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Guest
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 Posted:
Sun Nov 13, 2005 5:35 pm Post subject:
maintaining constant temperature? |
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Say I have a metal plate the size of an eraser head. I want it to stay
at 140 F using 4 AA batteries. It seems like I could simply use a
thermister, but I really don't know much about the electronics world.
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Rheilly Phoull
Guest
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| Posted:
Sun Nov 13, 2005 5:35 pm Post subject:
Re: maintaining constant temperature? |
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<leon.park@gmail.com> wrote in message
news:1131884703.848833.208870@f14g2000cwb.googlegroups.com...
| Quote: | Say I have a metal plate the size of an eraser head. I want it to stay
at 140 F using 4 AA batteries. It seems like I could simply use a
thermister, but I really don't know much about the electronics world.
|
Yup, you have said it all !!
--
Regards ......... Rheilly Phoull |
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Rich Webb
Guest
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| Posted:
Sun Nov 13, 2005 5:35 pm Post subject:
Re: maintaining constant temperature? |
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On 13 Nov 2005 04:25:03 -0800, leon.park@gmail.com wrote:
| Quote: | Say I have a metal plate the size of an eraser head. I want it to stay
at 140 F using 4 AA batteries. It seems like I could simply use a
thermister, but I really don't know much about the electronics world.
|
What's your target run time? What kind of thermal isolation is there
between the metal plate and any other heat sinks? What is the ambient
temperature? What are the consequences of failing to maintain 140 F?
What other loads are presently being, or are being considered for, the
4 AA cells?
--
Rich Webb Norfolk, VA
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Guest
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| Posted:
Mon Nov 14, 2005 1:35 am Post subject:
Re: maintaining constant temperature? |
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I was just going to put the thermistor next to it. There wont be other
loads, and it will be in room temp.
But I don't know what kind of thermistor I need or anything. I'd like
it to stay at 140F +/- 2F. |
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Rich Webb
Guest
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| Posted:
Mon Nov 14, 2005 8:16 am Post subject:
Re: maintaining constant temperature? |
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On 13 Nov 2005 13:43:24 -0800, leon.park@gmail.com wrote:
| Quote: | I was just going to put the thermistor next to it. There wont be other
loads, and it will be in room temp.
But I don't know what kind of thermistor I need or anything. I'd like
it to stay at 140F +/- 2F.
|
As a swag, a CMOS 555 hooked up as a variable duty cycle astable driving
a saturated transistor switch might work. Try a 10K thermistor from Vcc
to DISCH, a 22K between DISCH and THR/TRIG paralleled with a switching
diode, and a 0.1 uF cap at TRIG. This won't ever turn completely off,
which might be a good thing depending on ambient losses.
Alternately, use the thermistor in a voltage divider compared to a
setpoint pot, both input to an LM339 comparator as a bang-bang
controller.
Haven't run the numbers on what kind of heating to expect with only
4 AA cells to work with, nor the expected run time.
--
Rich Webb Norfolk, VA |
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Guest
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| Posted:
Tue Nov 15, 2005 9:12 am Post subject:
Re: maintaining constant temperature? |
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I see...
No, wait-I'm confused. DUMBFOUNDED as it were... |
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Guest
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| Posted:
Tue Nov 15, 2005 9:35 am Post subject:
Re: maintaining constant temperature? |
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Ok, nevermind all the IC, I can just tweak until a simple thermistor
and source work properly. Say for a constant input of 5V and up to 1A,
happen to know what sort of thermister would I want to get at radio
shack to be able to reach 140F? |
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John Popelish
Guest
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| Posted:
Tue Nov 15, 2005 9:35 am Post subject:
Re: maintaining constant temperature? |
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leon.park@gmail.com wrote:
| Quote: | I see...
No, wait-I'm confused. DUMBFOUNDED as it were...
The simplest power controller is an infinite gain (on-off) type that |
turns the power full on if the temperature is low and turns it off it
the temperature is too high. A comparator is a device that switches
off or on depending on which of two signals is more positive.
Based on the power of the heater, the heat stored in the heater
element (that keeps coming out for a while after the power is turned
off, and takes a while to come out when the heat is turned on),the
thermal mass of the object being kept hot, and the time delay between
that mass and the temperature sensor, the temperature will ramp back
and forth through the desired temperature. Whether or not it stays
within the required bounds under this method of control depends on the
the factors I mentioned earlier.
