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Bad Fluorescent Night
Light March 30, 2010 |
The wife knows that I’m constantly searching for that perfect night light design.
When she saw this blister pack with two compact fluorescent night lights, she knew
they had my name on them. I really liked the style and size. The
package said that they consumed only 1.6 watts, which was 40% of the typical 4
watt incandescent night light style lamp. Being fluorescent, they should
also produce more light and last longer than the typical 6 months I usually got
from the standard incandescent 4 watt lamps. |
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| I marked the
installed date on the side of the lamps with a pen and then plugged one into the
laundry room, where it would run 24/7. I plugged another lamp in the dark
upstairs hallway, where it would also run day and night. In only two months both
units died within days of each other with the telltale smell of burned electronics.
I cracked open the assembly to have a look. Sure enough, there was a burned resistor.
Clearly, its power dissipation value had been exceeded, which eventually caused the
failure. |
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I traced out the circuit.
The schematic is shown below. Note that the resistor R6 is 100 ohms. It
appeared to be only a ¼ watt size. To dissipate 250mw of power the average
current would only have to be 50ma. There are two 0.68uF capacitors in the
circuit, which are supposed to limit the average current. The capacitance
reactance of a 0.68uf cap is 3.9K at 60Hz. In theory, with a 120vac line, the
average current should only be about 30ma max. But, the voltage rating of the
part is only 160v. The peak of a 120vac line is closer to 170v. So, I
suspect that the capacitor was breaking down, causing current spikes to flow through
the poor 100 ohm resistor, which over time, finally killed the part. I think the
100 ohm resistor should be increased in size to at least a ½ watt and those two
capacitors should have had a 250v rating. After those change, perhaps the lamp
would last longer than just 2 months. Yet another bad Chinese product. |
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| 1.6W
Fluorescent Night Light Circuit Board |
Burned ¼
watt 100 Ohm Resistor |
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