So, I was going to continue to work on the lantern project, but fate intervened and caused my dryer to die…again! I’ve found replacement parts online TWICE now and they just keep failing.
The part that keeps dying is known as the thermal cut-off fuse. When the internal temperature of the dry gets too hot, the fuse blows. As with most fuses, this thermal cut-off is a one shot deal. I’m guessing that the temperature regulator is not working correctly and is not keeping the temperature constant.
The dryer has three temperatures setting (low, medium, and high.) I’m going to have to read the temperature of the dryer and find out how hot the different temperature settings are. Since we are talking about temperatures in excess of 300⁰F (150⁰C) we have limited measurement devices at our disposal.
A K-type thermocouple is a really easy way to read high temperatures, especially because there are several easy ways to read thermocouples. A good number of multimeters, like my trusty Fluke 197, can read K-type thermocouples. All I need to do is attach the thermocouple to the location that formally held the thermal fuse, short the blown fuse, run the dryer, and read the max temperature!
Again, to get the dryer running again, the blown fuse must be shorted. Since this creates an extremely unsafe situation (the fuse was a protection device that I’m now defeating) I’m going to babysit the dryer and watch the values on the multimeter to make sure things don’t get too hot.
To short the fuse, I just used an alligator clip to hold the two ends of free-hanging wire together. Super safe!
After running the dryer with the thermocouple in the heater enclosure, I found out that the max temperature of the dryer is around 80⁰C. That’s nowhere near the rated temperature of the thermal fuse of 309⁰F (80⁰C is 176⁰F.) On top of that, the temperature settings on the dryer appear to do nothing from what I can tell. All three seem to have the same performance.
Well, I’ve been burned by two aftermarket parts now, so it seems that this looks like a chance to design my own temperature safety device!
Let’s put together some requirements before we get started.
- The circuit needs to protect from an over temperature event. The previous thermal fuse was supposed to trip at 309⁰F (which is 153⁰C.) To make things simple, I’m going with 150⁰C, (I’m going with centigrade from here on out, and never look back at Fahrenheit again!)
- I’d like this to be resettable. I don’t want to have to remove the back of my dryer every time there is a problem with the circuit and replace parts (which I’ve done a half dozen times now!)
- The circuit needs to be able to withstand the relatively hot and humid environment of the back of the dryer.
- The power needs to come from either a battery or the 210V AC wall voltage. I haven’t determined which way I want to go. The battery is super simple to design, but I’d have to replace the battery every so often and I don’t want to do that. (see requirement 1) On the other hand, tapping into the 210V line would require some more parts to get it to work. Also, I’d have to deal with high voltage, which I try to avoid if possible.
Well, I’m off to the brainstorming phase of the design process next.