Diapason
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- Feb 8, 2006
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- 48
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Switched-mode supplies (which most modern consumer electronics use) are notoriously bad for having a large IV phase difference due to the inductors used as part of the switching process. Very few consumer manufacturers bother with active PFC as it adds significant expense to the cost of the power supply.
Absolutely. As you say, it costs to get it right, and the legacy of a poor supply is a large IV phase difference - with effectively 'wattless' current - but still not chargeable! The problem is that the cables are still taking the load and - quite rightly, fuses need to be specified accordingly.
Another interesting point is the IV phase of a tungsten filament lamp (the ones before these new-fangled energy saving things). When attached to a dimmer, for part of the mains cycle they act as an inductor, and for part as a resistor (due to the way most modern dimmers work, switching at the zero-crossing point of the AC waveform).
Gosh, this is interesting. The point you make about the inductive nature of a coiled tungsten lamp is fascinating. I hadn't heard/thought of that before. I can quite see why the lamp would behave as an inductor when the current slew rate was high.
However, surely that's not quite right about the dimmer. Modern lighting dimmers switch midway through the half-cycle, the exact point determined by the level of lighting required. For full lighting, the dimmer switches at the beginning of each half-cycle, and remains on until the end. When at half power, the device switches on when voltage is at peak, and then - as before - remains on until the end. It is the switch at peak voltage, presenting dI/dt at maximum when the dimmer is in 'Grand ol' Duke of York' mode (neither up nor down!) that causes interference and characteristic dimmer buzz - and often, in theatres etc, cross couples to audio circuits if cable routes have not been selected carefully. And surely, it's this switch at peak voltage, with a very high dI/dt, that brings out the inductive characteristic of the light bulb that you bring to our attention.
Zero-voltage switches are used in order to ensure that any device is only switched on at the beginning of a half-cycle, and are used - for example - for heating loads. Proportional heating control can be effected by cycle counting with a zero-voltage switch, so - for example - a heater can be set at half power by switching 'on' for 25 half cycles, and then 'off' for the next 25. Zero voltage switching does, in this way, reduce dI/dt to the minimum - improving the capability of the triac, and reducing the need for suppression components and cable isolation. However, such a device couldn't be used on a lighting circuit because the flicker over the resultant (1/2 second) duty cycle would be absolutely intolerable. Heating applications are fine, because of the thermal inertia of the element and room.