Greenhouse Heater Temperature Control Project

Posted on Feb 4, 2014

This is simply (4) 1N4148 diodes connected in series with a thermal anticipation resistor (R1) heat shrunk together at the end of a (3) wire signal cable ”it is visible on some of the photos. The use of a thermal anticipation resistor is an old HVAC thermostat technique that adds negative feedback to the system by immediately heating the temperat

Greenhouse Heater Temperature Control Project
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ure sensing device slightly. It forces short cycles and prevents temperature overshoot. Because the amount of power to apply to the thermal anticipation resistor was unknown, I incorporated a power level pot (R6). I later determined that it works quite well at the maximum setting, so the pot is not required. Although the temperature measurement can be accomplished via a single diode, four diodes in series are used to get the signal out of the mud.  The inexpensive LM324 op amp has a maximum input offset voltage of 7mV, so the higher level voltage signal helps to improve performance. The diodes are biased at approx 4mA via R2. U1C is connected as a voltage comparator with positive feedback via R5. Positive feedback prevents relay chatter. C3 is an input noise filter capacitor. As the temperature cools, the voltage drop across the probe diodes increases. When it reaches the set point, the output changes states and the positive feedback through R5 further increases the non-inverting input by 5mV to assure that it remains latched until the temperature increases and the voltage drops below the set point. In a previous circuit we see that the temperature coefficient of silicon diodes is approx. -2mV / °C. (4) in series boosts it to -8mV / °C. I dropped the probe in ice water and measured the voltage ”2. 773V in my case. Then I calculated what the voltage would be at 4. 4 °C (40 °F). Then I adjusted the voltage at the calibration...

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