This has taken a while to design and build as I have other things to do as Spring approaches.
The controller is a complete stand alone unit. It has very repeatable temperature control with a precision of about +- 10 degrees F. Accuracy is dependent on the thermocouple and how well the calibration is carried out. The circuitry is very simple although the theory of operation is less simple.
The unit is generally easy to build although it has a lot of internal wire connections. There are also a couple of parts that I used because I had them that may be hard to obtain. In that event there are alternatives that may work even better at the cost of a little more complexity.
The main features:
Fail safe control of both legs of the 240 vac supply. If the controller loses power the 240 ac supply is disconnected. Also, the main power relay is never called on to switch under load. This prevents wear and tear on the contacts and the possibility of contact welding.
Very flexible ramp up and ramp down from a rate of 100C per hour to the maximum furnace capability.
Easy addition of computer control using any version of Windows. The additional circuits for computer control are minimal and no special adaptor or cables are required beyond possibly a cheap USB to serial port adaptor and a standard serial cable. The software is not dotnet based.
With computer control complex staged heat, hold and cool down cycles can be easily implemented. Cycle programming is in natural English.
The circuit is so simple that I haven't designed a circuit board for it as yet. I probably will but for this prototype I used a small bit of prototyping board.
The box I used is steel rather than aluminum as I feel that is a safer option when dealing with 240ac power. It also helps to shield the low power portion of the electronics.
The high voltage components are inside the black steel shield. They are the main power relay, the solid state relay and the small 110 ac step down transformer that powers the analog logic. The rear connector block for the high voltage has a cover that screws on to prevent contact with the terminals.
The knobs on this unit are one of the very hard to find items I mentioned. They are planetary geared with a ratio of 10 to 1 for very precise setting of the temperature and ramp. Another item that may be difficult to find is the potentiometer that sets the temperature regulation. It is a 500 ohm, 1 ohm per winding turn wirewound potentiometer. This means that it is extremely repeatable and unaffected by changes in the environment. The knobs are calibrated with one hundred and one index marks from zero to 10. This allows for easy and repeatable manual setting to within ten degrees Celsius over a range of 1000 degrees Celsius.
The top switch controls the main power relay and the power to the logic. Since the SS relay cannot be turned on until the main power is turned on it is impossible to close the main power relay into the load under power.
The centre switch enables the SS relay and turns on the heat. The bottom switch sets either ramp up or ramp down operation. The bottom knob sets the charging or discharging rate on a .1 farad supercapacitor that has virtually zero leakage and will give a very accurate time delay even at extremely slow charge rates.
Each of these items may be substituted although the supercapacitor will have to be substituted by a slightly more complex timing circuit. The meter may be subtituted by an ordinary digital voltmeter and the circuit provides for calibration so that the volt meter reads temperature directly in the units of choice.
The calibration was performed using a high quality kiln pyrometer with a platinum/palladium thermocouple. Several heating runs were made and the values plotted in Excel to determine the tracking profile of the type K thermcouple. This profile can be used by the software to provide a real time correction.
This is the schematic for the controller. I haven't shown the power components as the hookup of those is very straight forward.