Electronic ballasts used in fluorescent lighting systems can experience high failure rates, resulting from end-of-life (EOL) issues. The IEC 61347-2-3 standard specifies three tests to simulate the effect of the lamp’s end of life: the asymmetric pulse test (Clause 17.2); asymmetric power dissipation test (Clause 17.3); and the open filament test (Clause 17.4). Any one of these three tests can be used to prove the eligibility of the electronic ballast. At the end of the lamp’s life, the standard requires that the ballast shall not make the lamp holder overheat at any voltage between 90 percent and 110 percent of the rated supply voltage.
One of the protective measures recommended for electronic ballasts is to connect a resettable PPTC device in the resonant circuit, in series with a resonant capacitor to achieve abnormal-state protection. With this solution, when the fluorescent tube is in a normal state and the electronic ballast is powered on, the inductor, capacitor, and PPTC thermistor form a series-resonant circuit to make the fluorescent lamp function normally. At this point, the low resistance of the PPTC device will not affect the resonant circuit. If an abnormal state occurs due to deactivation of the filament caused by an aging filament or tube leakage, the PPTC device will activate within seconds.
A typical circuit utilizing a PPTC device for protection of IC integrated ballasts.
The figure shows a protection scheme utilizing a PPTC device in a T5 14W electronic ballast with integrated ICs. The PPTC device is connected in the LC series resonant circuit, shown as R1. During normal operation, the resistance of the PPTC is very low and does not affect the normal operating resonant state. If an EOL abnormal state should occur, the current flowing through the resonant circuit will increase to approximately five to six times the normal operating current, causing the PPTC device to trip and device resistance to increase.
The resonant circuit formed by the PPTC device’s increased resistance will cause the inductor and capacitor to change resonance, and the IC will change the resonant frequency of the ballast. The increase of the resonant frequency of the ballast will reduce the power of the ballast, thereby reducing the current flowing through the filament and providing the required ballast protection. After the fault has been removed and the power is cycled, the resistance of the PPTC device will return to its original state and the ballast can return to normal operation.
When specifying the resistance of a PPTC device in these applications, the maximum operating temperature and maximum operating current must first be obtained. Next, the appropriate PPTC device can be identified by using the temperature derating curve, and, finally, it must be determined that the PPTC device complies with the standard for the shortest time required for protection.
The ballast referenced is a T5 14W circuit built for the linear tube fluorescents in commercial and shop applications. They have end-of-life issues different from CFLs
Though related, I think this circuit is chasing problem of low population as I understand new fluorescent ballasts have requirements to be more EMI and otherwise future modulation distribution compatible. Hopefully, this problem is addressed by smarter self-protection circuits.
The takeaway should be that as engineers with low wattage circuits we need to understand that PWM and boost power supplies can create fire hazards that need to be anticipated early in design.
However, their own refutation text is cause for alarm:
CFLs (compact fluorescent lamps) don't burn out the way incandescent light bulbs do. Instead, as they near the ends of their lives, they grow dimmer. While some CFL bulbs merely stop emitting light when they finally quit working, others kick the bucket with a dramatic "pop"! sound and then vent a distinct odor. A few even release a bit of smoke at their termination. Sometimes the bases of the bulbs turn black. This seemingly cataclysmic reaction has to do with the breakdown of the bulb's ballast, which is contained in the part of the bulb that is screwed into the socket. As the bulb ages and degrades, so does its ballast. Yet as scary as odors, smoke, and even blackening of the base of the bulb might be, these lamps are fireproof and are meant to fail safely at the end of their lives.
An incandescent lightbulb does not turn black, does not emit smoke or an odor when it fails naturally.
How is this better than an incandescent bulb? Maybe the technology described in this article prevents the failure symptoms listed above. I hope so; it might improve the image of the bulbs.
The alleged cost savings aren't there. The light given isn't as bright. They fail in a messy and annoying manner. I forget why they're being forced on us.
TJ, I had not heard that. My experience with CFLs is that they do not last nearly as long as rated. I expect that the reason is that they are cycled far more often than they are designed for. We do that with incandescant bulbs and think nothing of it. Might this overvoltage protection approach help with that problem?
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