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Teardown: A19 LED bulb

-January 22, 2018

My home has numerous LED bulbs, be them in recessed fixtures, hanging fixtures, and standing lamps. So far two have failed. The first was a BR30 that cycled on an off with temperature, the second was a A19 bulb that gave off far too few lumens. Both are manufactured in China, sold by Feit Electric, and purchased from Costco.

Reaching the power circuit of this bulb required destroying the base, nothing you can't do with a screwdriver and a wire cutter. Figure 1 shows how to pry off the "glass" from the heat sink.

Feit Electric failed LED bulb
Figure 1 A screwdriver separated the bulbs plastic "glass" from the heat sink.


With the glass removed, you can see 14 LEDs in Figure 2. Note how 10 of them are vertical mounted. That's how the bulb's emitted light becomes "omnidirectional."

LEDs inside an A19 bulb
Figure 2 Fourteen LEDs emit light in a more-or-less omnidirectional pattern.


With the LED board removed, the PCB was visible but not accessible (Figure 3). Using a wire cutter, I nibbled away the plastic base, which revealed the board. Note the 250 V aluminum electrolytic capacitor across the output to the LEDs. Aluminum electrolytic capacitor? They just fail. How can this LED bulb possibly last 25,000 hours? The apparent damage on the capacitor is superficial, caused when opening the bulb housing.

250 V capacitor
Figure 3 The power-supply board inside the LED uses a 250 V electrolytic capacitor across the LED drive voltage.


Before I proceeded with the teardown, I measured the voltage at the output with a DMM. The bulb's base was still intact at this point so I inserted it into a lamp fixture. The output was a surprising 234 VDC unloaded (Figure 4). There's no transformer on the board, hence no isolation of AC mains.

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Figure 4 The power board's output voltage was over 234 VDC unloaded. I didn’t expect that.


Next, it was time to completely remove the board. That meant destroying the rest of the bulb. Now, let's take a tour of the components. Figure 5 shows a 2 A, 250 V fuse across the AC line hot and neutral lines. The mustard-colored component on the left is TVR1, a 7N721K 270 V varistor.

circuit protection
Figure 5 Circuit protection starts with a 2A, 250 V fuse (F1). A 30 Ω, 1/2 W resistor (RX1) is clearly visible.


By bending the components, their markings became visible. Figure 6 shows C1, a 22 µF capacitor. The black canister at the bottom of the photo is an unmarked inductor designated L1.

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Figure 6 A 0.22 µF capacitor, designated C1.


Figure 7 shows capacitors CX1 and CX2, both of which are 0.047 µF.

capacitor
Figure 7 Capacitors CX1 and CX2 are both 0.047 µF.


Figure 8 shows Q1, a 4N60 N-channel MOSFET.

4N60 MOSFET
Figure 8 A 4N60 N-Channel MOSFET, designated as Q1.


While inductor L1 has no markings, inductor L2 is marked 821, indicating a value of 820 µH (Figure 9).

820 microHenry inductor
Figure 9 Inductor L2, 820 µH.

On the next page, we take a look at the underside of the board and the schematic diagram.


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