DIY temperature controller

[[email protected]]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

 

[Jim Thompson]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Indeed! One of my wife's pet peeves (except ours has OFF, LOW and
HIGH ;-)

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

...Jim Thompson

 

[Michael A. Terrell]

Indeed! One of my wife's pet peeves (except ours has OFF, LOW and
HIGH ;-)

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

Jim Thompson

Mine new one has three settings, the lowest being, "Keep Warm",
which use to really slow cook soup or chili.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[martin griffith]

Mine new one has three settings, the lowest being, "Keep Warm",
which use to really slow cook soup or chili.

Not the Salmonella setting, I hope


martin

 

[John Larkin]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Simpler to use a lamp dimmer, open-loop, or even one of those series
diode things.

John

 

[Michael A. Terrell]

Not the Salmonella setting, I hope

No, I've been using it close to a year that way. The food is
definitely hot enough when you serve it, but it loses less water while
cooking. I takes a couple hours to cook a pot of chili, and I have
cooked it as long as eight hours on an electric stove in the past.


--
Service to my country? Been there, Done that, and I've got my DD214 to
prove it.
Member of DAV #85.

Michael A. Terrell
Central Florida

 

[Spehro Pefhany]

Indeed! One of my wife's pet peeves (except ours has OFF, LOW and
HIGH ;-)

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

...Jim Thompson

I made a slow cooker from a deep electric saucepan a zillion years
ago. I used a 72mm x 72mm industrial thermocouple-input controller
with a power relay and hid the type J thermocouple inside the prod
that used to hold the bimetal thingie. Worked *very* well for stews
and such like. Time proportioning with about a 20-second cycle.

It's easy enough to get 3/16" SS thermocouple probes, but I'm not sure
you want the probe in your food, just from a convenience POV. It's
certainly not necessary from a control POV if the cooking pot is a
good quality heavy one.

This one had the original bimetal control in a bakelite plug thingie
that unplugged so you could wash the pan without the cord and control.
They may not make them that way anymore, I don't know, all our pots
are range top type these days.



Best regards,
Spehro Pefhany

 

[[email protected]]

On 19 Mar 2006 11:45:42 -0800, [email protected] wrote:

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

Farnell have three pages of LabFacility tnermocouple probes, mostly in
stainless steel sheaths. For better temperature stability, there's a a
page of stainless steel sheathed platinum resistance sensors (again
from LabFacility) a couple of pages furhtr on.

 

[John B]

[email protected] scrobe on the papyrus:

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a
temperature probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Maxim/Dallas have a whole range of parts. Start with the DS1626
standalone thermostat, or for better resolution use the MAX6674 or
MAX6675 thermocouple amplifiers with cold junction compensation and an
SPI interface.

 

[John Larkin]

I made a slow cooker from a deep electric saucepan a zillion years
ago. I used a 72mm x 72mm industrial thermocouple-input controller
with a power relay and hid the type J thermocouple inside the prod
that used to hold the bimetal thingie. Worked *very* well for stews
and such like. Time proportioning with about a 20-second cycle.

It's easy enough to get 3/16" SS thermocouple probes, but I'm not sure
you want the probe in your food, just from a convenience POV. It's
certainly not necessary from a control POV if the cooking pot is a
good quality heavy one.

This one had the original bimetal control in a bakelite plug thingie
that unplugged so you could wash the pan without the cord and control.
They may not make them that way anymore, I don't know, all our pots
are range top type these days.



Best regards,
Spehro Pefhany

One concern is that you don't convert a slow cooker to a bacteria
incubator.

John

 

[Spehro Pefhany]

One concern is that you don't convert a slow cooker to a bacteria
incubator.

John

It too long ago to remember the details, but I definitely did consider
that danger. IIRC, there's a large margin for error between a good
slow-cooking temperature and minimum safe storage temperature*. Of
course this was a properly calibrated instrument that we made in
volume, and with a one-off homebrew assembly everything, including
calibration of the instrument, would be suspect. The sensor will be
fine in any case, a thermocouple is perfect for this application
(except perhaps system cost for a mass-produced product).

*Okay, Illinois Dept of Public Health refers to the "danger zone" as
41°F to 140°F (5°C to 60°C), and that foods exposed to those
temperatures for more than 4 cumulative hours should be considered
suspect.

The production of steam at, say, 170ºF is *way* less than near to
boiling.

Hm.. I wonder if everyone started to do this if new bacteria would pop
up that could live at relatively high temperatures. Temperatures well
inside the 'danger zone' are used in some commercial meat cooking, but
they heat it up high enough later on to kill whatever has been
incubating away.

Aside: I wonder how I managed to look up that kind of stuff before the
internet. Library?


