Author Topic: Is a thermal fuse a current fuse as well?  (Read 5575 times)

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Offline Zero999

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Re: Is a thermal fuse a current fuse as well?
« Reply #25 on: August 09, 2017, 12:55:41 pm »
But the thermal fuse will blow, long before the transformer catches fire.
No it does not have to, read my previous post.
When the current is excessive the contacts in the thermal fuse might weld together before the thermal pellet was molten and the small spring could break the contact.

Now this can be also gotten indirectly from a datasheet where they state that you have to test yourself for DC operation where the chance of contact welding and arcing is much higher than with AC operation:

Quote
Current interruption capacity in DC circuits is highly application sensitive.
Therm-O-Disc recommends thorough testing of DC electrical applications using the testing guidelines in Therm-O-Disc’s MICROTEMP® thermal cutoff technical information section.
How the heck is that going to happen? The enamelled wire inside a small transformer will itself fuse, long before the current is high enough for that to happen.

It is not inside windings, it is located on top of the winding, under insulation tape. Why do you think that whole winding must heat up before it is able to catch fire? What if there is a short which bypasses the whole or most of the winding?
And how likely is that? The other end of the winding typically goes through a slot, to the centre of the bobbin.
 

Offline Kjelt

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Re: Is a thermal fuse a current fuse as well?
« Reply #26 on: August 09, 2017, 01:00:45 pm »
How the heck is that going to happen? The enamelled wire inside a small transformer will itself fuse, long before the current is high enough for that to happen.
Transformer maybe but TS has a heating cushion, no transformer in there, so I don't kwow why you are all babbling about transformers suddenly, has nothing to do with this topic now does it?
Anyway the whole point is that a thermal fuse should never be used as a current fuse, period.
 

Offline drussell

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Re: Is a thermal fuse a current fuse as well?
« Reply #27 on: August 09, 2017, 01:20:00 pm »
What is the rating of that normal fuse? You haven't told the most important part.

It doesn't matter.  As long as it is still working there is no reason to need to know, other than curiosity and as he said, he doesn't want to bother opening it to see.  (Neither would I :)

there's another "normal" fuse in there as well. I can't tell what that fuse is rated at as it's heatshrunk and I don't want to cut it open just to have a look, but it's no bigger than a 1/4W resistor.

It will probably be one of those little pico-fuses that look like a resistor.  They're available in a wide range of values but if and when it ever needs replacement he can figure that value out. 
 

Offline Zero999

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Re: Is a thermal fuse a current fuse as well?
« Reply #28 on: August 09, 2017, 01:30:15 pm »
How the heck is that going to happen? The enamelled wire inside a small transformer will itself fuse, long before the current is high enough for that to happen.
Transformer maybe but TS has a heating cushion, no transformer in there, so I don't kwow why you are all babbling about transformers suddenly, has nothing to do with this topic now does it?
You've obviously missed this reply:
https://www.eevblog.com/forum/beginners/is-a-thermal-fuse-a-current-fuse-as-well/msg1276121/#msg1276121
Quote
Anyway the whole point is that a thermal fuse should never be used as a current fuse, period.
I agree and in this case, there is also a current fuse.
 

Offline Kjelt

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Re: Is a thermal fuse a current fuse as well?
« Reply #29 on: August 09, 2017, 01:40:44 pm »
You've obviously missed this reply:
I didn't miss it but it is way offtopic, a transformer is built by another company than that builds the product that uses the transformer.
There is no way the transformer company can know what current the end product nominally uses and what current the fuse should blow except the max. the transformer is designed for.
Ergo the transformer company will never calculate the current use of the product in the design in of the thermal fuse and as stated earlier it can not since a thermal fuse is not a current fuse.
 

Offline wraper

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Re: Is a thermal fuse a current fuse as well?
« Reply #30 on: August 09, 2017, 01:54:42 pm »
What is the rating of that normal fuse? You haven't told the most important part.

It doesn't matter.
It does matter a lot. OP asked what thermal fuse to use, 1A or 10A. If the main fuse is, say, 0.5A, there is no reason to use 10A thermal fuse. if the main fuse is 2A, of course he shouldn't use 1A version.
 

Offline Zero999

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Re: Is a thermal fuse a current fuse as well?
« Reply #31 on: August 09, 2017, 03:23:42 pm »
You've obviously missed this reply:
I didn't miss it but it is way offtopic, a transformer is built by another company than that builds the product that uses the transformer.
There is no way the transformer company can know what current the end product nominally uses and what current the fuse should blow except the max. the transformer is designed for.
Ergo the transformer company will never calculate the current use of the product in the design in of the thermal fuse and as stated earlier it can not since a thermal fuse is not a current fuse.
So what if wraper's question and all replies to it were off topic? I know it can sometimes be a pain to sift through information to find what is relevant, but I find I often learn a lot from off topic posts. If it were a political or religious rant, then I'd understand it being against the forum rules but it was still vaguely on topic.

Current and thermal fuses both serve similar purposes, they protect against fire, even though they go about it in totally different ways. In the case of an current fuse: its job is to fail open circuit, before the current through the wire causes it to get hot enough to cause a fire. In the case of the thermal fuse: its job is to fail open circuit, before it reaches a dangerous temperature. When the thermal fuse is embedded in a motor or transformer, the windings do not normally need separate over current protection, because the thermal fuse will safely disconnect them before it catches fire. Now, I accept what you said before that too much current could prevent some thermal fuses from operating correctly, so in large transformers (where the short circuit current could easily exceed the thermal fuse's rating for long enough to cause a problem), additional over current protection may be required, but in a small transformer, it's not necessary.

Is it isn't always practical to provide over-current protection to everything. One could argue that inside a mains adaptor, the short piece of cable, from the mains pins, to the transformer, requires over current protection, but where do you stop? There's never any over current protection between the first short piece of cable to the fuse holder in a battery circuit because it's not practical.

What is the rating of that normal fuse? You haven't told the most important part.

It doesn't matter.
It does matter a lot. OP asked what thermal fuse to use, 1A or 10A. If the main fuse is, say, 0.5A, there is no reason to use 10A thermal fuse. if the main fuse is 2A, of course he shouldn't use 1A version.
Yes, I agree. The thermal fuse should have an equal, or greater, current rating than the over current fuse.
« Last Edit: August 09, 2017, 03:30:20 pm by Hero999 »
 


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