A resistor at 150°C...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...any ideas?

Thanks Francesco

--
Posted via Mailgate.ORG Server - http://www.Mailgate.ORG

"Francesco Piantedosi" wrote

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.

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.

-- Nicholas O. Lindan, Cleveland, Ohio Consulting Engineer: Electronics; Informatics; Photonics. Remove spaces etc. to reply: n o lindan at net com dot com psst.. want to buy an f-stop timer? nolindan.com/da/fstop/

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"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com

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

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