A Thermostat With Adjustable Hysteresis

Published:2012/12/3 0:11:00 Author:muriel | Keyword: Thermostat, Adjustable Hysteresis | From:SeekIC

A thermostat doesn't try to maintain a constant temperature. In order to do so - it would have to keep switching on and off every few seconds. Instead - it keeps the temperature within a specific range. When the preset temperature has been reached - it switches off. And it only switches on again - when there has been a significant change in temperature. The difference between the temperature at which the thermostat switches off - and the temperature at which it switches on again - is the hysteresis. Without this hysteresis - your central heating, refrigerator etc. would keep switching on and off every few seconds. This particular circuit energizes the relay when the temperature falls - and de-energizes the relay when the temperature rises again. If you replace the pnp transistor (BC557) with an npn transistor (BC547) - the circuit will operate the other way round. In order to minimize power consumption - choose the configuration that energizes the relay for the shorter time period. If it's going to be hot most of the time - choose the one that energizes the relay when the temperature falls (BC557). If it's going to be cold most of the time - choose the one that energizes the relay when the temperature rises (BC547).



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