Op-Amp Hot Tub Temperature Controller:

One evening late in the summer, I decided to take a little dip in the hot tub before bed.  I went out to the patio, swung back the cover and slipped in.  As usual, there was the sting of hot water that always goes away after a moment or two.  However, this time it just got worse!  I quickly got out and discovered that the temperature was 112 degrees and climbing!  The temperature controller had gone bad.

Now, this hot tub was purchased for $50 at a garage sale a few years ago.  It was five o'clock on a Sunday and there was a House For Sale sign in the front yard so a deal was to be had!  The tub worked fine when we got it home and I was able to buy a filter element that fit online.  However, it was clear that this tub had had issues in the past as there were many signs of repair in both the plumbing and electrics.  Within a month there was a small fire in the electrical box.  I scraped out the carbon, rewired the relays, and replaced all of the molex connectors with soldered joints.

Last year the bearings in the motor started to make a lot of noise.  These was no way I was going to invest $400 in a new pump motor so I took it apart and removed the bearing to see if I could find replacements.  After a fruitless online search I discovered them at my local Aubuchon Hardware store for $12 each. 

This year the filter housing started to leak.  I removed it and carried it over to our local Big K and start looking for something about the right size, shape and material.  In the sporting goods aisle I found the perfect thing; a polypropylene bait bucket!  Within a couple of hours the bucket was fitted and the tub refilled!

So, the tradition was established, no ordering the appropriate replacement part for this tub!  a hack was required.

I pulled out my dog-eared Radio Shack Engineer's Mini Notebook by Forest Mims and got to work.  I used a LM 339 comparator and a BUZ11 FET for the output stage.  The IC may have been able to drive the relay directly but I wanted a robust design.  I bought a 6 volt relay and a 10K thermistor from Radio Shack and the rest came out of the parts box.

I bread boarded the circuit first and tested it on the dining room table.  My wife kept asking me why I couldn't just buy the part! ;-)

Everything fit on a bit of perf board I had and I found some lovely little gold-plated connectors in my junk drawer for the sensor.  I installed the components and let the tub come up to temp.  A little calibration and the tub was keeping temperature better then ever!  The thermometer never waivers from 104 degrees.

A couple of months passed and I started to have trouble.  The temp went up to 110 again.  Some quick tests determined that the sensor was several K-ohms less then it should be at room temp.  I had made the sensor by encasing the thermistor in gum-wall heat shrink tubing but the hot-glue lining in the tubing seemed to be breaking down from the heat and chemicals in the tub.  I made a new sensor by soldering the end of a piece of radio antenna closed, filling it with heat sink grease and inserting the thermistor.  The water now only touches the tubing and can't get close to the sensor.


The only other trouble I've had was when the filter cap in the wall-wart power supply went bad, the pump relay chattered furiously and the lights in my house flickered alarmingly every time the heater came on.  A new cap fixed that.

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