Heating appliance

A method of generating a clock signal 232 from an AC power supply includes receiving, by a comparator, a reference signal 236 at a first input and the AC supply signal 246 at a second input. A clock signal is output 252 by the comparator based on a comparison of the input signals, such that transitions of the clock signal take place while the reference signal is at a trigger voltage, which may be the average voltage of the AC supply. Following each clock signal transition, the reference signal is changed to a hysteresis voltage 254 to reduce the chance of a spurious clock transition due to noise. The reference signal is returned from the hysteresis voltage to the trigger voltage 266 prior to the AC supply signal returning to a level intended to cause a further clock signal transition at the comparator output. A clock generation circuit (Figure 7) on which the method is operated is also provided, as is an apparatus comprising a heater, heater drive circuit and clock generation circuit; the clock generation circuit includes a comparator and reference voltage generator. Furthermore, methods and apparatus for sampling, low-pass filtering and estimating an AC mains voltage, and for testing a relay by testing a voltage at a node connecting it to a heating circuit (e.g. Fig.17), are provided.