[SOLVED] Replacing a thermal cutoff - Ensure replacement is safe. - Page 1
Forum section: electronics / repair

author: CaptainObvious
author_rank: Contributor
timestamp: May 24, 2015, 02:24:50 am
content: I purchased a wireless "lamp switch" if you will a few months back, didn't have a use for it until just a few days ago.. but of course, when I try to use it, it doesn't appear to work!It's basically got a 433MHz receiver and transmitter kit (maybe 315MHz) with the PT2262 pair chips and a small relay to flip the mains, just using the 1 channel on the PT chips. Push button on transmitter once, its powers on.. push it again, it goes off. After troubleshooting last night, I found the issue, appears there is an on-board thermal cutoff fuse that's blown. (Not sure if it blew when it was first plugged in, or months ago when purchased)Long story short, I've found some similar replacements online but there's a couple digits off on the markings and I'm not sure if it's just a date code.. or something more important.I know I need 102 degree C cutoff, 250v at 3 amps, but the rest of the model is what I'm unsure of. As seen in the picture below, "A1-3A-FX4" but when referencing this link:http://www.aupo.com.hk/article-22-1.htmlandhttp://www.ebay.com/itm/5-Pcs-Aupo-Thermal-Fuse-Cutoff-TF-102-250V-3A-A1-3A-F-/181212613669I'm not able to find a "FX4" version.Really the question I'm after, am I safe replacing this fuse with the eBay link? Just not sure if that "X4" really makes a huge difference. I tested the amperage across the fuse pins, the max draw I was getting was about 43mA, so I'm sure temperature is the biggest factor, as it was laying under the mains lines (red n white cables)) Rather safe than sorry, thanks in advance for your insight!(Here's a direct link to the JPG, if it's beneficial at all:http://lvls.org/images/thermal-cutoff.jpg)

author: wagon
author_rank: Frequent Contributor
timestamp: May 24, 2015, 11:43:49 am
content: Just get a 102 degree celcius thermal fuse that looks like that. That's what I'd do if I was repairing that device. Cheap, easy to get.Where are you located?

author: CaptainObvious
author_rank: Contributor
timestamp: May 24, 2015, 06:40:03 pm
content: That's what I kind of figured, guess the "X4" just threw me for a loop! I found 5 of the fuses for $4 (free shipping) so I'll jump on those and get this bad boy repaired.Northwestern part of the states here, Washington to be exact.

author: Gyro
author_rank: Super Contributor
timestamp: May 24, 2015, 06:56:36 pm
content: Difficult to tell from your photo - I can't see what's under the sleeving, but these cutouts are normal crimped rather than solder (for obvious reasons!).If it is soldered then you'll need to heat-shunt the leads very well and minimize contact time.

author: CaptainObvious
author_rank: Contributor
timestamp: May 24, 2015, 07:12:32 pm
content: Yeah, it was soldered on directly to the board. (Potentially why it failed before even putting a load on it?) It's not a very big brand by any means, so I didn't expect the best quality (AmerTac) but I did want it to work out of the box. Alas, maybe my expectations are too high.I haven't had to deal with soldering a lot of heat sensitive components, I'm guessing just a decent heat sink across the leads while soldering and minimizing time should be okay? (Don't have any real "sinks" so planned on using some pliers holding onto the leads.)Oddly enough, just searched for this model.. and while I don't see an exact match, there are a few comments of this brand coming in DOA. For anyone curious in the states, here's a link to Amazon, appears to be same receiver just a different transmitter. (Mine is handheld)http://www.amazon.com/Westek-RFK100LC-RFK101LC-Mounted-Receiver/dp/B000HJBE68/ref=cm_cd_al_qh_dp_tAppreciate the responses!

author: Gyro
author_rank: Super Contributor
timestamp: May 24, 2015, 07:39:17 pm
content: Hmm tricky. I suspect you're right about the reason for failure.Hard to think they would get away with anything other than hand soldering it, even if the rest of the board was wave soldered.Pliers are probably about your only option for heat-sinking. Keep the leads as long as possible consistent with safety - sleeving may help but leave enough clear to get your pliers in (obviously), slide it up and glue afterwards if necessary (pliers close to the fuse will probably be the most effective).It might be worth gripping each lead firmly and quickly tinning the end first to minimize soldering time on the PCB joint. Check continuity before fitting to the board and leave the pliers on for about 10s longer than you think you need to.At least it sounds as if you'll have some spares - Good luck!

author: CaptainObvious
author_rank: Contributor
timestamp: May 24, 2015, 08:18:54 pm
content: That's quite the failure to make coming out of a "factory" if that was the case, they should know better. Either way, I'm glad it was a quick/easy fix!I'm going to stick a DS1820 (temperature sensor) inside and put an Arduino off to the side to get some temperatures while I'm waiting for these fuses to come in, curious to see what what the operating temperatures really are. I'll be using it as it was intended.. then include an ATMega8 (have some extras) for IR control (control from Samsung cell phones) and add timer functionality, so I can be lazy when I go to bed, but still have the light on!Thankfully it's in an easy location to get some pliers into without bumping into many components. I'll have a few extra just in case I fail, part of learning as we all know! I think I will tin them before tossing them into circuit, for exactly that reason make it a quicker iron application.Thanks again guys!

author: wagon
author_rank: Frequent Contributor
timestamp: May 26, 2015, 04:33:46 am
content: Keep the lead length long & be quick with your joints. Do one joint, let it cool. Then do the other.

author: Ian.M
author_rank: Super Contributor
timestamp: May 26, 2015, 04:56:25 am
content: Freezer spray! pre-chill the heat sink pliers and thermal fuse body to give you a better chance of success.