A resistor at 150°C...how?

[Francesco Piantedosi]

Hi all,
I have to buil a circuit to use a resistor as a heater; what I want is
to use this resistor to heat a surface at 150 °C.

My first problem is to choose right resistor(I have ONLY 15 mm diameter)
able to dissipate this heat power without crash!

The second problem is design ctemperature control circuit...the
sensor(LM35 or lm45) should be in contact with surface I have to heat
and give feedback to power supply of my resistor...any ideas?

Thanks
Francesco

 

[Fred Bloggs]

Hi all,
I have to buil a circuit to use a resistor as a heater; what I want is
to use this resistor to heat a surface at 150 °C.

My first problem is to choose right resistor(I have ONLY 15 mm diameter)
able to dissipate this heat power without crash!

The second problem is design ctemperature control circuit...the
sensor(LM35 or lm45) should be in contact with surface I have to heat
and give feedback to power supply of my resistor...

http://www.iprocessmart.com/sunrod/sunrod.htm

 

[John Fields]

Hi all,
I have to buil a circuit to use a resistor as a heater; what I want is
to use this resistor to heat a surface at 150 °C.

My first problem is to choose right resistor(I have ONLY 15 mm diameter)
able to dissipate this heat power without crash!

 

[Rich Grise]

Hi all,
I have to buil a circuit to use a resistor as a heater; what I want is
to use this resistor to heat a surface at 150 °C.

My first problem is to choose right resistor(I have ONLY 15 mm diameter)
able to dissipate this heat power without crash!

The second problem is design ctemperature control circuit...the
sensor(LM35 or lm45) should be in contact with surface I have to heat
and give feedback to power supply of my resistor...any ideas?

I've been looking into something very similar to this, to warm a 5-10 l/m
air stream to 155 C. The LM35 doesn't go that high. The thermistors I've
seen aren't guaranteed at that temp. range. This leaves either a
thermocouple, which is a PITA, or an RTD, which I don't even know how to
search for - all I come up with is platinum and stuff, which cost more
than I'm budgeted for the whole project.

For the resistor, I found some thin nichrome wire on Ebay, which has a
resistance of approx. 28 ohms per foot, which gives me a "blank check" as
far as what I want the heating element to look like, since I'm making it
from scratch. I also have a 7.5V 1A wall wart that somebody threw away -
one of these days, I'm going to see how hot a 3" piece of this nichrome
gets with the wall wart, but the bugaboo, of course, is sensing the temp.

The controller shouldn't be too hard at all - there are a lot of suggested
circuits that you could just copy. You could search on, say, "voltage
controlled dimmer" and such.

Good Luck!
Rich

 

[John Larkin]

I've been looking into something very similar to this, to warm a 5-10 l/m
air stream to 155 C. The LM35 doesn't go that high. The thermistors I've
seen aren't guaranteed at that temp. range. This leaves either a
thermocouple, which is a PITA, or an RTD, which I don't even know how to
search for - all I come up with is platinum and stuff, which cost more
than I'm budgeted for the whole project.

Several people make thinfilm platinum rtd's on little ceramic slabs,
and they cost a few bucks each. google "thinfilm rtd"

John

 

[John Popelish]

I've been looking into something very similar to this, to warm a 5-10 l/m
air stream to 155 C. The LM35 doesn't go that high. The thermistors I've
seen aren't guaranteed at that temp. range. This leaves either a
thermocouple, which is a PITA, or an RTD, which I don't even know how to
search for - all I come up with is platinum and stuff, which cost more
than I'm budgeted for the whole project.

For the resistor, I found some thin nichrome wire on Ebay, which has a
resistance of approx. 28 ohms per foot, which gives me a "blank check" as
far as what I want the heating element to look like, since I'm making it
from scratch. I also have a 7.5V 1A wall wart that somebody threw away -
one of these days, I'm going to see how hot a 3" piece of this nichrome
gets with the wall wart, but the bugaboo, of course, is sensing the temp.

The controller shouldn't be too hard at all - there are a lot of suggested
circuits that you could just copy. You could search on, say, "voltage
controlled dimmer" and such.

Good Luck!
Rich

Glass encased thermistors go quite a bit higher than 150C and have
large signal outputs. Take a look at:
http://dkc3.digikey.com/PDF/T043/1044.pdf

Nichrome is great for heating a small air stream, because a small coil
of it is self supporting in a ceramic tube and can go to red heat
quite reliably.

 

[Nicholas O. Lindan]

I have to buil a circuit to use a resistor as a heater; what I want is
to use this resistor to heat a surface at 150 °C.

I take it this is a one-of research laboratory or hobby experiment.