Best regards,
Spehro Pefhany

 

[artie]

Indeed! One of my wife's pet peeves (except ours has OFF, LOW and
HIGH ;-)

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

...Jim Thompson

I've a very nice Fluke k-type probe, stainless with a very pointy end.
Doesn't have a part number on it anywhere.

But why not use one of the Dallas single chip sensors (DS1620)? They
even have programmable setpoints and an alarm output, so you can set
your own hysteresis points. -55 to +125C.

 

[DaveM]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Take a look at some of the articles at
http://www.discovercircuits.com/T/temp-cont.htm and take your pick. Lots of
ideas there, maybe you'll find one suitable for your crock pot.

Cheers!!!
--
Dave M
MasonDG44 at comcast dot net (Just substitute the appropriate characters in the
address)

Never take a laxative and a sleeping pill at the same time!!

 

[ehsjr]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Conceptually: use the resistance of the thermistor to
replace (or modify) the resistance of the pot in a lamp
dimmer. The dimmer is already set up for line voltage.
But (there's always a "but") you need to be sure the
wattage of the crock pot is within the rating of the
dimmer.

With the warnings others have given you might be
better off buying one that has a temperature
control built in.

Ed

 

[Frithiof Andreas Jensen]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

Do not bother - when i was distillin ..... err ... experimenting with an
environmentally sound disposal method for failed wine productions I used a
thermostat from a scrapped washing machine. Which worked fine.

One of the mechanical types with a capillary tube and all. That particular
device had contacts rated for the load so it was self-containen and also
accurate to within 2 deg. C (Good I.M.O.).

A spare thermostat for an oven or washing machine will be easy to get. Much
easier than designing one.

 

[GregS]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

I just posted about a analog controller. I need noise free DC opperation.

I would suggest buying this unless you really want to fool around.
The cheapest unit I have found, and the price just went up $5, is
http://www.coleparmer.com/catalog/product_view.asp?sku=9352000

greg

 

[Michael]

I'm trying to figure out how to make a simple feedback (on-off)
temperature controller.

The basic problem that I want to solve is that I have an electric
cooker ("crockpot") that unfortunately only has two temperature
settings: on and off. I would like to have it hold a specific
temperature, a temperature that is lower than the temperature it
normally when in the "on" setting, and keep it there within a
tollerence of a few degrees. My initial thought is to use a temperature
probe to do simple on-off control on the device.

What I do not know is some of the circuit details:

I've never worked with thermocouples or thermisters before. Do I need
to worry about amplifiying them?
My thought is to use an op-amp in inverting mode to compare the
measured temperature against a setpoint. How do this in practice? I'm
not completely clear on how to "tune" the assembly.
What would be a good way to switch the power on and off? I'm nervous
about interfacing with the AC coming out of the wall.

many thanks!

Others have offered suggestions for sensing temperature, so I won't.

As for switching AC, I suggest you consider a solid state relay (SSR). They are
dead simple to use, controlled by a low voltage and low current signal, and in
an easy-to-integrate package. A SSR that can switch 25A costs .... $15 or so?
I'm not sure about price because I bought three 25-ampers 15 years ago, put only
one into a project (cycling a 1500 watt space heater), and that one is still
working just fine.

 

[Spehro Pefhany]

Others have offered suggestions for sensing temperature, so I won't.

As for switching AC, I suggest you consider a solid state relay (SSR). They are
dead simple to use, controlled by a low voltage and low current signal, and in
an easy-to-integrate package. A SSR that can switch 25A costs .... $15 or so?
I'm not sure about price because I bought three 25-ampers 15 years ago, put only
one into a project (cycling a 1500 watt space heater), and that one is still
working just fine.

With an SSR, you need a heatsink capable of keeping the SSR mounting
plate temperature at a reasonable temperature while dissipating about
1W/A of switched current.

BTW, especially with SSRs, but in any case, you should make sure that
any safety cutoffs are not bypassed or eliminated-- especially if this
thing is to operate unattended. SSRs generally fail 'on', and even
relays have been known to do so on occasion. Solid state control
circuits can fail or latch up too (eg. soft failures of
microcontroller systems).


Best regards,
Spehro Pefhany

 

[Rich Grise]

Indeed! One of my wife's pet peeves (except ours has OFF, LOW and
HIGH ;-)

Provokes the question: Where do you get a probe like comes with my
remote meat thermometer? Stainless steel, thus food safe.

...Jim Thompson

http://www.omega.com/newsearchengin...=rank&search=thermocouple+probe&submit=Search

Cheers!
Rich

 

[Rich Grise]

One concern is that you don't convert a slow cooker to a bacteria
incubator.

John

No problem - make yogurt! That's just big lumps of rot, after all,
isn't it?

Thanks!
Rich