for the solder to melt.

[m k]

I'd say that last on has its original colors.

Alternative reading could be something where white band is a separator.
Last one doesn't need it, distance difference is enough.

Thermistors have different color codings, and can have two different values.
So brown and brown black, based on measured value.

One thermistor coding is two colors.
There black black is 150 degrees and brown brown 110.
Black brown and brown black are missing.

One other thermistor coding has only colors from black to orange.

Maybe it is read from other direction.
So tolerance of 1% and value of 10 with standard multiplier.

[Wallace Gasiewicz]

Usually if the last band is BLACK. it indicates a wire wound resistor; whether that is non inductive or flameproof would be unknown unless you have the manufacturer's datasheet.   I suppose these are flameproof if they got so hot as to melt the solder without self immolating.         

If the last band is WHITE it would be a fusable resistor, but again you would need manufacture's specks to determine the fuse value.   

It is inherently possible that the last band color is designed to drive repair people NUTs........

[cybermaus]

It is inherently possible that the last band color is designed to drive repair people NUTs........

Active repair sabotaging is something I would absolutely see the likes of 21th century Apple or John Deere do, but these are circa 1995 Swedish house thermostats. Not saying they would go out of their way to enable repair, but probably not sabotage. Not yet anyway.

[Ian.M]

The thermistor hypothesis would be easy enough to check - heat or cool one of the 'resistors' while measuring its resistance, and see if there is a significant change in resistance.

[inse]

That would be an uncommon form factor for a thermistor