Posted on Oct 20, 2012

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.

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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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