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Temperature controller

Temperature controller

The TempRat is going to be a little device to switch things off if the ambient temperature rises above a definable level. A certain time before switching off, a message is sent down the serial link to the host computer telling it how long until the power will be switched off.
The temperature sensing in the TempRat is done using an LM334Z. This is little device that outputs the current temperature in volts, with 10mV per degree Kelvin. This means that at 0 degrees centigrade, the device will be showing about 2.73 volt s. This voltage is then compared to the reference voltage (2.5 volts from an LM335-2.5) and amplified through a differential amplifier with a differential gain of about 220/47 = 4.69.

This gives, from an input voltage swing of between 2.73V and 3.43V, a (thoeretical) output voltage swing of from 1.07V to 4.35V, working out to a 46.9mV per degree (which is a lot easier to measure, than 10mV/deg).

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