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Changes to Tempmaster

I have just completed building the Tempmaster Mk2 (SILICON CHIP, February 2009), a very efficient design as I previously had an incubator that had a Triac and ramp generator problem with the IC; it was a UK design. My question is, can the temperature range be up to 40°C, as I need 37.5°C for incubating chickens? Currently it can only run up to 19°C. What components need changing? (G. F., via email).

The temperature range of the Tempmaster Mk2 can be extended up to a maximum of about 40°C by making the following changes:

(1). Reduce the value of the resistor from pin 3 of REG1 and test point TP1 from 2.7kΩ to 2.4kΩ.

(2). Increase the value of the resistor from TP1 to the trimpot VR1 from 3.3kΩ to 3.6kΩ.

With these values changed, use VR1 to set the voltage at TP1 to 3.105V to achieve a temperature threshold of 37.5°C.

Increasing hysteresis
in the Tempmaster

I have recently built the Tempmaster Mk2 and while it basically worked as described, one or two problems have arisen. When constructed and initially tested it was incapable of achieving the temperature ranges described in the original SILICON CHIP article. My unit was only capable of reading between 4°C and approximately 15°C as built from the kit.

I’ve pondered the problem and after doing the arithmetic I ultimately replaced the 500Ω 10-turn potentiometer (VR1) with a 1kΩ unit, this certainly cured the temperature range problem. My intention was to run the unit in the heating mode and temperatures beyond 19°C were required and with that change the unit worked well up to a temperature over 22° C.

My next problem occurred with the unit being unable to switch cleanly and a given temperature setting. It appears to me that the rate of change of temperature/voltage in the LM335Z, is too slow to enable precise switching of the relay, resulting in a constant chattering as the sensor reacts to very small temperature changes. Can you suggest alternate values for the positive feedback resistors (10MΩ and 1.2kΩ) that would be suitable to create a sufficient differential between the reference voltage and the comparator? (W. G., Dunedin, NZ).

• Replacing the 500Ω trimpot with one of 1kΩ is one of a number of acceptable ways to increase the temperature range, so you chose well.

To provide the comparator with a larger hysteresis band in order to reduce ‘chattering’, simply increase the value of the resistor between pin 3 of the comparator and LK1, from 1.2kΩ up to 1.5kΩ, 1.8kΩ or even 2.2kΩ.

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