For a product design I would recommend differently.

But, first, some questions need to be answered before an intelligent
response can be given:

1) How much hotter than 150C does the resistor have to be to
hold the plate at 150 in the coldest expected environment?
This is a function of plate insulation, plate size,
airflow, dynamics: thermal capacity & resistance,
transient thermal load, initial temperature (or, don't really care) ...

2) How much steady state and transient _power_ needs to be fed to
the resistor to accomplish #1? A function of thermal resistance.

My first problem is to choose right resistor(I have ONLY 15 mm diameter)
able to dissipate this heat power without crash!

That means nothing until the amount of power is known

The second problem is design temperature control circuit...the
sensor(LM35 or lm45) should be in contact with surface I have to heat
and give feedback to power supply of my resistor...any ideas?

Wrong move. Use a high temperature epoxy and attach a platinum
RTD - you can also get iron ones, but they are best used around
room temperature.

Use a synthetic bridge (fancy name for a resistor of about the
same R as the RTD in series with same) and measure the voltage
across the resistor and the RTD. As usual, ratio and curve fit in software.

If there is no software, use a four arm bridge with the RTD in one
arm and a pot in another arm. Adjust pot till the plate is 150C.

 

[Rich Grise]

Several people make thinfilm platinum rtd's on little ceramic slabs,
and they cost a few bucks each. google "thinfilm rtd"

^^^^^^^^^^^

Like I said - more than I'm budgeted for the whole project. ;-)

Thanks!
Rich

<alter_ego>
Yes, I know, I'm spending it all on booze, drugs, and harlots. They say
"easy come, easy go" - well, I've had the "easy go" part down pat for
quite some time now - somehow, that "easy come" part has been elusive for
me. And for those of you who wish to blame booze, drugs, or harlots for my
"sorry" "financial" "state", I say, "So what?" ;^j

TA!
R.
</alter_ego>

 

[Rich The Philosophizer]

Glass encased thermistors go quite a bit higher than 150C and have
large signal outputs. Take a look at:
http://dkc3.digikey.com/PDF/T043/1044.pdf

Nichrome is great for heating a small air stream, because a small coil
of it is self supporting in a ceramic tube and can go to red heat
quite reliably.

Just curious - would you have been this forthcoming with neat information
like this if you'd already known that it's for a THC evaporator? %-}

;^j
Rich

 

[Rich Grise]

That means nothing until the amount of power is known


Wrong move. Use a high temperature epoxy and attach a platinum
RTD - you can also get iron ones, but they are best used around
room temperature.

Use a synthetic bridge (fancy name for a resistor of about the
same R as the RTD in series with same) and measure the voltage
across the resistor and the RTD. As usual, ratio and curve fit in software.

If there is no software, use a four arm bridge with the RTD in one
arm and a pot in another arm. Adjust pot till the plate is 150C.

I must be missing something here. I know I've never really "understood"
the theory of bridges, other than that they try to get balanced. But in
the one you've just described here, where does the heating element fit in?
Provided that the problem of the element design itself has already been
solved, of course.

Thanks,
Rich

 

[Spehro Pefhany]

I must be missing something here. I know I've never really "understood"
the theory of bridges, other than that they try to get balanced. But in
the one you've just described here, where does the heating element fit in?
Provided that the problem of the element design itself has already been
solved, of course.

Thanks,
Rich

The bridge is balanced when the ratio of the resistors on each arm is
the same. Suppose you have a Pt100 sensor, we know that the resistance
at 150°C will be 125.37 ohms.

If you have a 5V supply you might like to use 4K99 precision resistor
as the resistor in series with the RTD to allow about 1mA to flow so
that the self-heating is not too bad, yet there's a bit of signal to
work with.

Then the other side of the bridge could be any pair of resistors
provided the ratio is 39.8:1 provided that the bias current doesn't
cause too much error. Offset voltage sensitivity is about 220uV/°C.


Best regards,
Spehro Pefhany

 

[John Fields]

I must be missing something here. I know I've never really "understood"
the theory of bridges, other than that they try to get balanced. But in
the one you've just described here, where does the heating element fit in?
Provided that the problem of the element design itself has already been
solved, of course.

They don't _inherently_ try to balance, but surrounded by the proper
circuitry, they can be forced into it.


