The other day, my wife was making some Easter decorations for the house and as usual, she had her hot glue gun turned on. The gun was on for about half an hour and out of nowhere, it exploded while sitting on the table.
Luckily, the breaker immediately tripped so most of the explosion was confined inside of the gun and nothing caught on fire.
Watching the entire thing happening in front of my eyes I was puzzled at what happened. My initial reaction was that the cable somehow broke and the AC wires touched inside but since the gun wasn’t handled at the moment, that theory seemed unlikely.
To investigate further I set to open the gun and my first challenge was finding a bit that matched the screw. The case was held together by six Torx screws that had a pin in the middle. This is a special type of Torx screws, also called security Torx or pin-in Torx.
As the name implies, since it is not a common type the purpose of it is to be difficult for regular people to have that bit to open it up. However, we are not those regular people and I managed to find the right bit in one of my sets.
When the screws were out, the case still didn’t want to release because of all the hot glue that spilled inside. I used two screwdrivers to pry open the case and after some fiddling around I managed to split open the case.
On the inside, the entire construction of the hot glue gun is very simple where there is a metal casing where the AC wires go into and a mechanism that pushes the glue sticks inside in order to heat up and melt.
The entire assembly was glued to the other half of the case so I used my torch lighter to heat it up and release it from the glue.
Since there are no electronics inside to regulate the temperature, the hot glue gun resorts to a trick that the heating element has in order to prevent being overheated and it self-regulates.
The heater is made out of a special material that has a positive temperature coefficient, meaning that the resistance of the material increases when the temperature increases. This material is either a form of rubber or more commonly a form of a ceramic that is then enclosed between two metal plates.
When current runs through it, it starts to heat up and when it does, its resistance will go up limiting the current that It can go through it. In normal operation, this will come to a point of balance where the temperature can not rise any more so the PTC element will self-regulate.
Since I had the heater out of the enclosure and there wasn’t an obvious short that might have caused the explosion initially, I decided to give it a go and try it once again. I thought that maybe the problem was temporary as if something got stuck in between the plates while operating.
Be warned that this is live AC that we are dealing with right now. If mishandled, it can kill you so only proceed if you really know what you are doing.
Having said that, I placed the heating element separate from any metal around and I plugged it in, only to be greeted with a nice fireball and pieces of molten metal flying around my desk.
Luckily, the silicon mat caught most of the particles and prevented any damage and I was well aside so I was uninjured but very intrigued. The explosion was so violent that it also tripped the breaker in my house so I had to reset it.
What was causing this seemingly short circuit conditions?
I had to find out, so I first took out my other hot glue gun and measured the resistance of its heater. This gave me the first clue as you can see, its resistance is roughly 10 times greater than the resistance on the broken one. This one measures close to 10 kOhms while the faulty one is just 1 kOhm.
Whatever happened to the damaged one must have permanently altered the ceramic material and made it lose its properties.
To test this theory, I removed the cable, wrapped a piece of Kapton tape to isolate the element and hooked it up to my multimeter. While connected, I started heating it up and to my surprise, the resistance started going down instead of up.
I repeated the process several times and the same thing was happening over and over again. The material was no longer acting as a PTC but as NTC. It switched properties that now it had a negative temperature coefficient which explains the short circuit and the explosions.
While the current runs, the element gets hotter and hotter making it reduce its resistance allowing for even more current to flow. At some point, the current is so high that it starts to arc across the plates and causes the explosion that trips the breaker.
By doing some searching I also found some other fellow YouTubers that experienced the same phenomenon so it seems that this is probably a common way how PTC heaters fail but at the moment I have no idea what caused this so if you have any suggestions, please leave them down in the comments.
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