ok to short the thermal fuse?

[questron]

I have a wall adapter that's not working, took it apart and discovered the thermal fuse on the transformer's primary winding is not continuous, it's an M20 Tf115 12J.

I don't have any for replacement, so the question is, is it ok to just short it, jump it?

tks for helping!

[c4757p]

No.

[Riotpack]

For brief testing you could replace it with a low value fuse (worked out by the transformers rating) to test if the enamel coating on the copper wire has been damaged. In the long term these fuses are usually non replaceable and you can not be certain of the damage to the insulation of the transformer due to the high temperature which caused the fuse to trip. I would replace the adapter.

[questron]

thank you Riotpack!

when i was using it, it did get warm, i thought somewhat warm is no biggy, but then it quit on me, so it did not get really hot. i can't see the whole winding, but the narrow edge of the winding's end looks ok, as new, no visual sign of burning or anything, i did not smell anything when using it either.

also, it's a
AC/DC Adaptor
Class 2 transformer
input 120AC
output 12V DC 400mA

and the primary reads 165 ohms.

does that sound like that winding is ok?
if the winding is ok, no problems with the insulation, then is it alright to just jump the thermal fuse?   

[Zero999]

Just measuring the resistance with a multimeter isn't an accurate way of determining whether the primary is good or bad. It may have one or more shorted turns which will cause excessive current to flow, result in it overheating and fire if the thermal fuse has been bypassed.

No it's never OK to just jump the thermal fuse. Although it may work, the thermal fuse is there to cut the power, if it overheats, preventing a fire. If you bypass the fuse and it overheats it could burn your house down.

How much would a new adaptor cost? If your house burns down and the fault is traced to the mains adaptor with the fuse bypassed then the insurance company won't pay out. Is the risk really worth it?

[questron]

hi Hero999,

besides burning down my house, there are at least 2 more things involved in here.

one, learning and understanding.

two, i need it to power a small tool now, finding a new one will take a while. also there's no quarantee a new one will have a thermal fuse in it, most places don't specify that, as far as i know.

so as Riotpack suggested, i bypassed the thermal fuse to give it a test, didn't notice anything wrong, except the DC output measured 20.1V, as opposed to 12V. that's too high, yeah? what does that suggest, as a simple resistance measuring could, primary shorts? the thermal fuse shouldn't have voltage drop across it, correct?

to go one more step further in investigating and gaining understanding, what if i do have a correct replacement thermal fuse? simply replacing it won't eliminate possible dangers of a damaged primary, yeah?

what experienced people do in such cases and what is practical is one thing, learning about and understanding all these stuff is quite another. buying a new one is simple, cheap, and easy, but what understanding and knowlege does that give me?

so, the real world is always eludiing us yeah? it's not as simple as not burn my house down per se, do we really believe people are that studpid as not capable of that calculation? it's about learning and understanding mainly and really. oh well on a second thought, maybe we ARE that stupid, we made a huge hole in the ozone layer didn't we.

[SeanB]

20VAC open circuit is out of spec even for the cheapest transformer, it is likely your one has a shorted section of the primary winding. That both causes it to heat up and also reduces the turns ration so that the output voltage goes high.

That it looks good on the outside is no surprise, the cooking parts are likely right by the core inside the unit, where it is likely that a winding rubbed to another layer during manufacture, and the thin layer of insulation left finally shorted out from heating and thermal cycles, shorting out the hundred or so turns on the layer, and causing them to heat up a lot more than designed. The outer layers stay cool as they lose heat, the inner ones do not cool so easily.

The thermal fuse is typically a 115C unit, and will normally never fail unless the unit inside reaches between 112 and 115C surface temperature on the surface exposed to the fuse case. The case will be cooler because it is insulated, and has a larger surface area to dissipate heat.

Time to get a new transformer.

[wraper]

I have repaired few transformers with a blown thermal fuse. Replaced them with identically rated thermal fuse. Apparently there were no shorted turns as those transformers still work without any problems after many years. Thermal fuses are not too expensive, around 30-50 euro cents in the local shop.

[Riotpack]

Just measuring the dc resistance on the primary will not indicate if the insulation is weakened and could break down (even if slowly) when used at mains potential. Unless there is something very special about the plug pack I would replace it. You would have to do a breakdown test and a shorted turns test to be certain it was safe to be placed back into general use.

[questron]

20VAC open circuit is out of spec even for the cheapest transformer, it is likely your one has a shorted section of the primary winding. That both causes it to heat up and also reduces the turns ration so that the output voltage goes high.
...

very nice, detailed, and useful explanations Sean, tks a lot!

the thermal fuse in this adaptor was laid flat on the surface of the primary winding, just fyi. i'll be careful with it, i'm not insisting on using it unsafely, i'm just wanting to understand it, that's all.

[questron]

I have repaired few transformers with a blown thermal fuse. Replaced them with identically rated thermal fuse. Apparently there were no shorted turns as those transformers still work without any problems after many years. Thermal fuses are not too expensive, around 30-50 euro cents in the local shop.

it's really nice to hear from some one who actually went through such scenarios, thank you wraper!

thank you also for your info on the cost! i did check that out, it's about us$10 or us$12 for a bag of 50 from web stores, then there is shipping, then i need only one, and then there is also no electronic part stores to be found locally nowadays, plus there is no tell whether the transformer is actually safe or not without finding that out by actually using it, therefore i didn't go that route. so a seemingly minor thing is actually and really not that simple. but i'm learning, now i know, backed up by people's actually experiences as well, no complaints from me. thank you very much for your help!

