| 摘要 |
A thin-film windshield heater is energised with a high voltage supply derived from a low DC voyage supply via a DC-DC converter (3, 7, 8, Figs 1, 2). The high DC voltage may be applied to the heater via a commutator (10) to periodically reverse the polarity of the DC voltage applied to the heater; e.g. every 15 seconds. The high DC voltage may be progressively reduced to zero prior to commutation and subsequently returned progressively to its former magnitude. The RMS value of the voltage applied to the heater may be increased relatively slowly over several reversal cycles on initial switch-on so as to minimise thermal stress on the windscreen. The absence of high frequency components from the supply voltage applied to the heater eliminates interference therefrom. The voltage variations may be obtained by pulse width control of switching transistors in the DC-DC converter, by varying the resistance of a control element connected in series between the converter and the heater, or by varying the conductivity of switching elements in the commutator (10). <IMAGE> |
