Is a thermal fuse a current fuse as well? - Page 1
Forum section: electronics / beginners

author: Lupin III.
author_rank: Contributor
timestamp: August 08, 2017, 12:41:46 pm
content: I have a heating cushion that has a blown thermal fuse in the control circuit. My guess, someone "reset" the 90 minute heating limit (done by a microcontroller) by cycling the power all the time. The fuse is rated 102°C, 250V, 2A. At my local electronics shop they got either a 1A or a 10A version. I'd rather buy it there than pay 2000% handling and shipping costs for a 20-30 cent part. Can I use the 10A version or is a fuse like that doubling as a current fuse as well? (in my case the circuit has another fuse directly at the input as well).

author: DTJ
author_rank: Super Contributor
timestamp: August 08, 2017, 12:48:52 pm
content: Check to see that there is a second current limiting fuse in the unit, if so fit the 10A unit.Be careful soldering the replacement. I heard of someone replacing one in a pedestal fan, the soldered in 2 before they realised they were killing them by soldering. They needed to be crimped.

author: Pseudobyte
author_rank: Frequent Contributor
timestamp: August 08, 2017, 12:52:26 pm
content: No do not use a 10A in the place of a 2A fuse. If you do something like this, the whole point of having that 2A fuse is lost, and the circuitry is no longer protected in the way it was designed to be. Most of the thermal fuses that I have encountered have not been single use, and are usually resetable PTC style. If the fuse has truly failed and is not just tripping as soon as you turn the DUT on, then it should be replaced by the same or equivalently rated part. You could just put a piece of wire in it's place, but that is a terrible idea, this is how you burn your house down.

author: drussell
author_rank: Super Contributor
timestamp: August 08, 2017, 01:01:17 pm
content: Quote from: Pseudobyte on August 08, 2017, 12:52:26 pmIf it is a normal thermal fuse, that is incorrect.Standard thermal fuses are intended to open at a certain temperature only, not interrupt based on current. The rating on a thermal fuse is the maximum continuous current handling capability, not some kind of current interrupt rating.Obviously if you pass too much current through one it will eventually get hot and open itself but I've never seen one characterized and rated to do that intentionally at a certain current, though I suppose specialty ones may exist.

author: Pseudobyte
author_rank: Frequent Contributor
timestamp: August 08, 2017, 01:18:03 pm
content: Quote from: drussell on August 08, 2017, 01:01:17 pmYes that may be what they are designed to do, but when he says it is rated at 2A, that is the continuous current rating, right? So I don't think he wants to go putting in a non thermal fuse that is rated at a higher continuous current.Now I am thinking that I misinterpreted a bit. I thought he was talking about replacing his thermal fuse with a standard slow blow fuse. As long as the 10A fuse is rated to trip at the same temperature it should be OK. But it still opens up a current path that is rated much higher than the original.

author: wraper
author_rank: Unknown
timestamp: August 08, 2017, 01:30:18 pm
content: Quote from: Pseudobyte on August 08, 2017, 12:52:26 pmAll PTC fuses are low voltage. 250V thermal fuses are single use. Most likely 1A fuse will be fine. I guess, 2A fuse was used there only because such fuses are the most common type. Generally they are used as single protection device without any additional fuses, almost every small mains transformer has such inside.

author: Kjelt
author_rank: Super Contributor
timestamp: August 08, 2017, 01:41:45 pm
content: Quote from: wraper on August 08, 2017, 01:30:18 pmYou don't know for sure unless you know the current usage of the device.If you look at the datasheet for a thermal fuse there is an average operating time with nominal current and that is usual 1000 hours.So you do want the thermal fuse nominal current to be higher than the nominal current of the device or after 1000 hours you can again replace the fuse.Then since it is a heating cushion and heating elements usually have a way lower resistance when cold than warm you also do need to take the inrush current when the element is cold into consideration.

author: wraper
author_rank: Unknown
timestamp: August 08, 2017, 02:00:06 pm
content: Quote from: drussell on August 08, 2017, 01:01:17 pmThen why small transformers (used in power bricks and similar) have only a thermal fuse in them (usually 2A), and are connected directly to mains without any additional fuses?

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2017, 04:35:03 pm
content: Quote from: wraper on August 08, 2017, 02:00:06 pmBecause no other fusing is necessary?A fuse is there to protect the cable against fire. The wire inside a small transformer is much thinner than the wire going to it and if it gets too hot, due to a short circuit, then the thermal fuse will blow, thus safely disconnecting it. The thermal fuse is always, just on the outside of the coil, with one leg going to the enamelled wire and the other straight to one of the terminals or flying leads. It's fairly well insulated with tape, so there's very little chance of a short circuit before it.

author: drussell
author_rank: Super Contributor
timestamp: August 09, 2017, 01:04:52 am
content: Quote from: Pseudobyte on August 08, 2017, 01:18:03 pmWho said he was replacing it with a non-thermal (regular current-interrupting type) fuse? That would be insane.QuoteWhat? No... I believe he was just asking if he could replace a 2A max thermal fuse with a 10A max thermal fuse. Yes. Of course you can. The 1A is quite possibly inadequate depending on the load but he didn't supply any information about the actual current consumption of the device. Maybe the 1A version would be fine as well. That doesn't matter, though, this isn't an overcurrent device so the 10A version should always be an appropriate but more robust substitute.

