first i have a little understanding of circuits etc (enought to be
dangerous) and am wanting to make a temperature controller for a hot tub. I
have reworked my tub so it runs on a hot water heater element. currently i
have a hot water heater element attached to the heater to control
seems most of the controllers are now giving a good 10-15 degree spread from
on to off to on!!!
1>- I wish to have a set temperature, no variation, so a simple on off
controlling maybe a relay??
2>- are their currently chips off the shelf to run as a sensor.
3>- maybe a 4 degree variation is okay.
KEEP it simple!!!
pls let me know.
remove the numbers
sounds like a do it youself electrocution kit to me. is the heater on a GFI
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
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.
. 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.