Hi, I would like to build a small wax heater to make a batik like technique.
The wax should be between 170 and 230°C and I would like to add a thermostate and a dimmer to fine tune the temperature.
Initially I thought to use an arduino shield Relay shield, but I think it should be possible with a single relay like this
do you think the relay is the right one? I assume that the heater, obtained from a small VAPE MAT for mosquitoes and a small vat to melt the wax into, should absorb a very low energy. Unfortunately I can't find the absorption rates on the Vape, but I guess it is very low, and I don't need a fast heating device, but only a stable one.
I assume the VAPE MAT is mains-powered. I suggest you use a small zero-crossing SSR to control it from the Arduino. By using burst-file control (effectively slow PWM), this will allow you to control the amount of heat precisely.
You'll have to experiment with the heater to make sure you've got enough heat. Remember that a certain amount of energy gives you a certain heat rise, so make sure you are getting enough heat to work in your coldest-expected environment.
The single relay should work, but check the coil current (or resistance) to make sure you're not exceeding the Arduin's 40ma rating.
With the Thermistor, you'll need to make a voltage divider (another resistor in series). Again, you'll probably need to experiment to determine the best value for the other resistor.
If you connect the output of the voltage divider to an analog input, you can use that to regulate the voltage. I
If you connect a potiometer to another analog input, you can compare the two readings and use the pot-setting to vary the target temperature. (One end of the pot goes to 5V, the other end to ground, and the center to the analog input. The valuse isn't critical... Anything between 10K and 100K should be fine.)
...I don't need a fast heating device, but only a stable one.
The stability comes from your thermistor & feedback system. Like any heating system, you turn it off when it gets too hot, and on when it gets too cold. There is usually some hysterisis (the turn-off temperature is slightly higher than the turn-on temperature) so that the system doesn't oscillate rapidly and so your relay doesn't "chatter".
If the "VAPE MAT for mosquitoes" you refer to is one of those tiny mains powered heaters then it will have a tiny heat output - whatever cost savings or convenience benefits you get from re-using the heater will likely be dwarfed by the extra hardware and complexity needed to regulate mains powered devices with an Arduino. I suggest you decide what maximum power output you want and get a low voltage DC supply of around 12V that can deliver that much power with a suitable safety margin. You can use any handy resistor as a load - incandescent light bulbs for example. I suspect you'll end up needing Watts rather than tens of Watts in which case any of the small motor drive circuits would be all you need - you can even get inexpensive Arduino clones with the driver circuits integrated onto the main board.
Having got the heater hardware sorted out, you should really consider whether you need the complexity of a microcontroller and temperature sender - a simple mechanical thermostatic controller would be cheaper and more reliable.