This temperature controller operates as a pulse snatching device, which
allows it to run at its own speed and tum on at the zero crossing of
the line frequency. Zero crossing tum-on reduces the generation of line
noise transients. TMOS Power FET, Ql, is used to tum on a heater.
Temperature sensor D6 provides a de voltage proportional to temperature
that is applied to voltage-tofrequency converter Ul. Output from Ul is a
pulse train proportional to temperature offset that is applied to the
input of triac optoisolator U2.
Simple Proportional-Temperature-Controller Circuit Diagram
The anode supply for the triac is a 28 V pk-pk, full-wave rectified sine
wave. The optoisolator ORs the pulse train from Ul with the
zeroTrossing of U2`s anode supply, supplying a gate tum on signal for
Ql. Therefore, TMOS power FET Ql can only tum the heater on at the zero
crossing of the applied sine wave. The maximum temperature, limited by
the sensor and the insulation of the wire, is 130°C for the components
shown.
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