|Figure 1. Simple Temperature Controller|
I found this circuit at a site devoted to Barometers, of all things. You may well ask what is a TL431? It is a programmable Zener diode. Whatever voltage you place from the cathode to the connection coming out of the side will make it a Zener of that voltage. It covers a range of 2.5 to 36 V. The point where it starts to work, 2.5 V, is what we take advantage of to use it in a temperature controller. Simply choose resistor values along the left side to make 2.5 V at the control electrode and it will switch on as the temperature drops below the set point and off as it goes above. NTC thermistors are produced to a standard calibration curve and this is it for the NTC103 Thermistor:
|Figure 2. Calibration Curve for NTC103 Thermistors|
On the above schematic the values not in ( ) are those of the original article and those in ( ) are the ones I selected based on a 9 V source and an operating temperature of 120 deg F/48.5 dec C. From the curve in Figure 2 the resistance of the thermistor at 48.5 C is 3846 Ohms from which I calculated the other resistance to be 10k.
I installed these components on the short crystal in the new QRP Labs 80 kit which I was soldering up at the time. Note that only two leads are required for the 9 V source and can be taken off a convenient location on the QL board. Figures 3 and 4 shows what this looks like and Figure 5 shows the crystal assembly installed in the QRP 80. I placed a 1 cm thick slab of Styrofoam between the crystal and circuit board as part of the thermal insulation. To complete the insulation I whittled out a matching block of Styrofoam to place over the top. When soldering up the QRP 80 it helps to move aside some of the components surrounding the crystal to accommodate the Styrofoam insulation.
|Figure 4. Connection of Components|
It seems to work as well as the controller I described in Part 1. When switched on the frequency quickly drops to a steady value and remains there for hours on end. The big advantage is that no external circuitry is needed. This is the way I plan to go when I build future MEPT's.
Before closing this post I'll mention yet a third temperature controller which is used by Johan, PA0TAB. It consists of discreet components, two transistors and four resistors, which can be mounted on or at the crystal. I'll describe that in a future post.