Hot tub heater control
This circuit controls the heating element of a water jet spa tub. My job was to write up a Theory of Operation and quality control test procedure for the assembly line. I also took care of a few quality control issue that came up after the start of production.
Production reported that rarely one of the resistors on the power line would spark and open when the unit was first powered on. But the unit did pass the UL required surge and line noise testing. So I simulated continuous relay contact bounce by powering the unit through a probe that was scratched across a rusty plate. This created sparks and powerful and random noise in the power line. And I discovered that all of those types of resistors would burn open after about 20 seconds. So we tried a different type of resistor and found some that would not burn open even after 20 minutes of scratching.
There was also an issue with the thermistor that measured the water temperature. This thermistor was epoxied into an indentation in the steal tube. It was used as part of a safety switch to turn the heating element off if the water got too hot. Production reported that some units would turn off at too low of a temperature. So I measured many samples at the same room temperature and formed a histogram (See the graph below). I found that our supplier of thermistors was including thermistors of a lower value in the bags they were selling to us. I showed them the histogram and asked them to better sort the products they were selling to us.
The pressure switch also had some problems. If the water pump shut down unexpectedly, the water pressure would suddenly decrease and the pressure switch would engage. The electronic controls would turn off the heating element when the pressure switch detected that the pump was turned off. The set pressure at which the switch would engage would abruptly change after running in the system for a while so that the pump would unexpectedly turn off. There is a spring inside that pushes against the snap dome and the snap pressure is set by the number of turns of an adjustment screw. I had a number of switches X-rayed, and it was found that the spring was getting caught on the threads inside the shaft. Any vibration would loose the spring from the threads and change the set snap pressure. After removing some threads, the unexpected change in the set snap pressure no longer occurred.
Histogram of thermistor resistance
Pressure switch
