As mentioned in another post I got an industrial (well semi industrial anyway) temp regulator from conrad.de the other week. The regulator is in fact a PIC microcontrolled piece of electronics with a diode as the sensor. A diode’s PN-junction has a temperature dependency of about 2 mV/C and can be pretty precise. The output of the regulator is a relay capable of 2 A, which is enough for driving a solid state relay for better power control.
Unfortunately the relay only opens or closes. I had to modify the circuit so that it gives voltage enough to control the solid state relay.
A peek inside the conrad-controller shows the PIC16 microcontroller, a 7805 regulator and the output relay.
The relay is operated by 12 V and since there is a 5 V regulator inside, a guess was that the transformer would give something in the 12 V range. The assumption turned out to be correct!
The PCB modified for controlling the relay coil with the unit’s internal 12 V. Modified like this the controller gives +12V when heating is required and 0V else.
The controller and solid state relay was fitted in a simple plastic enclosure:
First trial run with mains attached. On the table to the left is the actual sensor tube.
Side view showing the solid state relay. I find it quite impressive that this little blighter can switch a current of 40A at 240VAC!
The manual to the controller doesn’t specifically say that it can be calibrated. But in the menu system there actually is an option for offsetting its readings. The snag is finding an exactly 0C reference. Ice is below 0C and water above 0C, so somewhere in-between must be 0C? I made some ice cubes in the freezer and put them in a pitcher with cold water, about a 50/50 mix. Under constant stirring, the ice melted as the cubes produced 0C water and after a while the temperature of the mix stabilized to exactly 0C. That is when I took the corrective reading to the controller.
Calibrating the controller. Here the water is 0C and the controller reads exactly that!
Now where do I get a better heater? That would complete the C-41 temperature controller.