Our Voltimum Experts answer your questions on a daily basis in our Technical Expertise area. This one, about low wattage lighting loads and flicker problems when using electronic switching, is answered by Elkay:
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| Use an Elkay PFCC-1 4uF capacitor. |
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| Figure 1. |
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Question: Why do low wattage loads flicker or stay on when using electronic switches?
Answer: When low energy lamps, compact fluorescent, LED etc. are controlled with electronic timers and switches, the lamp may flicker or even stay illuminated when the switch is in the off state.
The main reasons for this are:
- Electronic timers and switches need energy to power their own electronics. If the switch has no neutral connection this energy can only be obtained via the load.
- Compact fluorescent lamps and similar low energy lighting products present no load at very low voltages.
Consider the first point. Electronic timers and switches generally have a microprocessor or other electronics to monitor the activating button of the switch, keep a battery charged if fitted, and any number of tasks required for correct operation. The energy to perform these tasks ultimately comes from the supply and as previously stated, if the switch has no neutral connection this energy can only be obtained via the load. The power supply circuits in such switches and timers require a low voltage to operate, typically 5 - 24V and a relatively low current of a few milliamps.
Second point - compact fluorescent lamps (CFLs) and electronic fluorescent ballasts use an inverter or other voltage converting device to be able to strike a fluorescent tube from a wide operating supply voltage. This also gives CFLs and ballasts the ability to stabilise the tube current for greater lamp life and stable light output. It also implies the ability to turn off the lamp and inverter if the input voltage drops below a workable voltage its dropout voltage. Below this voltage, because the tube and inverter are not functioning, the lamp presents no or little load. LED lighting has a similar characteristic although the electronics are more usually a switching down converter but still, a dropout threshold will be used.
Assume an electronic switch or timer is connected to a CFL or electronic ballast and the supply is turned on. Up to this point the voltage across the CFL or ballast internal capacitor in zero. The switch becomes active and its quiescent supply current begins to flow.
See Fig 1 - Assume that the current is 5mA and the capacitor has a value of 33uF. If the CFL or ballasts inverter was not present, the voltage would rise to the peek of supply, 340V as in Fig 1. In practice, this would be a curve, as the current would tend to reduce as the voltage approached the maximum.
If you take a point on the graph that represents the start voltage of the inverter, say 254V DC, and a point that represents the drop out voltage, say 127V DC. Then the downward slope of this line would be a measure of the operating current of the CFL or ballast. A steep slope would indicate more current than a shallow slope. In any case the voltage would drop to the dropout voltage, the inverter would shut down and the capacitor would begin to charge once more. This cycle would continue at a rate that is dependant on the start and dropout threshold values, the value of the capacitor, the current flow from the switch and even the supply voltage. Each individual start, dropout would be a flash from the lamp.
Solution: Lamp and ballast manufacturers continue to make lower and lower power devices that will work satisfactorily on lower and lower voltages. Switch and timer manufacturers make more and more complicated devices with new customer features, illumination buttons, standby supply batteries etc.
All this requires current to operate and, as previously stated, if the switch has no neutral connection, this energy can only be obtained via the load.
For electronic timers and switches to work correctly with CFLs and electronic ballasts, a method of keeping the voltage across the load as low as possible during the off state needs to be found. Some ballasts need the off state voltage to be below 10V AC to reset their internal circuitry.
In most cases the Elkay PFCC-1 4uF capacitor will bring the voltage across the load to less than the CFL's threshhold voltage and eliminate any flickering or load staying on in the off state.
To see many more Q & A in Voltimum UK's Experts Area, please click on the link:
www.voltimum.co.uk/consult.php?universe=consult.index.questions
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