help with making heat controller for hot-tub ?

sounds like a do it youself electrocution kit to me. is the heater on a GFI protector?

there has to be some hysteresis or the circuit will "chatter" (rapidly turn on and off)

a digital temperature control with all parameters programmable can be found at places like Granger.

a pool a spa outlet will have products specific to your needs. see here for exmple

First, as Tim asked, are you on a GFCI, if not stop right now and buy a GFCI breaker. It is code and also common sense. The next thing is you need to be sure that element base is bonded to the other spa equipment and grounded with a #8 solid copper wire. Also you need to bond any other metal within 5' of the spa with a #8 solid. Omega will sell you a real pretty 2"x 1" stainless reducing bushing with enough "meat" to install a bonding lug for $15. You could use galvanized but it will rust out in the chemicals. Brass is virtually unobtainium in this size. Don't do anything until you get your elements solidly bonded and you get the GFCI on it. They do go bad and when they do they usually short to the case. Your water heater is bonded too.

OK that said, your thermostat is not really that hard to make if you can work with a 324 op amp. There are actually 4 on a chip but it is an easy, single power supply chip to work with. Make a whetstone bridge with a terrmistor sensor a 25 ohm pot (plus a resistor and a trimming pot) to match the sensor and 2 other resistors about the same size. There are a lot of articles on the web about these bridges. Connect the op amp across the sensing corners of the bridge and use a solid state relay from the output to hit your elements Guys like Hosfelt or Allelectronics will sell you a 40a unit fairly cheap. This is better than a chattering relay and you can hold the hysterisis very low. It is also just a LED as far as the op amp is concerned. It also gives you good isolation if you make a seperator baffle between the line and low voltage sides. Remember the SSR will generate some heat. You need a heat sink. A good source for a sensor is the auto parts store. The temp sending unit for most cars is about 250 ohms for mechanical guage units and some thousands of ohms for a computer sensor (at 100f)

I have a thermostat on my spa that is made like this.

ground fault intalled.

all items are connected with #8 solid copper, i am about 10 feet from the outdoor distribution box and i have added another 15' grounding rod to the other end. i have used galvenized 2' pipe for my heaters.made in a u shape with two 5500 watt heaters, one only works when i fill the tub, the other is used to maintain temperature.

maybe someone knows of a good quality lower heater for hot water tanks. I have tried a couple new one and they are just too much fluctuation.

many thks bg

. Brass is

You are not going to get the accuracy you want with a water heater thermostat. In a spa a degree or two is very noticable. I can hold mine within a degree with the OP amp/car sending unit.

You will end up rusting out that galvanized pipe pretty fast.

I also interlocked mine with a 110f thermal sensor that shuts down the whole system on a thermal runaway.

The only simple answer is DO NOT FOOL WITH THIS STUFF.

Get a licensed electrician experienced in hot tubs.

I would not answer any questions in case I might get blamed after the event.

-- John G

Good luck on that

good point, i do my own legal work too!!