Sciencemadness Discussion Board
Not logged in [Login ]
Go To Bottom

Printable Version  
Author: Subject: Temperature Controller for hotplate etc
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 1-12-2005 at 22:31
Temperature Controller for hotplate etc


Here is a schematic for a temperature controller for mains powered hotplates. It is a simple proportional controller, using an NTC thermistor as a sensor. Despite the simplicity, the circuit works very well and will hold to within a degree or two of its setpoint. I built one to control a hotplate I use for making PCBs by toner transfer. Note that the TRIAC will require a heatsinking if your hotplate pulls more than about 5A. With the components shown the controller will easily handle a 2kW heater. Enjoy!

[Edited on 2-12-2005 by Twospoons]

Attachment: Temperature Controller.pdf (13kB)
This file has been downloaded 1531 times





Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
IrC
International Hazard
*****




Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline

Mood: Discovering

[*] posted on 1-12-2005 at 23:53


Nice circuit. If you could find a 45 volt (or close) varistor you could use the one part to replace all 4 of those zeners. You could also use two seperate 1N4007 diodes to replace D5 the BAV99. Also any good diac around 20 volts could be used for Q3. I love circuits like this since I can build them with stuff laying around in the junkbox. Saves looking around and buying stuff. Good post!
View user's profile View All Posts By User
chromium
Hazard to Others
***




Posts: 284
Registered: 27-6-2005
Member Is Offline

Mood: reactive

[*] posted on 2-12-2005 at 09:53


Good work, i have to make something like this for my lab other day.

Why not put this into Prebublications forum as this surely could be in SM article collection?
View user's profile View All Posts By User
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 4-12-2005 at 15:19


IrC a varistor wont work - its a breakdown device and wont regulate the way zeners do (thats what they're there for - as regulators).
I only used a BAV99 because I had some, its a SOT-23 suface mount device, so 1N4007's would be easier to handle.




Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
jimwig
Hazard to Others
***




Posts: 215
Registered: 17-5-2003
Location: the sunny south
Member Is Offline

Mood: No Mood

[*] posted on 4-12-2005 at 16:16


without offending anyone these are available off the shelf -

600 watt to 2000 watt wall controllers for resistive loads at Home Depot, Loews et al. They come ready to go as a series type device.

Lower wattage phase controllers (probably zeo crossing circuits) for fan motors or motors in general. I am not familiar with the available power ratings.
View user's profile View All Posts By User
bio2
Hazard to Others
***




Posts: 447
Registered: 15-1-2005
Member Is Offline

Mood: No Mood

[*] posted on 4-12-2005 at 19:13


.........600 watt to 2000 watt wall controllers for resistive loads at Home Depot, Loews et al. ..........

What type of temp sensor do they have? Sold for heaters I quess?

Hey twospoons, if I use a 100K ohm NTC the pot and fixed resistors can simply scaled is that right? Just wondering if I'm missing something. I calibrated the NTC with a cool program I found using three measurements with a mercury thermometer. When I then compared the curve to actual ohm readings it was amazingly close! If
you need something like this let me know.


I was just about to put together a similar unit using a diode as temp sensor but I like this NTC circuit better as now I don't need a op-amp comparator and optoisolator.

For symmetrical firing 4 diodes are needed (inductive loads) although the smoothness is noticeable on resistive loads compared to the 2 diode version as it resets the capacitor every half cycle. Both 2 and 4 diode versions are hysteresis free.
View user's profile View All Posts By User
IrC
International Hazard
*****




Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline

Mood: Discovering

[*] posted on 4-12-2005 at 20:02


"IrC a varistor wont work - its a breakdown device and wont regulate the way zeners do (thats what they're there for - as regulators)."

