Help with thermal fuse specs?

[DaveC]

Found a white, square, 2-lead device located in a small transformer, under
the insulation tape, soldered in series with the primary winding of a
120v/12v small transformer in a wall-wart power supply. It is open (no
infinite cont.)

It has these markings:

M20
Tf (that's T sub-f) 115 deg C
11F

It is a little smaller than a TO-220 package (no tab heat sink, 'tho).

What is this? I presume it is thermal cutout or fuse of some kind. What specs
do I need to know to replace it with kind unit?

Can anyone cross this number to a current product?

Thanks,

 

[DaveC]

It is open (no infinite cont.)

Of course, this should say "infinite continuity."

 

[Alan McClure]

Of course, this should say "infinite continuity."
--
DaveC
[email protected]
This is an invalid return address
Please reply in the news group

Of course, it should REALLY say "infinite resistance"
or "no continuity".

ARM ;-)

 

[DaveC]

Of course, it should REALLY say "infinite resistance"
or "no continuity".

Blast and damn those early-morn posts!

 

[Jim Adney]

Found a white, square, 2-lead device located in a small transformer, under
the insulation tape, soldered in series with the primary winding of a
120v/12v small transformer in a wall-wart power supply. It is open (no
infinite cont.)

It has these markings:

M20
Tf (that's T sub-f) 115 deg C
11F

It is a little smaller than a TO-220 package (no tab heat sink, 'tho).

It certainly sounds like a 115C thermal fuse, but all the ones I've
ever seen were cylindrical, about the size of a 3A diode, but with a
metal shell and axial leads. One lead insulated.

I used to be able to buy replacements at Radio Shack, but I believe
NTE sells a wider assortment.

-

 

[DaveC]

I used to be able to buy replacements at Radio Shack, but I believe
NTE sells a wider assortment.

Yeah, I got an NTE replacement. It is larger, and cylindrical, as you say.
But there was room, and after ruining one by soldering it in without using a
heat sink (Doh!!), I successfully installed one. Works fine, now.

Thanks,

 

[Asimov]

"Jim Adney" bravely wrote to "All" (15 Dec 03 22:14:50)
--- on the heady topic of "Re: Help with thermal fuse specs?"

JA> From: Jim Adney <[email protected]>

Found a white, square, 2-lead device located in a small transformer, under
the insulation tape, soldered in series with the primary winding of a
120v/12v small transformer in a wall-wart power supply. It is open (no
infinite cont.)

It has these markings:

M20
Tf (that's T sub-f) 115 deg C
11F

It is a little smaller than a TO-220 package (no tab heat sink, 'tho).

JA> It certainly sounds like a 115C thermal fuse, but all the ones I've
JA> ever seen were cylindrical, about the size of a 3A diode, but with a
JA> metal shell and axial leads. One lead insulated.

Yes, I've seen square thermal fuses...

.... You may be a tech if you're entertained by a 6-pack and sparking HV.

 

[Jim Adney]

But there was room, and after ruining one by soldering it in without using a
heat sink (Doh!!),

BTDT

Isn't it amazing how quickly they respond to temperature? ;-)

-

 

[GPG]

Short the "thermal" fuse and wire in a conventional one.
These fail because of repeated transformer currents on switch on.
Use an appopriate value fuse.

 

[Sofie]

Short the "thermal" fuse and wire in a conventional one.
These fail because of repeated transformer currents on switch on.
Use an appopriate value fuse.

--------------------------------------


GPG:
Not really a good plan......
........a conventional fuse will not provide over-temperature protection and
fire hazard prevention.
The thermal cut-out fails not because of "repeated transformer currents on
switch on" but rather because of over-temperature due to the windings in the
transformer over-heating.
Usually the thermal cut-outs, in addition to the temperature rating, have a
CURRENT rating well in excess of the transformer application..... 2 amps
for the small thermal cut-outs and 10 amps for the "common" ones available
from Radio Shack, NTE, ECG, etc.

 

[Ben Miller]

Short the "thermal" fuse and wire in a conventional one.
These fail because of repeated transformer currents on switch on.
Use an appopriate value fuse.

Exactly how will this protect the winding against over-temperature
conditions? What is an "appropriate value"? Have you run the UL temperature
tests to confirm the effectiveness of your recommendation? Modifications
such as this are not a good idea.

Ben Miller