Like this:

+V
|
+--------+--------+
| | |
| [R1]- - - --|
| | |
[POT]<---[METER+]---+ |
| | ---> |
| [HEATER]----> O THERMOMETER
| | --->
+--------+--------+
|
GND


What happens is that +V is fixed and R1 is adjusted until the heater
gets to the desired temperature as indicated by the thermometer, then
the pot is adjusted to cause the zero-center meter wired across the
bridge to indicate zero. Now, if the heater has a positive tempco,
its resistance will increase if it gets hotter, causing the voltage
dropped across it to increase, causing the meter to deflect upscale.
when that happens, whoever's minding the store decreases

 

[Spehro Pefhany]

On Wed, 24 Nov 2004 18:06:49 -0500, the Spehro Pefhany

Grrr.. I used the °F values by accident. The numbers below are for DIN
standard curve sensors.

The bridge is balanced when the ratio of the resistors on each arm is
the same. Suppose you have a Pt100 sensor, we know that the resistance
at 150°C will be 125.37 ohms. ^^^^^^
157.33


If you have a 5V supply you might like to use 4K99 precision resistor
as the resistor in series with the RTD to allow about 1mA to flow so
that the self-heating is not too bad, yet there's a bit of signal to
work with.

Then the other side of the bridge could be any pair of resistors
provided the ratio is 39.8:1 provided that the bias current doesn't ^^^^^^
31.72:1

cause too much error. Offset voltage sensitivity is about 220uV/°C.

^^^^^^^^
370uV/°C


Best regards,
Spehro Pefhany

 

[John Fields]

I must be missing something here. I know I've never really "understood"
the theory of bridges, other than that they try to get balanced. But in
the one you've just described here, where does the heating element fit in?
Provided that the problem of the element design itself has already been
solved, of course.

They don't _inherently_ try to balance, but surrounded by the proper
circuitry, they can be forced into it.


Like this:

+V
|
+--------+--------+
| | |
| [R1]- - - --|
| | |
[POT]<---[METER+]---+ |
| | ---> |
| [HEATER]----> O THERMOMETER
| | --->
+--------+--------+
|
GND


What happens is that +V is fixed and R1 is adjusted until the heater
gets to the desired temperature as indicated by the thermometer, then
the pot is adjusted to cause the zero-center meter wired across the
bridge to indicate zero. Now, if the heater has a positive tempco,
its resistance will increase if it gets hotter, causing the voltage
dropped across it to increase, causing the meter to deflect upscale.
when that happens, whoever's minding the store decreases

Aaaaarghhh!!!!

Hit "send" by mistake...

 

[John Popelish]

Just curious - would you have been this forthcoming with neat information
like this if you'd already known that it's for a THC evaporator? %-}

;^j
Rich

Why should I worry about evaporation?
Accidents waiting to happen and weapons are what bring my help to a
halt.

 

[John Larkin]

^^^^^^^^^^^

Like I said - more than I'm budgeted for the whole project. ;-)

Thanks!
Rich

Good grief, are you Francesco Piantedosi too?

John

 

[Rich Grise]

Good grief, are you Francesco Piantedosi too?

Actually, no.

But thanks for asking!

Cheers!
Rich

 

[Francesco Piantedosi]

[CUT]

Good grief, are you Francesco Piantedosi too?

John

Absolutely NO!
BTW Thanks for the answers!

Francesco

 

[Guy Macon]

Given the limited budget, I wonder if it would be possible to
derive the temperature from the resistance of the nichrome wire.
It all depends on the tempco of the wire, of course. Just a thought.

 

[Robert Baer]

I've been looking into something very similar to this, to warm a 5-10 l/m
air stream to 155 C. The LM35 doesn't go that high. The thermistors I've
seen aren't guaranteed at that temp. range. This leaves either a
thermocouple, which is a PITA, or an RTD, which I don't even know how to
search for - all I come up with is platinum and stuff, which cost more
than I'm budgeted for the whole project.

For the resistor, I found some thin nichrome wire on Ebay, which has a
resistance of approx. 28 ohms per foot, which gives me a "blank check" as
far as what I want the heating element to look like, since I'm making it
from scratch. I also have a 7.5V 1A wall wart that somebody threw away -
one of these days, I'm going to see how hot a 3" piece of this nichrome
gets with the wall wart, but the bugaboo, of course, is sensing the temp.

The controller shouldn't be too hard at all - there are a lot of suggested
circuits that you could just copy. You could search on, say, "voltage
controlled dimmer" and such.

Good Luck!
Rich

Please be advised that the LM35D may not be speced beyond 125C (or
whatever), but I have found that it is linear and as repeatable as a
scientific thermometer or calibrated thermocouple at least to 185 C.
One could use surface mount resistors for the heater, and surround the
LM35; mounting everything on a sheet of copper for good thermal
coupling.
**
How else do you think i characterize parts for high temp use?
I have designed a number of electronic circuits, and guarantee
parameters to 175 C and useable operation (at minimum) to 186 C.