The only way to keep the temperature in much narrower bounds is to
pulse the heat faster than the time delay between the heater and the
temperature pickup, and vary the duty cycle of those pulses with a
more complicated scheme than the simple one I first described. One
such more complicated scheme is the PID algorithm (a combination of
Proportional gain times the temperature error, another gain times the
integral of the error, and a third gain times the derivative of the
error. |
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John Popelish
Guest
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| Posted:
Tue Nov 15, 2005 5:35 pm Post subject:
Re: maintaining constant temperature? |
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leon.park@gmail.com wrote:
| Quote: | Ok, nevermind all the IC, I can just tweak until a simple thermistor
and source work properly. Say for a constant input of 5V and up to 1A,
happen to know what sort of thermister would I want to get at radio
shack to be able to reach 140F?
Radio Shack isn't going to have much that is helpful. But Digikey |
sells a wide range of thermistors , including some that switch from
conducting to insulating over a very narrow temperature range, as
temperature rises. This type is called a PTC thermistor for Positive
Temperature Coefficient. I use a few of these soldered between a pair
of brass ribbons mounted in the front of my motorcycle helmet to act
as a self regulating defroster bar. You may be able to use one or two
of these connected in parallel and mounted around your heated mass to
act as both heater and control system. But it may take more than 5
volts to get it up to its switching temperature, since they have about
50 ohms resistance when cold.
The Digikey part number for a 50 ohm PTC thermistor that switches off
at 60 C ( 140 F) is 235-1123. |
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Peter Bennett
Guest
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| Posted:
Tue Nov 15, 2005 5:35 pm Post subject:
Re: maintaining constant temperature? |
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On 14 Nov 2005 20:33:30 -0800, leon.park@gmail.com wrote:
| Quote: | Ok, nevermind all the IC, I can just tweak until a simple thermistor
and source work properly. Say for a constant input of 5V and up to 1A,
happen to know what sort of thermister would I want to get at radio
shack to be able to reach 140F?
|
I think that you may misunderstand what a thermistor is, or does.
A thermistor is a component that varies its resistance depending on
temperature. Thermistors are normally used as temperature sensors.
They may be used in a circuit that controls a heater, but they don't
do the heating themselves.
--
Peter Bennett VE7CEI
email: peterbb4 (at) interchange.ubc.ca
GPS and NMEA info and programs: http://vancouver-webpages.com/peter/index.html
Newsgroup new user info: http://vancouver-webpages.com/nnq |
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Ken Moffett
Guest
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| Posted:
Wed Nov 16, 2005 1:35 am Post subject:
Re: maintaining constant temperature? |
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John Popelish <jpopelish@rica.net> wrote in news:yNOdnWRrbdT_YuTeRVn-
tw@adelphia.com:
| Quote: | leon.park@gmail.com wrote:
Ok, nevermind all the IC, I can just tweak until a simple thermistor
and source work properly. Say for a constant input of 5V and up to
1A,
happen to know what sort of thermister would I want to get at radio
shack to be able to reach 140F?
Radio Shack isn't going to have much that is helpful. But Digikey
sells a wide range of thermistors , including some that switch from
conducting to insulating over a very narrow temperature range, as
temperature rises. This type is called a PTC thermistor for Positive
Temperature Coefficient. I use a few of these soldered between a pair
of brass ribbons mounted in the front of my motorcycle helmet to act
as a self regulating defroster bar. You may be able to use one or two
of these connected in parallel and mounted around your heated mass to
act as both heater and control system. But it may take more than 5
volts to get it up to its switching temperature, since they have about
50 ohms resistance when cold.
The Digikey part number for a 50 ohm PTC thermistor that switches off
at 60 C ( 140 F) is 235-1123.
|
The devce you are using is a "Switching PTC Thermistor", a name given to
it by the manufacturer: Thermometrics.
http://www.thermometrics.com/assets/images/ptcnotes.pdf
Most people in the field would not generally refer to this as a "PTO
thermistor". The "silister" in the above URL is what most people refer
to as a thermistor. The Switching PTO Thermistor curves look like those
of "PTC resettable fuses". The are probably the same technology under a
different name. |
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Guest
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| Posted:
Fri Nov 18, 2005 1:35 am Post subject:
Re: maintaining constant temperature? |
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lol. no comment.
Anyway I was intending to just use the thermistor as a heat source, I'm
not melting an iceberg. Oh well, I'll figure it out later. |
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Rich Grise, Plainclothes
Guest
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| Posted:
Fri Nov 18, 2005 1:35 am Post subject:
Re: maintaining constant temperature? |
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On Sun, 13 Nov 2005 04:25:03 -0800, leon.park wrote:
| Quote: | Say I have a metal plate the size of an eraser head. I want it to stay
at 140 F using 4 AA batteries. It seems like I could simply use a
thermister, but I really don't know much about the electronics world.
|
Concert pipe? You need 155C to evaporate the THC. ;-)
Good Luck!
Rich |
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