LED lamps are readily obtainable direct from China, but how safe, effective and reliable are they? Can you believe the published specifications?
I have been testing a quantity of the 230V 6W B22 5050 LED type lamps and G9 type lamps which come in cool white (c. 5000°K-6000°K) and warm white (c. 3000°K) colour temperatures. One thing to note is that the warm white versions are not quite as efficient as the cool white ones, but the 6000°K ones can be a little too harsh for comfort.
Firstly, are they electrically safe?
No. If they have a cover on they are safe from a shock point of view, although a cover increases the operating temperature. If they don't have a cover on then some of the exposed soldered LED joints are at mains potential due to the unisolated nature of the simple capacitive coupled mains dropper/rectifier. Touching the soldered joints on the LEDs could result in a full mains voltage electric shock. The ones I have been evaluating have no input fuse protection or fusible resistor so there is nothing to limit the current on a serious fault. That could result in a fire.
Secondly, can you believe the specifications?
Definitely not. The measured DC input power to the series connected LED stack is much less than the published figure. When you see nonsensical specifications. like "Light Wattage: Max. 3W Electricity Wattage: Max. 6W" from the Chinese retailer then you know there is something seriously wrong. Even the best LEDs only approach 15% efficiency at converting electrical energy into light energy in the visible spectrum, (incandescent are less than 3% efficient). LEDs have some way yet to go. No way are they 10 times as efficient as halogens as some of the Chinese retailers would have you believe. These Chinese ones are roughly on a par with compact fluorescents for efficiency, i.e. about 5 times more efficient than ordinary incandescents. They could be more efficient if they weren't so overdriven, which brings me to:
Thirdly, are they reliable?
Once again, NO! Three of the four B22 21 5050 LED "6W" bulbs failed within three months usage at about 5 hours per day. Failure was due to one LED going open circuit and the rest of the LED boards showed charring evidence of overheating. When you see this silly statement:- "To ensure the using life of the bulb, we suggest that do not keep the bulb continue working for more than 16 hours" from the Chinese retailer it should alert you to the fact that something is wrong. If the published figure of 6W mains input power was correct, (the loss in the simple capacitive coupled rectifier system is negligible), then each of the 21 LEDs is dissipating roughly 6/21 watts = 285mW apportioned as c.14% light and the rest as heat which is way over the 5050 LED rating, especially in the environment where they are used, i.e. they are way overdriven and can never last very long.
Evidence of overheating:
The boards were removed from a lamp where two 5050 LED failures had occurred. The fibreglass pcb has clearly been running pretty hot as it is showing marked 'charring'.
In the latest 22 LED batch of lamps that I have received the manufacturer has reduced the value of the 'dropper capacitor' C1 from 0.82 uF to 0.68uF which has reduced the single LED chip dissipation to just over 100mW, but it is still rather high considering the operational environment. This has resulted in a measured lamp LED power of only 2.3W, (VStack=64V, IStack=36mA at 240V AC mains), which is less than half of the supposed 6W which is clearly bogus.
I produce similar genuine 5W versions with 84 5050 LEDs, (IStack of 21mA @ a VStack of 240V DC), which gives a single 5050 chip dissipation of less than 60mW and better efficiency as there is less LED 'droop'.
Construction of Modified Version
Note the ventilation holes in the top of the cover. The existing ventilation holes in the base can also be drilled out to match. The LEDs are assembled into lengths of seven by sticking them face down, pins-to-pins,
observing polarity, on a strip of double sided tape stuck to a suitable work surface and soldering them together. The strips of seven are then removed from the double sided tape and glued to the lamp body using thermosetting epoxy adhesive, (e.g. Araldite), and all connected in a series stack making 84 chips in total, (12x7), for a 230V AC system. The dropper capacitor, C1 needs to be increased to c. 1.2uF, (class X 400V), to give a stack current of c. 20mA for a 5W lamp. A fusible resistor should be added to the PCB in series with one supply leg as detailed in the modified lamp circuit below.
Generic Chinese LED Lamp Circuit
Note two safety issues:
1) There is no input fusing, i.e. neither a fuse nor fusible resistor.
2) In an uncovered lamp the soldered LED joints are accessible. Between point y, (which could be mains live with a bayonet connector) and point x there is only a diode, (D4), so point x can be at full mains peak potential with respect to earth. That is clearly an electric shock hazard.
The circuit can be modified to introduce a fusible resistor to fulfil the function of R3 and also provide circuit protection as follows:
Modified Lamp Circuit
The new resistor R3 must be a fusible metal film type, typically 10-47 Ohms 2W would be suitable. Note: This simple type of driver is only suitable for a covered lamp as one end of the LED stack is at mains potential. R1 and R2 are optional and can be omitted without affecting the operation of the circuit, (they were present in the original Chinese design & have just been left in situ).
Many of the cheap Chinese LED lamps using 5050 LED chips that are available are not electrically safe if they are uncovered or unfused and their specifications are exaggerated. They are also not reliable or even particularly effective as they use too few chips, (21/22 for the B22 230V 6W versions I have been evaluating), and they are overdriven which means that the chips run too hot, suffer from LED 'droop'
and do not last long, especially as the lamp covers do not have ventilation holes. A small number of chips also means that the lamp power factor is very low. A roughly optimum number of chips with the above simple driver is Vmains(RMS)/3, (where 3 is the nominal single LED voltage drop), thus allowing a 'charge pump' operation towards Vpeak to operate the LEDs. Stack current should be limited to c. 20mA if the LEDs are going to last a reasonable time.
Increasing the number of chips to c.70-80, (230V system), reduces the dissipation per chip, increases efficiency by reducing LED 'droop' and increases the lamp power factor. The lamps
have to be covered to be safe, (many are not covered), but it is possible to retain safety and also improve ventilation by drilling small ventilation holes in the top of the covers and the lamp base to allow a through air flow.
DISCLAIMER AND WARNING - mains electricity is potentially dangerous. Do not interfere with these lamps unless you are qualified to do so and fully understand the principles involved in their design and construction.
Any modifications are done at your own risk - no liability can be accepted for any event that occurs as a result of such work. A suitable fuse or fusible resistor
must be fitted in one of the lamp supply leads.
Beware, one end of the LED stack can still be at full mains voltage - do not touch an uncovered lamp with the power on.
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© bobbybear2 2013
LED Lamps using 5050 LED chips
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9 February 2014
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