Finding a 44 volt varistor might be hard, but I disagree if you had the right part (voltage), as they stop breaking down below their voltage. I have used varistors for many years to provide high voltage regulation in my geiger counter circuits. A varistor does not turn on at it's rated voltage and then stay turned on below its rated breakdown voltage when operated within it ratings. As to whether or not there would be some specific problem using them with this circuit I am not sure, I would have to build and try it. Then again, it is far easier to find zeners in this voltage range so why bother I guess. In my geiger circuits I used a series string of 130 or 150 volt varistors as they were common and rated for more dissipation than typical 1 watt zeners, and work on both half cycles.

They are commonly used right across AC mains to supress surges, and if they did not shut off below their rated voltage they would blow themselves out of the circuit. As to your specific application you may be right, but the reason you have zeners back to back is to cover both half cycles at 44 plus 1.2 or roughly 45.2 volts. In similar regulation schemes I have used varistors and the circuit worked very well. I am sure this is all moot as 45 volt varistors may be a little hard to come by.

As to buying the circuit at Lowes or Home Depot this may be easier but it does take away the mad science doing it yourself aspect, kinda boring. Unless you are in a hurry to get some other cool idea going using the temperature regulator.


[Edited on 5-12-2005 by IrC]
View user's profile View All Posts By User
bio2
Hazard to Others
***




Posts: 447
Registered: 15-1-2005
Member Is Offline

Mood: No Mood

[*] posted on 4-12-2005 at 21:06


Back to back zeners emulate a diac or SBS and a MOV varistor should work
very well as long as the curve and frequency (moot @ 60Hz) characteristics
is somewhat similar .

MOV's found in phone circuits are usable for this voltage range 20-45 or so depending on the input (120 or 240). MOV's can also be used in series so automotive ranges might be easy to scrounge.
View user's profile View All Posts By User
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 5-12-2005 at 13:30


Honestly, zeners are just easier! They come in a hundred different flavours and they're dirt cheap!

bio2: yes you can use a 100k NTC and scale the pot - though this means the transistors have less base current to work with and you may not get full power. This could be compensated somewhat by reducing the size of the timing cap.

As for calibration, really what you need to do is build the circuit and calibrate it in use. Its only using proportional control so calibration aint going to be stunningly precise. Variations in transistor gain, and other components, will all affect the relationship between pot value and final temperature. The real point of this controller is that once it is set, the temperature is very stable, with none of the flicking up and down you get with an on/off type thermostat.




Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
Thomas Winwood
Harmless
*




Posts: 43
Registered: 16-12-2004
Location: United Kingdom of Great Boredom
Member Is Offline

Mood: Anhydrous

[*] posted on 5-12-2005 at 14:45


Your circuit diagram represents the heater as having an inherent resistance. Would that resistance have any correlation to the heat of the implement? Would it be possible to obtain a good heater which has a thermistor attached to measure the temperature?



I\'ve been told having a sig is banned, despite the facility being available. Um...contradiction?
View user's profile Visit user's homepage View All Posts By User This user has MSN Messenger
IrC
International Hazard
*****




Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline

Mood: Discovering

[*] posted on 5-12-2005 at 16:55


"zeners are just easier! They come in a hundred different flavours"

I agree they are better, but now you have me really lost. I spent all day going through a box of zeners tasting them one by one and I could not find any difference. There was one drawer of them that had a flavor so I thought you might be right. Then I remembered one year the cabinets were on the floor in a move and catching my wolf pissing on my parts cabinets. Therefore the test may be in error.
View user's profile View All Posts By User
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 5-12-2005 at 19:44


Its your cellaring technique! They need to be kept at -113C, and sealed in anti-static bags with some dessicant if they are to retain their flavour for any appreciable time. A well aged, properly cellared zener is a real treat!

Meanwhile, back in the real world...

Yes the resistance of the heater element correlates with its temperature. Its just damn inconvenient to try to measure a small signal like that when there's 2kW of mains pumping through. Not impossible though.
You would be unlikely to find a heater with a thermistor already attached. But thats no problem, as thermistors also come in many styles - some with rings or threaded bosses to make attachment easier.

A note on thermistor position: closer to the heater element will result in better stability. If the thermistor is too far away the transport lag may make the control loop unstable, and the temperature will oscillate. This is something peculiar to each installation - so you are just going to have to guess the best position. I had no trouble, but I epoxied (steel loaded epoxy putty!) mine only 2cm from the heater.




Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
IrC
International Hazard
*****




Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline

Mood: Discovering

[*] posted on 5-12-2005 at 23:44


Savin or Ricoh copy machines around the 770 model liquid toner machines of the 80's had the perfect thermistor. It was built in a neat 6 mm wrench hex shape with threads on end so you could just screw it in a hole you drilled and threaded. Also had very good mechanical ruggedness and heat insulated leads. I am sure they are not very costly and still available from a copy dealership. I am also sure powdered toner and much more modern machines as well as other brand machines use nearly identical screw in thermisters. Good place to start looking for the perfect part for all your heating control needs. Non-kiln of course, drying ink didn't get that hot but they would work for any normal hotplate temperature you would ever see.
View user's profile View All Posts By User
12AX7
Post Harlot
*****




Posts: 4803
Registered: 8-3-2005
Location: oscillating
Member Is Offline

Mood: informative

[*] posted on 6-12-2005 at 15:23


Quote:
Originally posted by Twospoons
Yes the resistance of the heater element correlates with its temperature. Its just damn inconvenient to try to measure a small signal like that when there's 2kW of mains pumping through.


Well, not really. Something called a current sensor. Given a relatively constant (i.e., +/-5%) voltage supply, P = V/I is inferred. (Or V and I variable if you vary voltage.)

This is used in omnidirectional anemometers where a nichrome wire is heated by a current and the voltage is read off directly as a function of wind speed.

Tim




Seven Transistor Labs LLC http://seventransistorlabs.com/
Electronic Design, from Concept to Layout.
Need engineering assistance? Drop me a message!
View user's profile Visit user's homepage View All Posts By User This user has MSN Messenger
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 6-12-2005 at 23:20


I said "inconvenient" not "impossible". The technique is also used in Pirani pressure gauges, where the heater forms one arm of a Whetstone bridge. Neither this, nor anemometers, usually use 2kW heaters run from AC mains!



Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
lordmagnus
Hazard to Self
**




Posts: 92
Registered: 10-1-2006
Location: Webster, TX
Member Is Offline

Mood: No longer annoyed ( I found a new girlfriend)

[*] posted on 10-1-2006 at 20:53


Depending on the wattage rating of the hotplate, get a wall mounted light dimmer from home depot or loews that is bigger than the wattage of the hotplate (for durability), and install it in a handy box inline with the power cord. bypass the bi-metal temperature controller that is built into the hotplate, and use the dimmer to get the heating element to the temp. you want. IT works pretty good, I use it on my ABSINTHE REFLUX STILL all the time. allows me to get the boiling flask up to about 190 deg. F. and hold it there so that the alcohal, and the thujone evaporate without much water boiling out as well into my vigereaux column.



I\'ll kill a man in a fair fight, or if I think he is gonna start a fair fight, or if he\'s bothrn me allot, or if I am getin payed good, or probably over a good woman.

-Jayne Cobb (Serenity)

View user's profile View All Posts By User This user has MSN Messenger
Twospoons
International Hazard
*****




Posts: 1326
Registered: 26-7-2004
Location: Middle Earth
Member Is Offline

Mood: A trace of hope...

[*] posted on 10-1-2006 at 21:44


Thats a very different beastie - you have no temperature feedback. All you are doing is providing an adjustable constant-power input. If your thermal load changes, then your temperature will too.



Helicopter: "helico" -> spiral, "pter" -> with wings
View user's profile View All Posts By User
IrC
International Hazard
*****




Posts: 2710
Registered: 7-3-2005
Location: Eureka
Member Is Offline

Mood: Discovering

[*] posted on 22-1-2006 at 00:57


Some good circuits on this page :

http://ourworld.compuserve.com/homepages/Bill_Bowden/page2.h...
View user's profile View All Posts By User
Texium
Administrator
Thread Moved
19-11-2023 at 10:54

  Go To Top