Temperature controller

Michael, Typically, the thermostat has a relay rated for the 24 VAC that is used for control of HVAC systems. What you would need is a small transformer to step down the voltage from your power line to say, 12 VAC, and a relay that works at that voltage rated for 120 VAC on the contacts. You are basically using the relay in the thermostat to trigger the relay that controls your lights.

Charlie

When you get tired of the temperature bounce you will get with a thermostat (unless you select a bulb that just barely can overheat the box) you should get a cheap PID controller from eBay and use its DC pulse output to drive a solid state relay or the internal relay contact to operate the heater. I use these for all sorts of temperature control applications and they work very well. You can tune the PID parameters to hold the temperature very stable. They also work with a thermocouple sensor, so the controller can stay outside the controlled temperature box.

Here is an example of the sort of controller I am talking about:

How big of a temperature bounce are we talking about here? +/- 3 degrees F should be fine for my application. (There will be a circulating fan in my box, also.) I thought the thermocouple will stop once measured temperature exceeds one degree F above the setpoint.

Thanks for the eBay link. Those look just like the temperature controllers one of my professors used in his high-temperature plastic pyrolysis experiments, when I used to work in his lab.

Can the PID controller be tuned? Or is that not necessary?

Thanks for the replies,

Michael

How fast will it heat up, relative to the temperature stability you need?

I think a thermostat is unsuitable for this application--- it's designed to work with a system that cannot tolerate rapid switching but the users can tolerate some temperature variation, so it trades off one against the other.

Are you trying to control the temperature of the air? Is there air circulation in and out? This could be a dead simple control problem or fairly difficult, depending on the details of your application. The sensor and output device are important, as well as the controller.

Best regards, Spehro Pefhany

When the bulb is operated in 'full power on, till temperature setpoint is exceeded' mode, there will be energy stored in the hot bulb that continues to heat the box, after the bulb is turned off. There will also be a delay between the air temperature and the thermostat measured temperature that lets the bulb ob longer than necessary, and delay its next power on phase. The fan will help both of these, but not eliminate them. Only experiment with a small fast response thermocouple will tell you exactly what the temperature is doing with your setup.

Each thermostat (not a thermocouple, but a bimetallic switch) has a particular temperature hysteresis that must take place to turn it on when it is off and off once it is on.

It is necessary. The tuning informs the controller the key thermal time constants and gain of the the system being controlled, so that it anticipates the overshoot and begins to power down before the setpoint temperature is crossed (or just barely crossed) and pulses the power to hold the temperature acceptable steady. The pulse period is a separate setting from the PID tuning. You set the pulse period to be a lot shorter than the response time of the system so that a train of power pulses produces about the effect of a steady but lower power. The average power is set by varying the duty cycle of the pulses.

But the tuning is very forgiving, if you only want to hold a steady temperature, not follow rapid setpoint or load changes (like opening the box and expecting rapid return to the temperature setpoint.

If you want to read a beginners tutorial on PID tuning, I wrote one that is available at:

John Poplish has given his usual comprehensive answer, but note that some digital PID controllers offer auto-tuning - I used one like that back in 1993.

It worked fine, but the temperature didn't settle all that rapidly - we ended up disabling the auto-tuning and using the Ziegler-Nichols manual tuning procedure to get much faster settling.

I haven't run the numbers yet, but I'm guessing maybe it'll take an hour or so to reach setpoint temperature. (It'll also be blowing in cold air from outside, but at a low flow rate.)

I could use a higher-power bulb, but I don't want my plastic box to catch fire, either.

Idea is to place this outdoors, so that the experiment won't stink up my garage. (Wife has complained about this before.) It gets awfully cold at night, though - maybe 30 F out here in Sacramento now. (If I'd run this in the summertime, 110+ F temperatures, I wouldn't even need a hot box. ;-)

Thanks for the replies, all. You've given me lots to think about.

Michael

One and two and

Come on microbes, feel the burn!

Three - Work it!

C'mon ladies, sweat it!

:-)

One point with a thermostat solution that hasn't been mentioned so far is the wear on the bulb/s involved. Turning them on and off constantly will result in a higher burnout rate. Running then at a steady, lower power will make them last longer.

Hoo yeah! Before you get too excited, though, my microbes are asexual.

"Aspergillus oryzae is an asexual, ascomycetous fungus used for hundreds of years in the production of soy sauce, miso and sake without recorded incidents. "

;-)

Michael

Okie doke, one cheap 60W (or 75W, or 100W) bulb it is. I've got plenty of those lying around, after we replaced all major incandescent bulbs in the house with compact fluorescents.

No point in stressing out all those 5W Christmas tree lights my wife bought (and then decided we didn't want to run because they consume WAY too much power).

Michael

If you are worried about the bulbs failing and ruining a long run, just connect two in parallel, and select them so that the duty cycle is less than 50%. If one fails, the duty cycle will rise, and maintain the temperature pretty well, till you replace a bulb. Or connect two equal wattages in series so that they will last pretty much to forever.

Eventually this can be made to work, but... why not go to a pet store and look into a terrarium heater? It's not a new problem to thermostat a small box.

Regular resistance heating is better than a light bulb. 60W lamps have a service life of about 1000 hours, then they burn out and your box dies. That'll happen probably several times a year.

One could make a good thermostat from a solidstate temperature sensor (that means a $0.12 transistor with its base connected to the collector), and suitable amplification/comparison to a triangle-wave generator/ solid state relay driving a heater, but it'd take longer than returning the house thermostat and driving to Petco.

Interesting... thanks! I'd never heard of a terrarium heater before.

Michael

I would be seriously tempted to tighten up my control loop, maybe even to the point of SCR/triac phase control, or at least proportional integral-cycle control, but with PRF rates that make the bulbs not have to flash on and off.

Just another 2 cents. ;-)

Cheers! Rich

My old boss did a lovely temperature controller, (of the air inside a barometer capsule).

It was simply several yards of 48-gauge enamelled copper wire, loosely scrunched into the capsule, and forced (by a resistance bridge) to a constant resistance.

Through Hollywood's eyes watching women in an aerobics class is a good thing.

But in the real world the situation is much closer to "asexual".

And then there is the fungus...

:-{

I think that this is a great idea for you. It should be slower at heating the experiment than the light bulb.

With a light bulb snapping on and off you would be heating/cooling on off on off on off. The terrarium heater's that I've experienced are a bit more gradual.

Give some consideration to what you are heating. Just the air in the enclosure? Would you be heating a small thermal mass that would even out the temperature changes?

A thermostat might give you too much hysteresis, but it depends on your requirements.

If you want a good simple DIY version, look to the old Motorola App note AN-466 "Circuit Apps for the Triac" There's a couple of proportional ZVS temp controllers in there, using a Triac, tansistor or two and a UJT with a small handful of R, C and diodes.

I built a few of them years ago for controlling the temp of photo processing baths, they gave excellent results and could hold the temp to under 1 degree, assuming the water was stirred to avoid themal layering.

Barry