insides of the led fixture, a multimeter

El-cheapo China DMX LED Spots

I recently got me a set of 4 DMX-Controllable LED Lights of ebay for 35€ including shipping.

ebay-listing picture
ebay-listing picture

But they are rather dimm. The article page states

Power Consumption: 15W
LED: 12 pcs * 1W LED (Red*3, Green*3, Blue*3, White*3)

So, lets open one up and check the insides.insides of the led fixture, a multimeter

When setting all LEDs to maximum (RGBW) the multimeter reads 185mA DC. At just 12V thats not barely enough. Interestingly though, setting it to AC+DC mode reveals a stunning consumption of 340mA, thats almost 4Watt.

But wait, why is there even AC voltage when everything is turned up to full brightness?

Scope to the rescue!

I put a 1Ohm resistor in series to measure current, so 1mV -> 1mA. The voltage across the LEDs is always 12V, as output by the PSU.

scope showing the current waveform for the blue led

You can see 478us PWM-Period with a on-time of 126us. Thats a DutyCycle of 26% at 100% brightness. That explains the AC current measured earlier. The peak current is ~275mA.

  • Red: 780mA, DutyCycle 17us/318us -> 0.5Watt RMS
  • Green: 350mA, DutyCycle 108us/480us -> 0.95 Watt RMS
  • Blue: 275mA, DutyCycle 126us/480us -> 0.86 Watt RMS
  • White: 487mA, DutyCycle 98us/480us -> 1.19 Watt RMS

This roughly correlates with the 4Watts measured with the multimeter.

But what about the overall quality? I guess you get what you pay for.

  • The LED PCB is made out of aluminium and is a decent heatsink. But it is rather thin. The LED-lenses are as described and work properly.
  • Plastic case seems nice. No problems here
  • The Control-Knobs are easy to use, the 7-Segment-Display shuts off after a few seconds. Premium feature!
  • No gamma adjustment. 50% Brightness equals 50% (of max) DutyCycle.
  • The DMX In- Out-Port are just connected to each other. No RS485-driver to improve signal quality
  • The internal plugs are JST-XH (clones?), but they saved a few cents by just populating pins instead of complete receptables on the controller board.
  • The PWM-Base-Frequency of red is different than the other colors. WTF?
  • Since no DC is > 25% one would expect that the PWM-Periods are interleaved so the current does not add up. But nope, thats not the case.
  • No resistors in front of the LEDs. So the 3 red LEDs which have a combined Vf lower than 9V are run at the full 12V.
  • There is no sign of any PFC on the primary side of the PSU.
  • The PSUs isolation between primary and secondary is garbage. The feedback-opto has all its 4 terminals pretty close to the primary side. Thats less than 1mm.

Conclusion – Next steps

So this is not as bright as I want it to be. And the PSU looks dangerous. So the next logical step is to fix that. I used a lab power-supply to check if each LED actually can handle 1Watt. The answer is yes, but then it gets hot rather quickly. So some sort of cooling is required.

I think the mod will be made with series-resistors in front of the LEDs so they can run of 12V DC. Then all 4 fixtures controlled by one KaratLight Device. Not sure about cooling yet, running this at 12Watt will be to much heat for the aluminium PCB to handle.

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