[questron]

Just measuring the dc resistance on the primary will not indicate if the insulation is weakened and could break down (even if slowly) when used at mains potential. Unless there is something very special about the plug pack I would replace it. You would have to do a breakdown test and a shorted turns test to be certain it was safe to be placed back into general use.

thank you again Riotpack for your expert input!

i know a DC resistance measurement doesn't mean much, it's just that that is the only thing and the best i could do as of now.

i'm pursuing this, not because i'm insisting on keeping using it, but rather, i'm trying to learn about these stuff. and that's why a simple "replace it" comment would be meaningless to me, my understand about it will still be zero. meanwhile, pursuing the issue does not necessarily mean i insist on unsafe practice either.

now i'm really curious, because we are getting to the meat of it now. could you point me to somewhere for details on "breakdown test" and "shorted turns test" of transformers? or, if it's not too much of a hassle, could you give a brief description of these tests here, is it doable by normal people, at home, without sophisticated equipment?

thanks a lot!

[miguelvp]

Reminds me to the thread where some one asked for help after falling into a well

"So I should keep on digging? I do see the ropes you all threw in to help me climb up but I think I'll go with my original plan to keep on digging."
"No just climb up, you'll drawn if you keep on digging"
"Ok, I'll keep on digging"

or something like that. 

Edit: found it:
https://www.eevblog.com/forum/chat/advice-thread-humor/

It is unsafe, it way out of spec so that indicates a short and it's a fire hazard. Only way to repair it would be to unwind the transformer fix the shorts and put the thermal  protection back.

Edit: and winding a transformer by hand it's an ordeal btw, and if you don't know what you are doing will make it worse.

[SeanB]

I have fixed a lot of fans and such simply by replacing the thermal fuse ( not just remove it) and along with the new one stripping the bearings to remove the old hard grease from them to replace with nice new grease along with some oil in the felting pad. That way the new fuse will last a few years before failing again from the bearings slowing the fan down so that it both overheats from poor cooling air flow and excessive loading. Some went for a decade or more between servicing, often the first time was within 2 years of being new.

[Riotpack]

A form of breakdown testing is explained here:

http://electrical-engineering-portal.com/what-is-hipot-testing-dielectric-strength-test

Basically you are testing the integrity of the isolation of the transformer by applying a high voltage to test the insulation. This insulation is your protection against electric shock on the extra low voltage output of the transformer from the mains input.

This is to ensure that the enamel on the enamel coated copper transformer windings have not deteriorated due to the excessive heat.

The second test for shorted turns are where the insulation may have broken down on say the primary side of the transformer, but the isolation between the primary and secondary may still be OK (due to use of insulating tape such a kapton tape still providing insulation, or only a few turns are shorted etc.

[questron]

I have fixed a lot of fans and such simply by replacing the thermal fuse ( not just remove it) and along with the new one stripping the bearings to remove the old hard grease from them to replace with nice new grease along with some oil in the felting pad. That way the new fuse will last a few years before failing again from the bearings slowing the fan down so that it both overheats from poor cooling air flow and excessive loading. Some went for a decade or more between servicing, often the first time was within 2 years of being new.

thank you Sean, that's absolutely valuable real world experiences to me for sure. now my mind is not empty about such things anymore, i have real world references about them now.

i did a little digging, and it seems the tube radio community is rather familiar with this issue. people there mentioned simple circuits, neon lamp, halogen lamp, and even tungsten filament lamp tests, where the results won't be accurate down to the number of turns shorted of course, but would be good enough to indicate whether a transformer is usable or not. and all such techniques and knowledge are accumulated and passed down for generations since the dawn of radios, and are still being used by radio fans today. now, armed with that kind of knowledge, whether a particular transformer is deemed to be good or bad for use is up to an individual's good judgement, which is preferred over "replace it and still be dumb" mentality, to me lat least.

Riotpack has recommended link to various tests below. i'll go read those now. great help form you guys!

 

[questron]

A form of breakdown testing is explained here:

http://electrical-engineering-portal.com/what-is-hipot-testing-dielectric-strength-test

Basically you are testing the integrity of the isolation of the transformer by applying a high voltage to test the insulation. This insulation is your protection against electric shock on the extra low voltage output of the transformer from the mains input.

This is to ensure that the enamel on the enamel coated copper transformer windings have not deteriorated due to the excessive heat.

The second test for shorted turns are where the insulation may have broken down on say the primary side of the transformer, but the isolation between the primary and secondary may still be OK (due to use of insulating tape such a kapton tape still providing insulation, or only a few turns are shorted etc.

thanks a whole lot Riotpack, that's most interesting, getting right at the issue, what i wanted to know about, and the whole point of asing questions here, just great, never imagined there'd be so many transformer experts lurking around in here!

well, reading times now, thank you all for the great helps!