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!
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!chromium - 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?Twospoons - 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.jimwig - 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.bio2 - 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.IrC - 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]bio2 - 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.Twospoons - 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.Thomas Winwood - 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?IrC - 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.Twospoons - 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.IrC - 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.12AX7 - 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.
TimTwospoons - 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!lordmagnus - 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.Twospoons - 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.IrC - 22-1-2006 at 00:57