author: drussell
author_rank: Super Contributor
timestamp: August 09, 2017, 01:12:35 am
content: Quote from: wraper on August 08, 2017, 02:00:06 pmI think you are confusing a regular current limiting fuse (which IS a form of thermal fuse, I will concede) with what we are talking about here. We are talking about the kind of fuse that cuts off when the body temperature of the device is raised above a certaintemperature.Like, for example, in a coffee pot so that in case the main thermostat fails and the element gets above 250 degrees, or whatever, it opens so that the unit must be serviced. (Hopefully by someone who knows what they're doing)They are also used in motors (at a lower temperature that a heating device, for example) and about a zillion other things with many different one-time-blowout maximum temperature ratings.This has nothing to do with the current through the device!

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2017, 08:07:37 am
content: Quote from: drussell on August 09, 2017, 01:12:35 amI don't think he was confusing a thermal fuse with a current limiting fuse. He was noting, that in small transformers, the type found in wall warts, there's a thermal fuse, but no current limiting fuse.

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 08:27:06 am
content: Quote from: drussell on August 09, 2017, 01:12:35 amHave you disassembled/repaired a single transformer in a power brick made in last 20 years?

author: 2N3055
author_rank: Super Contributor
timestamp: August 09, 2017, 08:32:09 am
content: Thermal cutout fuses break at certain temperature.. Period.But since they have internal resistance, they will self heat with current going through them.So if you take 100°C /1A fuse and start pushing higher and higher current through it, at some current higher that 1A it will self heat to the point of thermal breakdown.1A rating means that at 1A it will not self heat enough to influence its 100°C cutoff temp much...Sometimes manufacturers combine self heating and temperature to get right temp cutoff. If your heater draws less than 2A, than only temperature is important and you can easily put 10A one.. If heater draws more than 2A, than you have take into account self heating, and also use 10A one but one that is less than 100°C.You can also try to find a bimetal based resettable thermal protection component (hair driers and electric heaters use them) Those will reset when they cool down. Of course find the right temp one...

author: Kjelt
author_rank: Super Contributor
timestamp: August 09, 2017, 08:51:46 am
content: Quote from: wraper on August 09, 2017, 08:27:06 amThey aren't made for the last 10 years, all smps which do have a primary current fuse unless it is ultracheap crap than they have a small pcb trace which should act as the fuseBut the older wallwart which do have a transformer it depends, I have seen both, current fuse on mains on the pcb and thermal fuse in the transformer.I think it is a question of $ because I agree with above posters that a thermal fuse should only be used to break contact at high temperature and the current is only to make sure that overheating the fuse does not falsely melt the temperature pallet.And another thing to think of , if in case of high shortcircuit, I do not think the thermal fuse is guaranteed not to latch, eg the current so strong that the contacts in the thermal fuse are melted together.So run 100 Amps through a thermal fuse and see if it breaks if it was designed to it would have been stated in the datasheet which I can not find in the standard temp fuses datasheets I just read

author: 2N3055
author_rank: Super Contributor
timestamp: August 09, 2017, 09:08:56 am
content: Permanent thermal fuses in a transformer are there to prevent fire and to make sure that transformer is rendered unusable after overheating that probably damaged electrical insulation which makes transformer unsafe...Overheating can happen without excessive currents. Overcurrent protection in form of current fuse is there only if transformer fails in such way to go into practical short circuit.You can have one on transformer, but your power socket fuse will handle it safely too...

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 10:04:18 am
content: Quote from: Kjelt on August 09, 2017, 08:51:46 amThey are still available, although not nearly as widely as before. Also such transformers are still built in into many devices.

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 10:08:58 am
content: Quote from: 2N3055 on August 09, 2017, 09:08:56 amWhich power socket fuse may I ask as I'm not aware of it's existence? If you are talking about 16A or 25A breaker in electrical box, it will happily stay ON while your small transformer catches fire.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2017, 10:17:05 am
content: Quote from: wraper on August 09, 2017, 10:08:58 amBut the thermal fuse will blow, long before the transformer catches fire.Quote from: wraper on August 09, 2017, 10:04:18 amYes, they're still used, especially for medical applications, as they have a lower leakage current. AC output mains adaptors are also still widely used because it's convenient to get bipolar DC, using two half wave rectifiers.I've also seen transformers like these with a bi-metal strip thermal cut-out, which has the advantage of being resettable.

