| 摘要 |
A transformerless electrical appliance with an AC heating element uses a triac in series with the heating element and a control circuit responsive to a thermistor to regulate flow of heating current. A power supply is formed by a resistive divider that steps down the AC line voltage and a fullwave diode bridge that rectifies the stepped-down voltage. The resistance of the divider is comparatively high, limiting power dissipation in the divider, but incidentally producing significant variations in the power supply voltage (roughly 100% from minimum to maximum levels). The impedance of the divider element which conducts the operating current of the control circuit is selected to ensure that the average minimum power supply voltage is above the minimum operating voltage required by the control circuit. Resistive dividers are used to produce temperature and reference signals from the power supply voltage, making switching of the triac relatively immune to the significant variations in the supply voltage.
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