Dimmable CFL’s

The CFL technology is very good for producing an energy saving light bulbs which are economically priced and will deliver significant savings in electricity costs to the consumer. However, it does have one big drawback when compared to the traditional incandescent bulb because it is not dimmable in its standard form. In applications where dimming is being used primarily to save electricity then this is not a major issue because the CFL is inherently efficient, but where people need to reduce the light level in a room then this is certainly a barrier.
Traditional dimmer switches used adjustable power resistors or adjustable transformers and whilst they worked they did not offer good efficiency as a large proportion of the power that was not directed to the light bulb was turned into heat causing inefficiency in the dimmer controller. During the 1960’s thyristors and triacs came to market which used semiconductor technology to make dimmers that offered good efficiency at a reasonable price. It is the triac thyristor which is used for dimming AC mains supplies and it operates by truncating each oscillation of the AC sinusoidal wave to reduce the power being passed to the light bulb.
The problem with the CFL is that with a standard electronic balast when the power is reduced there comes a point when there is insufficiemt power to activate the mercury vapour which then emits UV light to the phoshor which in turn produces the light.
As a consequence when a standard CFL is dimmed it may dim slightly but then it will go out completely.
The issue is that when the dimmer is turned down there is insufficient current through the burner and the filament temperature drops causing insufficient electrons to be emitted.
The technical solution is to add electronic circuitary for the following purposes:-

  • Control preheat current and time period for good ignition and to maintain burner filament temperature to ensure that the CFL can still operate when the triac dimmer is turned down.
  • Utilise a transformer which will supply sufficient current during low light settings to maintain a constant DC derived voltage
  • Use the right charge pump capacitor to pump power to the electrical capacitance at low dimming positions to prevent capacitive
    mode.
  • Modify input filter components to filter output noise from the dimmer to ensure a smooth input voltage is applied

The electronics of the dimmable CFL need to be designed to accomodate the above factors and with a good design the CFL light bulb will be dimmable in the range from 10-100%.
In order to achieve this the circuitry is fairly complex and as a result the cost of energy saving dimmable CFL bulbs is much higher than standard CFL’s but they still offer a sound return on investment and are invaluable where light level control is needed.

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