Sciencemadness Discussion Board

Re: Apparatus - Thermo-relay switch?

LSD25 - 28-12-2007 at 15:53

I am currently trying to design a way to modify a single hotplate available everywhere to a thermostat enabled hotplate w stirrer. I have found numerous kits using thermistors which throw a relay switch once a trimpot temp. is reached and throw back when the temp drops, however, I can find none that are rated for above about 130C...

Anybody got any ideas on what I could do? For instance, how would one wire a thermostat into the kit instead of the thermistor? Or what applications use higher-temp rated thermistors?

This is the final technical hurdle (as opposed to mechanical/electrical) facing the projected building of a stirrer hotplate (via modified hotplate), stirrer mantle and even a tube reactor (these two via a modified toaster) - so anyone out there with ideas on how to do this (I would like to be able to take all three to around 400-450C with a graduated dial/trimpot).

Obviously the idea is to be able to dial in both the stirrer speed and temperature in one unit.

not_important - 28-12-2007 at 21:23

Quote:
Originally posted by LSD25
...Anybody got any ideas on what I could do? For instance, how would one wire a thermostat into the kit instead of the thermistor? Or what applications use higher-temp rated thermistors?...


Without knowing what the kit you have is, its schematic an so on, there's no way to properly answer your question.

A thermostat is a controller, a thermistor simply a device that changes resistance with temperature. They're not directly interchangeable. And where are you going to get the thermostat you propose to use, and why not use it in place of the kit?

Most thermistors aren't rated above 150 C or so, some are rated to 300-350 or perhaps higher. You might be able to extend the range of the kit by finding a thermistor of the same type (NTC/PTC) and the proper resistance value.

From here
http://www.meas-spec.com/myMeas/default/index.asp

is a chart

http://www.meas-spec.com/myMeas/MEAS_download/datasheet/pdf/...

For higher temperatures you generally use thermocouples or RTDs/PRTs for very accurate measurement. Neither will work in a circuit designed for a standard thermocouple, even though RTDs are resistors.

This page http://www.omega.com/prodinfo/temperaturecontrollers.html is an overview of temperature controllers.

LSD25 - 2-1-2008 at 17:28

This is what I am after - seems I'll have to access this article:

http://www.siliconchip.com.au/cms/A_109007/article.html

Anybody got access?;)

Has anyone tried making a stirred hotplate? Seems to me that by splitting 240V/10A current one could put it through two dimmer switches - allowing for temp/stirrer control - one goes to an aluminium plate whilst the other goes to the stirrer motor...

Equally, one could just use two lots of 240V/10A current to run an aluminium hotplate and the guts of a cheap electric drill - making a very big, very hot, very high-torque, extremely heavy duty mag stirrer hotplate. I suspect an investment in rare-earth magnets would be a good idea - as would the external probe-heat controller from the article. The heat controller apparently can be set to a temp with +/- allowance of <0.1C making it perfect for so many applications.

-jeffB - 2-1-2008 at 17:59

Quote:
Originally posted by LSD25
I am currently trying to design a way to modify a single hotplate available everywhere to a thermostat enabled hotplate w stirrer. I have found numerous kits using thermistors which throw a relay switch once a trimpot temp. is reached and throw back when the temp drops, however, I can find none that are rated for above about 130C...


If you look at the guts of an "electric skillet", essentially a frying pan with a built-in heating element, you'll find a switch built around a bimetallic strip and a knob that adjusts the distance from the strip to an electrical contact. I salvaged the one from an electric skillet whose plug went bad, with just such an application as this in mind. It's not suitable for extremely fine temperature control, but it has the advantages of simplicity, reliability, and economy.

not_important - 2-1-2008 at 19:11

As the actual circuit design is tucked away behind a purchase filter, I can't say much about it.

Simple lamp dimmers are usually phased control triac switches, they control determines how far into the AC half cycle the triac turns on at. This gives a chopped sine wave, and may not work well with some motors, especially at the lower settings.

If the controller allows for a fine degree of setting, this implies that the sensor is calibrated or you must calibrate the device yourself. The simple integrated circuit temperature sensor ICs, such as the LM76, MAX6611, DS1626, or TMP125, have precision +-2 C down to +-0,25 C depending on which chip. Their accuracy runs from +-3 C down to less that a degree C error; you can check at one or two temperatures to apply a correction factor thus increasing the accuracy, after that the precision will determine how good temperature readings are. They have little drift, so recalibration generally isn't needed, some may be accurate enough that no calibration is needed. However these generally have a maximum temperature of somewhere between 100 to 140 C, depending on which chip you are looking at.

Here's one selection guide to give a feel for what's out there.

http://www.analog.com/UploadedFiles/Product_Highlights/43017...

Thermistors and cheaper thermocouples generally need calibration at several temperatures to get a goodly degree of accuracy. Some may need occasional recalibration. Mechanical sensors generally need calibration and recalibration at times, SFAIK.

The sensor ICs often are intended to be used with a microprocessor which does the actual controlling. Some ICs, such as the MAX6509, MAX6510, and AD22105, use an external resistor to set a trip point/temperature and provide an output for on/off control. You can use a variable resistor to allow adjustment of the trip temperature, but the resistor will limit thr precision of the setting.

Twospoons - 2-1-2008 at 19:54

The easy solution is to go on ebay and get something like this:
Temperature controller

These things will usually take an RTD (a platinum wire sensor), or thermocouple, or both. They will also do a much better job of temperature control than a plain old thermostat.
You should be able to get a basic one for under US$50.