Saturday, December 24, 2016

LED Bunker Light Teardown

I recently picked up this LED bunker light for $33 and I thought it'd make an interesting tear down.

Bunker Light
AT5700 bunker light

When you remove the diffuser from the front the first thing you see is the PCB that holds the 32 LEDs.  Ideally you're meant to be able to loosen the screws and turn the PCB slightly and then lift it out, the only problem with that is that there isn't enough room to get your fingers down the side of the circuit board.  As it took me a couple of minutes to remove the PCB I'd suggest that in their next design iteration they route some indentations into the edge to allow you to hold onto the board.

LED PCB
Surface mount LEDs

The light is just a low power bunker light for non task illumination and is rated for outdoor and indoor use.

Specifications
Box specifications

The LED driver is mounted to the back of the PCB and seems to be of the constant current type.  It's hard to tell but I think the voltage range of 12-25 volts indicates that it will regulate the output to 350mA and vary the voltage accordingly.  When measured the LEDs were drawing 350mA at 12V for a power of 4.2W.  The box indicates a power draw of 6 watts so the difference is likely to be what gets wasted in the driver.

LED Driver
LED driver specifications

The light was chosen because of its small size, almost too small in fact.  As we'll see later, the 70mm thickness doesn't leave much space for cabling.

Dimensions
Dimensions

The connector that supplies power to the board has a cover over its soldered terminals on the front side of the panel.  I assume that this is to prevent shorting occurring and not a safety thing as the voltages on the board are low and still accessible on the sides of the surface mount LEDs.  It's a good idea but I wish they'd put the same amount of thought into the installation process as it's quite hard.  Ripping a surface mount component off a board isn't that hard to do.

LED PCB
Insulator over connector solder points

The main problem I have is that the driver is mounted to the back of the LED PCB.  During installation you have to first connect the cabling to the terminal block.  This means that you need to leave about 8 inches of extra wire so you can get a screw driver in there before putting the board into the base and if you can't shove that extra cable into a wall or wherever the light is going, it has to be curled up inside the fitting and that's hard as there is only about 30mm of space behind the board.  I think a better solution would have been to mount the driver to the base of the fixture so that the driver can be wired in first and then connect the LED PCB via the header connector to the driver.  The connector on the PCB could be also moved to the front side to allow connection after mounting the LED board.  There is plenty of room, you just bring a small cable up the gap beside the PCB and connect it on the front side.

LED PCB
Rear of the PCB as it sits upside down in the base

Curious about how the LEDs were arranged I mapped out the layout of the board.  In the image below each different section of copper track has its own colour to clearly identify them.

LED PCB
Electrical layout of PCB

This shows that the 32 LEDs are arranged in 4 serially connected groups of 8 parallel lights.  I would usually have an issue with this because there isn't any form of load balancing, but I think the lights are under-driven enough that it isn't a concern.  I'll take a minute to explain that better.  If the LED driver is generating 12 volts with a constant current of 350 mA, you would assume that each component is getting 43.75mA at 3V, but due to manufacturing tolerances it's unlikely that the 350mA will get evenly split between the 8 LEDs in each group.  You then have a situation where one will be drawing more current than the others and if there isn't enough of a safety factor in the design it will eventually fail.  When this happens, the 350 mA will now be divided between the 7 remaining lights and another will fail this will continue until an entire group fails.  The reason that I'm not too concerned about this is that I think the packages are 2835 LEDs and they can usually handle more than the (0.04375 x 3) 131.25 mW per device.  The forward voltage of a white LED is usually considered to be above 3V.  This is what makes me think that to get the reliability they wanted, all they had to do was add extra LEDs to split the current and under drive each chip.  You would probably find that if there were only 7 LEDs in each group the higher current would cause the forward voltage to increase and instead of regulating at 12V, the driver may go to 13V.

Schematic
LED schematic

It's not too bad for $33 but I think the design could be improved with only some minor adjustments.

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