A couple of weeks ago, we reviewed the Heetech Low Voltage 12 – 85V DC/AC E27 Light Bulb. While it had a large input range of 12 to 85V, the luminous output started to dim as the voltage fell below 13.0V. If operated on a 12V battery, the reduction in light intensity was noticeable as the battery’s terminal voltage reduced throughout the night. It was also advertised and marked as 9W, but only consumed 7 to 8 watts.
At that stage, it was time to try a different product and see if we could find anything better. I ordered two Gatetop branded low voltage bulbs from Aliexpress (Nikkie’s Light Store). This time I ordered Natural 4000K, and assuming the output will be less than advertised, I opted for the 10W version. The listing suggested Luminous Intensity was between 80 and 100lm/W, thus 800 to 1000lm for the 10W version.
Testing
This bulb is physically smaller than the Heetech. This one measured 110mm high and 60mm wide. Printed on the base is “A60 10W 1000lm E27 AC/DC12-48V 50-60Hz”, confirming it is in fact a 10W version.
When connected to a bench power supply my two bulbs consumed the following.
Bulb Sample 1:
- 720mA @ 12.0V (Approx 8.64W)
- 340mA @ 24.0V (Approx 8.16W)
- 170mA @ 48.0V (Approx 8.16W
Bulb Sample 2:
- 730mA @ 12.0V (Approx 8.76W)
- 330mA @ 24.0V (Approx 7.92W)
- 170mA @ 48.0V (Approx 8.16W)
The marginally lower power consumption than stamped on the nameplate appears to be common with these bulbs and was expected.
Teardown
The bulb diffuser cover is glued down on this product, but with a bit of force, you can pop the top off without causing any damage.
The PCB is well designed for manufacture and hence cost. Both the driver and LEDs are included on a single aluminum clad PCB. This board has SMD mount wire spring clamp terminals that are loaded and soldered as part of the pick and place surface mount process and hence no further soldering of wires are required later in the production cycle.
The driver is based on a BP1808 DC/DC constant current LED driver from Bright Power Semi (Chinese manufacturer based in Shanghai). An English version of the datasheet can be found here.
The BP1808 supports a wide input voltage range of 3 to 60V (Absolute maximum 70V). The input cap is a 200uF 100V Electrolytic, branded H N, series CD11G.
The input bridge rectifier is made up of four discrete SS210 1.5A 100V Schottky Diodes. The power loss in the bridge rectifier is more significant when running at lower voltages than would be if operating at mains. When the bulb is operating at 12V (~720mA), the forward voltage drop measured across the diode was approximately 700mV. With two diodes in the bridge operational at a time, this means there is a 1.4V drop over the bridge. This dissipates approximately 1.0W.
The BP1808 is configured in buck mode and the circuit used is very similar to that of the reference circuit from BPS:
The BP1808 obtains its power from the VOUT pin. An internal regulator provides a 4.9V output on VDD.
To obtain the desired LED constant current, two high side shunt resistors have been placed in parallel. RS1 marked ‘R300’ is 300mΩ. RS2 marked ‘1R00’ is 1Ω. In parallel, this gives 230mΩ.
According to the data sheet the device maintains a 200mV difference across the shunt, hence with a 230mΩ shunt, should provide a constant current source of 870mA (I=V/R).
The dimming function (pin 3) or the Over Voltage Protection (pin 1) is not used. OVP is applicable to boost mode and protects the converter in the event the LED string goes open circuit.
LED Configuration
The board contains twelve 2835 LEDs (2.8mm x 3.5mm) all wired in parallel. When operating, I measured the forward voltage drop at 8.9V. Due to the single PCB design (no separate driver), it is difficult to measure the actual current being delivered into the LEDs, but assuming the current shunt resistor sets up an accurate 870mA, we can assume the LED array is drawing approximately 7.75W.
When bench tested, it appears the lamp begins to reduce brightness at around 11.2V (measured at the input to the PCB). The 8.9V LED forward voltage, 0.2V lost in the shunt and 1.4V drop across the bridge rectifier and losses in the converter (switch) suggest this is about the ball park.
Conclusion
This Gatetop branded low voltage bulb appears to be well designed and I’ll be purchasing a couple more units.
It is more suitable for the lower 12V or 24V battery banks, as there is insufficient safety margin to operate it from a 48V LiFePO4 battery than could see the output voltage reach 57.6 when fully charged. The Heetech appear to be better rated for 48V use.
If operated from a 12V battery, this bulb should perform better than the Heetech as the battery starts to go flat and the terminal voltage reduces.
As with the Heetech review, I don’t have the required equipment (integrating sphere) to accurately measure the luminous flux (lumens) to verify output.
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