An improved mini-oven temperature controllerDecember 2015
I recently bought an inexpensive mini-oven (benchtop oven). The oven was cheap, the store was having a special promotion day and it was sold to me for even less since it was the last of the model on the shelf. They practically paid me to take it! I will always be amazed at just how little one has to pay for simple appliances.
Part of the answer is of course, that these are unsophisticated in design and construction. You will have seen similar in appliance stores. Controls on this one include a temperature dial, an element selector for top, bottom, both or none and a clockwork timer (-it is the 21st century) that goes ding when time has expired. It also turns off the heating elements when it hits zero.
You know, it does work. One cannot argue about the functionality for the price paid. However, the temperature control was not good. When set to say around 150 degrees (celsius) or above, the elements would never turn off and there was a suspicion of the smell of overheating plastic. It would cycle on and off if the set temperature was below around 120 degrees, but the whole temperature control was unsatisfactory.
I had a look at it. The temperature dial was actually a simmerstat and the temperature of the oven is effectively only guessed, since there is no actual temperature measurement or feedback. Simmerstats are appropriate for hot plates, but ovens? I tried to readjust the simmerstat but this was difficult, and I only succeeded in making it switch on and off at too low a temperature even with the control at maximum. It did seem impossible to get just right. So, it was time for improved temperature control.
The new controller
I elected to use a small type-K thermocouple probe which was inserted into the oven cavity space just below the centre rack using a metal gland through the wall. The probe was to feed my controller board located in the 'control space' behind the main control panel. My controller board operated a small relay card, which has contacts rated at 250V and 10A. When both elements operate, the load is 6A. The board does need a source of 5 volts dc and in this case, a small 5V SMPS was used. The whole arrangement is powered only when the timer is set.
The schematic is below. I should add that this information is provided as a design idea rather than for a constructional project. I did not design a pcb for this one-off unit.
Although an instrumentation amp and comparator would do this job, I elected to use a dedicated thermocouple amplifier chip; the AD595. This chip is, admittedly quite expensive but is set up for thermocouples, including cold junction compensation. It keeps the parts count down and this is a one-off project.
When the oven space temperature is below the set point, pin 9 of the AD595 goes low and this turns on the relay via transistor Q1. The relay is a small unit from Jaycar which has an inbuilt driver. I did place a snubber network across the contacts. The AD595 can provide quite fine control but this is not necessary here and resistor R3 provides hysteresis so that the oven temperature has to fall some 10 degrees before the elements are switched on again after reaching the initial temperature.
The relay contacts were wired in place of the original simmerstat connections and the three boards; the PSU, relay card and controller board are housed easily behind the front panel. The 1kohm potentiometer was relatively simple to mount in place of the original simmerstat and with some filing of the shaft, the original control knob is also retained.
And the cost? Well the oven is now worth twice what was paid. Still cheap.
This information is not intended to be a step-by-step constructional project, although experienced constructors could easily assemble it. Please do not try to build this if you are not an experienced constructor. The dangers of building and testing such mains projects cannot be over-stated. I use an isolating transformer on the test bench and triple-check that things are un-plugged before making changes.
Axino-Tech Consulting & services; December, 2015
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