A friend reported that three of the four heating blankets he’s bought over the last several years have failed, so he sent the lot to me for teardown and maybe repair.
Looking inside one controller showed some obviously bad solder joints:
Hitting the joints with the soldering iron improved their outlook on life, but the controller remained dead; they weren’t really bad joints, they just looked that way.
If the “lot number” labels on the controllers mean anything, they’ve tried three different triac mounts over the years:
- A through-hole triac screwed to the board with no heatsink
- An SMD triac using the PCB copper as a heatsink
- A through-hole triac with a big aluminum heatsink
That’s in order of ascending lot number, suggesting the triac caused some reliability problems.
I’m still trying to figure out how to probe the circuitry without killing myself. An isolation transformer comes to mind, because the blanket dissipates only 85 W.
Surely the triacs have snubbers…