A resistor at 150C...how?

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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



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Re: A resistor at 150C...how?
On Wed, 24 Nov 2004 10:41:38 +0000 (UTC), "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!

---
I don't understand what you mean by 15mm diameter.  Is that the area
of a circle you have to heat or the diameter of a hole into which you
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Re: A resistor at 150C...how?


> 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



Re: A resistor at 150C...how?

> 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.

--
Nicholas O. Lindan, Cleveland, Ohio
Consulting Engineer:  Electronics; Informatics; Photonics.
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Re: A resistor at 150C...how?

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.



Re: A resistor at 150C...how?
On Thu, 25 Nov 2004 08:28:04 +0000, the renowned 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.

Yes, although NiCr is designed to have a relatively low tempco, and
usually controlling the heater temperature itself is a particularly
crummy approach from a controls pov.


Best regards,
Spehro Pefhany
--
"it's the network..."                          "The Journey is the reward"
snipped-for-privacy@interlog.com             Info for manufacturers: http://www.trexon.com
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Re: A resistor at 150C...how?
On Thu, 25 Nov 2004 08:28:04 +0000, 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.

Minco makes flexprint stick-on things that are simultaneously heaters
and sensors. Of course, they only sense their own temperature, not the
temp of the thing they're stuck to, so there's a coupling error. You'd
have the same problem heating air... the heater itself will be hotter
than the exit air stream.

John



Re: A resistor at 150C...how?
On Thu, 25 Nov 2004 09:29:16 -0800, John Larkin

>On Thu, 25 Nov 2004 08:28:04 +0000, Guy Macon
><http://www.guymacon.com wrote:
>
>>
>>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.
>
>Minco makes flexprint stick-on things that are simultaneously heaters
>and sensors. Of course, they only sense their own temperature, not the
>temp of the thing they're stuck to, so there's a coupling error. You'd
>have the same problem heating air... the heater itself will be hotter
>than the exit air stream.

---
And there _are_ budgetary constraints, I think, so even just one Minco
RTD/heater would cost more than a discrete heater - sensor solution.

--
John Fields


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