Controlling Resistance Heaters

So I've been thrown onto a project to make some resistance heaters work, gotta melt some plastic for my teacher. So the heater is 800W and runs on 120 V AC, Which that should be ~6.6Amps. Now thats low enough it should (keyword should) run off of a wall outlet correct.

How would one control the temp of the thing though, I was thinking of using the kind of knob off of an electric oven. But what is that itself, just a big potentiometer? From what I've been told, working with AC isnt as simple as dropping in a resistor to limit current.

Thanks ahead of time, and sorry for whats probably a dumb question, but theres not too much running aroudn in these forums about 120VAC electronics.

Edit. To note, this is a project, for an end of year, xtra credit essay. No actual heating system will be built.

You probably want a temperature controller. It would use some kind of sensor (probably a thermocouple) and would also have the internal logic (a self tuning PID loop) so that you wouldn't overheat/burn your plastic. It might also have a "Zero Crossing" switch (detects when the AC sine wave is at 0V to turn off)

You would hook up the sensor, set the temperature and wait.

I think you're going to have to buy something for this one man.

At my workplace, we use a lot of resistance heater "hot plates", called black bodies.

Anyways, i've seen two different controllers around to control the temp. One is made by this company called Arroyo, which looks REALLY nice and REALLY expensive. The other one is some rig that somebody made themselves, and judging from the amount of work it looks like they put into it, I don't think you're going to want to mess with trying to make one yourself.

Of course, the ones at work have nice little LCD outputs, rotary encoders, and RS232 ports for control, so maybe if you obviate those parts of the setup, making one might be more manageable.

Controlling 800 W ! I will use a relay rated to switch at least 15 A - 120 V ac. And work it like an electric thermostat, On and off, on, and off... A sensor - thermocouple ? That might work... I don't know where to find one of them.

It might also have a "Zero Crossing" switch (detects when the AC sine wave is at 0V to turn off)

That is a dimmer switch technique. Controlling a heather just like a dimmer is not to practical in my opinion. A simple high current rated relay will work just fine. I think the stove element work on that principle. I can hear clic and clic again when it is too hot. Hum, I wonder what type of sensor the stove use... maybe the same sensor the stove use "migh" work ... :roll_eyes:

but theres not too much running aroudn in these forums about 120VAC electronics.

That is because playing with mains when you don't know what you are doing is dangerous and can be fatal. I am surprised a teacher has given you this to do with so little knowledge of electricity. I would first check that your teacher is fully insured for when your parents try and sue over your death.

I would use a SSR solid state relay and turn it on and off at regular intervals to control the power. Get a relay that is rated at a current at least twice that you need to switch. The ratio of the on time to the off time determines the power. The code looks exactly the same as the code to flash an LED.

Search the forum for KILN controllers and PID library. Your answers will be there. I control a 2300 Degree F Kiln with the PID Library and and a 40 amp SSR (On a heat sink with a fan).

As was already stated, it's NOT a trivial project. I have 25 years of AC mains experience.

Grumpy_Mike:

but theres not too much running aroudn in these forums about 120VAC electronics.

That is because playing with mains when you don't know what you are doing is dangerous and can be fatal. I am surprised a teacher has given you this to do with so little knowledge of electricity. I would first check that your teacher is fully insured for when your parents try and sue over your death.

I would use a SSR solid state relay and turn it on and off at regular intervals to control the power. Get a relay that is rated at a current at least twice that you need to switch. The ratio of the on time to the off time determines the power. The code looks exactly the same as the code to flash an LED.

Perhaps I should note. This isn't an actual project. Its one of those kind of, end of semester. "Here go with this info and figure out how to do something like this". And get some xtra credit for doing a short essay about how you would do it. On of those deals. My teacher loves doing them. SO i figured I'd ask here, then over the weekend go find the guy who did my house's wiring and harass him for an hour. Basically what i had already was a relay controlled off of a switch, to turn the "heating units" off and on. But as far as temperature controlling. I understand its 1 thing to switch them on and off at certain setopints to keep the temp near constant. But with adjusting their output like a dimmer for lights. I'm sure there's more going on in a dimmer switch for lights in your house then just a potentiometer, so curiosity got the better of me and I didn't feel like waiting to ask the electrician so i threw it up here.

Gonna go up into the first statement and edit it stating its not actually something to build, shoulda probably mentioned it in the first place. However as it does stand. It seems a bit easier in the forums to get responses when asking about something that's over your head then for hypothetical projects.

@Nikarus

I agree with Grumpy_Mike here. Go buy a devise design to turn on / off 120 VAC @ 15 A. So you simply connected the low voltage section - like a 5 V on / off and the High Voltage section. Just screw the connectors and you are done. Connected the High Voltage in serie with the Power heater. Like : Live ---> Device ----> live heater --- > Neutral ( Canada / US is the same system ) But I recomment an electrician to do the High Voltage section. As for me, I did work on AC, so I know I am doing, but safety is a must - ALWAYS !!!

If you really have an idea how and work on AC line at home - Not talking about screwing a light bulb here - ie - you have experience working with AC - like wired a line from the breaker box , a plug and a switch and a light fixture - Than a high current rated SPST relay will work. The coil voltage may be high 12 V or higher, that you will need a voltage to control the relay - Automotive store may sell those high current type - 12 V coil, And use a transistor, base resistor and a diode and connect it right, than you are in the game.

In conclusion, No experience... let a pro do the job ! - I know, it cost money !!! :roll_eyes:

A simple solution would be using a 555 timer to adjust the "duty cycle" of the resistance. The 555 timer would cycle the resistance element on and off through the solid state relay previously mentioned.You could use a potentiometer to adjust the duty cycle. It would be nice to have some feedback to tell what the temperature is, but thermocouples are expensive. If you don't have feedback, then the 555 is just as good as the Arduino and not nearly as expensive. I have used this simple circuit to control a ceramics kiln, and it worked just fine. The grunt work come finding what duty cycle corresponds to what actual temperature... have fun