dc coil with problem

[Greek_engineer]

The problem I would like to mention has to do with a 12v dc coil.
To be more specific,the coil's reistance was 72 ohm at a normal condition.
After having energized for about 5 minutes the coil's resistance became 525 ohm but it was still able to hold the contacts close.
If you de-energized the coil and energized the coil again the latter was not able to close its NO contacts.
You had to wait for about 5 minutes for the coil to be cooled.
Since it was cooled (after 5 minutes) and its resistance altered from 525 ohm to 72 ohm the coil was able to function properly.
Has anyone faced any situation like that?

[chemi-cool]

How stable is your 12vdc supply? Does it have any fluctuations?

The coil might get shorted when temp rises. All coils are hot to touch when they are energized for some time.

[Greek_engineer]

the 12 vdc supply is stable.
when the temp rises coil's resistance alters from 72 ohm to 525 ohm.

[al]

is the transformer too small for the loads applied?

[Guner Edip Riza]

The DC type relay coil resistance varies due to ambient temperature as well as to its own heat generation to the extent of about 0.4%/°C, and accordingly, if the temperature increases, because of the increase in pick-up and drop-out voltages, care is required. (However, for some polarized relays, this rate of change is considerably smaller.)
Considering the above extract taken from a publication, there is no way that the ohms of the coil can go from 72 to 525 due to heating.
If your measuring is accurate there can only be an issue with the coil.
A break may develop in coil circuit due to heating and high ohms may be the value of the carbonisation developed due to arcing at this point which reverts back to original when cooled.

[Greek_engineer]

that may be the problem

[nike123]

Aso, problem could be at soldered joint of winding to coil terminals, if it is made that way.
https://www.quora.com/Is-a-dry-or-cold-solder-joint-conductive