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 10:54:21 am
content: Quote from: Hero999 on August 09, 2017, 10:17:05 amIf there is a short on the other side, it may catch fire before temperature of the transformer rises sufficiently to trigger thermal fuse by overheating.

author: 2N3055
author_rank: Super Contributor
timestamp: August 09, 2017, 11:37:16 am
content: Quote from: wraper on August 09, 2017, 10:54:21 amI don't understand what you try to say but here it is:1. Transformer won't catch fire before thermal fuse cuts primary current... Thermal fuse will cut at 100-120°C. It is inside windings. There is no fire at those temps.2. Electronics on secondary side of transformer must have their own current/thermal protection. It can catch fire at any time without taking trafo or any of it's fuses out of their specs for normal work.3. Primary side current fuse protects power socket and household installation from short circuit. Not transformer. That thing is long gone if it is in short circuit.4. Your household power sockets should not be connected to 25A fuses. 6A and 10A and 16A are ok if sockets are made for that current. They cannot be damaged with that. Smaller fuse on you trafo primary is good practice, because if done right, it might cut before your distribution box one, and prevent blackout for other devices connected to same circuit. But it doesn't protect your socket and distribution circuit. Your distribution circuit has to be designed to be protected on it's own. Otherwise it is not done properly.5. If you want to have protection from both thermal and all kind of overcurrent protection, you need to have thermal cutout, primary fuse (for short circuits) and secondary fuse (for overcurrent on secondary side). Thermal cutout will handle defects where current rises so much it doesn't blow any of the fuses but is still overheating trafo.6. Trafo insulation is specified for working temperature. There is such thing as max permissible working temp, max peak short term working temp, and permanently damaging temperature. If you reach that one, your trafo is permanently damaged, and must not be used anymore. Because of that, standards prescribe that a one time blow thermal protection element be put inside transformer winding, in such a manner that it cannot be replaced without damaging it completely.It is deliberate kill switch. Resettable one is strictly forbidden for that purpose.User or manufacturer are free to add additional resettable thermal protection element, set for max peak short term working temp, to protect transformer from occasional incidental overheating from user pulling marginally higher current from nominal, or when there are chances of working in extreme heat environments. Also it can be used to protect equipment specified for periodic intermittent work that is supposed to work short period and that should let certain period of time to cool down...With small transformers, most of the time resettable one is not used, but trafo is simply slightly over specified, and core is run at lower power density so trafo has reserve..

author: Kjelt
author_rank: Super Contributor
timestamp: August 09, 2017, 11:42:30 am
content: Quote from: Hero999 on August 09, 2017, 10:17:05 amNo 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

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 11:54:16 am
content: Quote from: 2N3055 on August 09, 2017, 11:37:16 amIt will blow, but not not necessarily because of wingdings overheating.QuoteIt 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?

author: Lupin III.
author_rank: Contributor
timestamp: August 09, 2017, 12:25:28 pm
content: Thanks for all the comments, although it's a little bit hard to find the answers between all the not so related stuff.To summarize, the amp rating on a thermal (as in reacting to a certain temperature) fuse does not mean it will also blow at that current. That's what I wanted to know. So picking the 10A rated thermal fuse is fine, especially since (as I've written in the first post) 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.The heating cushion is rated a 100W, on the highest setting I measured 125W (0.54A) at startup from cold and dropping a little with time. So maybe it would even be fine to use the 1A rated thermal fuse, which is physically smaller. Space is a little constrained since the original fuse was positioned between (and touching) two parallel 4.7k 1/2W carbon film resistors (that are visibly darkened, but fine).And, no I never wanted to place a current fuse in place of the thermal fuse.

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 12:38:24 pm
content: Quote from: Lupin III. on August 09, 2017, 12:25:28 pmWhat is the rating of that normal fuse? You haven't told the most important part.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2017, 12:55:41 pm
content: Quote from: Kjelt on August 09, 2017, 11:42:30 amHow 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.Quote from: wraper on August 09, 2017, 11:54:16 amAnd how likely is that? The other end of the winding typically goes through a slot, to the centre of the bobbin.

author: Kjelt
author_rank: Super Contributor
timestamp: August 09, 2017, 01:00:45 pm
content: Quote from: Hero999 on August 09, 2017, 12:55:41 pmTransformer 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.

author: drussell
author_rank: Super Contributor
timestamp: August 09, 2017, 01:20:00 pm
content: Quote from: wraper on August 09, 2017, 12:38:24 pmIt 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)Quote from: Lupin III. on August 09, 2017, 12:25:28 pmIt 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2017, 01:30:15 pm
content: Quote from: Kjelt on August 09, 2017, 01:00:45 pmYou've obviously missed this reply:https://www.eevblog.com/forum/beginners/is-a-thermal-fuse-a-current-fuse-as-well/msg1276121/#msg1276121QuoteI agree and in this case, there is also a current fuse.

author: Kjelt
author_rank: Super Contributor
timestamp: August 09, 2017, 01:40:44 pm
content: Quote from: Hero999 on August 09, 2017, 01:30:15 pmI 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.

author: wraper
author_rank: Unknown
timestamp: August 09, 2017, 01:54:42 pm
content: Quote from: drussell on August 09, 2017, 01:20:00 pmIt 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.

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2017, 03:23:42 pm
content: Quote from: Kjelt on August 09, 2017, 01:40:44 pmSo 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.Quote from: wraper on August 09, 2017, 01:54:42 pmYes, I agree. The thermal fuse should have an equal, or greater, current rating than the over current fuse.