Dim, flickering LED lamp: wrong dropper capacitor and missing bleed resistor
An NSQF batch assembles 15 identical LED lamps: 12 white LEDs in series (3.2 V each, 20 mA), a 0.33 µF/400 V metallised polyester dropper capacitor, a 100 Ω series resistor, a 1N4007 bridge, a 470 µF/63 V reservoir, a 1 MΩ bleed resistor across the dropper, all in a sealed plastic box fed from a two-pin 230 V plug. Design current is about 20 mA. One unit comes back from a staff room with the complaint that it is dim, strobes at switch-on, and glows faintly for a second after switch-off. On opening it, the trainee finds a 0.22 µF/400 V capacitor fitted and the 1 MΩ resistor position empty. Mains is 230 V, 50 Hz. All LEDs test good on the diode range, and the 470 µF reservoir reads 455 µF.
1. Calculate the design current and the current with the fitted capacitor. Does the capacitor alone explain the dimness, and does the 455 µF reading matter?
Read the worked answer
Design: Xc = 1/(2π × 50 × 0.33 × 10⁻⁶) ≈ 9.65 kΩ. Current ≈ (230 − 38.4)/9.65 kΩ ≈ 19.9 mA - about the intended 20 mA.
As fitted: Xc = 1/(2π × 50 × 0.22 × 10⁻⁶) ≈ 14.5 kΩ, so current ≈ (230 − 38.4)/14.5 kΩ ≈ 13.2 mA. That is only 66% of design, which directly explains the dim output; the LED string also sits closer to its knee, so small mains variations make it flicker.
The 455 µF reading is −3%, well inside normal electrolytic tolerance, and the reservoir does not set the LED current - the dropper does. The capacitor is therefore the primary fault, but the missing bleed resistor must also be corrected.
2. Explain the function of the 1 MΩ bleed resistor and the consequences of its omission in this unit.
Read the worked answer
The dropper capacitor is in series with the mains and stores charge when the lamp is unplugged. The 1 MΩ resistor gives that charge a deliberate discharge path: τ = 1 MΩ × 0.22 µF ≈ 0.22 s, so the capacitor falls to a safe voltage within a couple of seconds.
Without it, the capacitor can hold a peak of about 325 V for minutes. The stored energy trickles through the LED string and reservoir, which appears as the faint afterglow and the strobing at switch-on (the reservoir charges slowly through a high impedance until the string reaches its knee, then discharges). The real issue is safety: anyone touching the plug pins or the dropper leads after removal can receive a shock, and the plastic enclosure gives no warning. Never rely on the LEDs to discharge the dropper.
3. State the correct repair and the checks you would make before returning the lamp, including the component ratings that must not be substituted.
Read the worked answer
Replace the dropper with the specified 0.33 µF capacitor of the correct class - an X2-rated 275 V AC part or a 630 V DC metallised polypropylene - and fit a 1 MΩ, 0.5 W resistor across it. Do not fit a low-voltage electrolytic or a 50 V part: it will see close to 325 V peak and fail short, putting the full mains across the LEDs and the user.
Then verify: current in series with a true-RMS meter should be ≈20 mA; the lamp should light steadily with no visible flicker; after 10 minutes the enclosure should be only warm; insulation resistance from the mains pins to the outside should be high on a 500 V megger; and after unplugging, the dropper should discharge below 50 V within a few seconds. Check the 100 Ω resistor and bridge for overheating, and label the unit so the batch can be inspected for the same assembly error.