Temperature controller

[[email protected]]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

I went to homedepot.com, looked for the cheapest digital thermostat
they have, and found item 100047225. (If Home Depot allowed direct
links to their products, I'd post it; otherwise, you have to put the
item number into the Search box on the upper-right. Sorry about that.)

Now I'm wondering how I interface this thermostat with my 60W, 110VAC
lamp.

Specifications: Works With standard heating and cooling (millivolt to
24 Volt), 2 wire boiler

So, how do I use it? I'm guessing a simple relay from the thermostat's
outputs won't work (or would kill the battery inside the thermostat).

Any suggestions?

Thanks in advance,

Michael Darrett

 

[Charlie Edmondson]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

I went to homedepot.com, looked for the cheapest digital thermostat
they have, and found item 100047225. (If Home Depot allowed direct
links to their products, I'd post it; otherwise, you have to put the
item number into the Search box on the upper-right. Sorry about that.)

Now I'm wondering how I interface this thermostat with my 60W, 110VAC
lamp.

Specifications: Works With standard heating and cooling (millivolt to
24 Volt), 2 wire boiler

So, how do I use it? I'm guessing a simple relay from the thermostat's
outputs won't work (or would kill the battery inside the thermostat).

Any suggestions?

Thanks in advance,

Michael Darrett

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

 

[John Popelish]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

I went to homedepot.com, looked for the cheapest digital thermostat
they have, and found item 100047225. (If Home Depot allowed direct
links to their products, I'd post it; otherwise, you have to put the
item number into the Search box on the upper-right. Sorry about that.)

Now I'm wondering how I interface this thermostat with my 60W, 110VAC
lamp.

Specifications: Works With standard heating and cooling (millivolt to
24 Volt), 2 wire boiler

So, how do I use it? I'm guessing a simple relay from the thermostat's
outputs won't work (or would kill the battery inside the thermostat).

Any suggestions?

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:
http://cgi.ebay.com/Celsius-999C-Digital-PID-Temperature-Control-controller_W0QQitemZ250055834727

 

[[email protected]]

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:
http://cgi.ebay.com/Celsius-999C-Digital-PID-Temperature-Control-controller_W0QQitemZ250055834727

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

 

[Spehro Pefhany]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

I went to homedepot.com, looked for the cheapest digital thermostat
they have, and found item 100047225. (If Home Depot allowed direct
links to their products, I'd post it; otherwise, you have to put the
item number into the Search box on the upper-right. Sorry about that.)

Now I'm wondering how I interface this thermostat with my 60W, 110VAC
lamp.

Specifications: Works With standard heating and cooling (millivolt to
24 Volt), 2 wire boiler

So, how do I use it? I'm guessing a simple relay from the thermostat's
outputs won't work (or would kill the battery inside the thermostat).

Any suggestions?

Thanks in advance,

Michael Darrett

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

 

[John Popelish]

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.)

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.

I thought the thermocouple will stop
once measured temperature exceeds one degree F above the setpoint.

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.

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?

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:
http://www.tcnj.edu/~rgraham/PID/popelish.html

 

[[email protected]]

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?

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.

 

[[email protected]]

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

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. ;-)

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

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

Michael

 

[Huey]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment.

One and two and

Come on microbes, feel the burn!

Three - Work it!

C'mon ladies, sweat it!

 

[Bruce Varley]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

I went to homedepot.com, looked for the cheapest digital thermostat
they have, and found item 100047225. (If Home Depot allowed direct
links to their products, I'd post it; otherwise, you have to put the
item number into the Search box on the upper-right. Sorry about that.)

Now I'm wondering how I interface this thermostat with my 60W, 110VAC
lamp.

Specifications: Works With standard heating and cooling (millivolt to
24 Volt), 2 wire boiler

So, how do I use it? I'm guessing a simple relay from the thermostat's
outputs won't work (or would kill the battery inside the thermostat).

Any suggestions?

Thanks in advance,

Michael Darrett

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.

 

[[email protected]]

One and two and

Come on microbes, feel the burn!

Three - Work it!

C'mon ladies, sweat it!

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. "

http://www.epa.gov/biotech_rule/pubs/fra/fd007.htm

;-)

Michael

 

[[email protected]]

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.

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

 

[John Popelish]

Bruce Varley wrote:


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).

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.

 

[[email protected]]

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

I went to homedepot.com, looked for the cheapest digital thermostat

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.

 

[[email protected]]

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

 

[Rich Grise]

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.

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

 

[Tony Williams]

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. "

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.

 

[Huey]

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

"Aspergillus oryzae is an asexual, ascomycetous fungus used for

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...

:-{

 

[Huey]

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

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?

 

[Barry Lennox]

Hi guys,

I'd like to build a closed hot-box for an aerobic microbiology
experiment. The heater will basically be a 60W light bulb (or a string
of 5W Christmas tree lights), and I'm looking to run experiments at
around 90 degrees F.

The controller: at first I'd thought of using a microcontroller of
some sort, then I thought maybe I can use an off-the-shelf thermostat.

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