Thursday, March 23, 2017

Dance, Dance...ZOT! Fixing a Dangerous Disco Light

Having Fun Shouldn't Be Deadly, But It Easily Could Have Been...

The Shocking Story of the FX LAB Disco Light

The other day a friend told me he had a light that shocked him when he touched it.  Intensely curious, I asked him if I could take a look at it.  He was more than happy to pass it to me, and I spent a little time with it this afternoon figuring out why it was unsafe.  The short answer is that the light was badly designed - and cheaply manufactured.  This combination resulted in a device that leaked 230VAC to its external enclosure, at up to 10VA.  This is more than enough power to be lethal

Here's a shot of the rear of the unit, showing its brand name, working voltage and so forth:

Ocular Inspection

An external inspection of the unit revealed very little.  The power cord and plug were undamaged.  There were no obvious cracks, splits or other signs of damage to the body of the unit.  

"Cold" Power Check

I next performed a "cold" power check on the device, which means I checked it out while it was unplugged.  Using my beloved Fluke 12B Multimeter, I checked for continuity between the body of the device (which had been the shock vector) and the pins of the power plug.  Plugs in Hong Kong look like this:

They are wired like this:

Power in Hong Kong is color codedThese days, we follow the "New Cable Color Code":


  • There was zero resistance between the body of the device and ground (#3)
  • There was infinite resistance between the body of the device and line (#4, #5)
  • There was infinite resistance between the body of the device and neutral (#2)

These results are consistent with a normally functioning device.

"Hot" Power Check

My next move was to take the disco light apart.  In terms of the powered components of the unit, I discovered that it was composed of two modules connected by a wiring harness:

  • A POWER module
  • A LIGHTING module

Securing the unit safely on a level, non-conductive surface, I very carefully plugged it into a switched power bar outlet (with the outlet turned OFF).  I made sure that the unit was not in contact with me or anything conductive, and turned it on. 

Here's what I saw:

Once again, the unit appeared to be functioning normally.  I tested various points around the unit for voltage leaks by placing the black lead of my multimeter on the grounding bolt and probing around the unit with the red lead.  While doing so, I detected 0V at the steel base of the POWER module, 3V at the steel base of the LIGHTING module, and 12V at the back of the motor, which is responsible for an "extra" disco effect.  While detecting any leaking voltage is undesirable, what I found was insufficient to cause the kind of shock my friend described.

Mechanical Inspection

Increasingly puzzled by this situation, I took a closer look at how the unit was mechanically constructed.  

  • The first thing I noticed was that the LIGHTING module fit into the body of the device by friction alone; it slid under two flanges.  It could therefore be accidentally displaced within the device by a physical blow or by being oriented at too steep an angle.
  • The second thing I noticed that the front plate of the lighting module was supposed to be spot welded to a front plate that held a glass lens.  At some point, the welds had failed.

Mechanical Failure + Design Flaw = Death?

This led me to an interesting conclusion.  The hypothesis I came to was that my friend had been shocked because the LIGHTING module had slid too far back into the device, coming into contact with the transformer mounted on the POWER module.  This would have conducted 230VAC through the back of the motor (which was metal) through to the external shell of the device (which was metal). 

The Solution

Once I knew what was wrong, I knew what I needed to do to make this unit safe again.  I took a plastic lid (which I don't need) from one of the containers I use to organize my work bench and started cutting it up:

 I hot glued a circular piece of plastic to the back of the motor, isolating it electrically.

Then, I traced out a larger circle on the plastic lid...

...and hot glued it to the POWER module, leaving a notch for the wires that connect it to the LIGHTING module:

At this point, the unit was very well isolated electrically, but I still wanted to do something about the mechanical failure.  

So what I did was put the unit together in such a way that the connecting flange was on the outside of the unit rather than in its normal position, which is inside.

I then drilled a hole through both the flange and the face plate:

I then I took the unit apart once more.  I colored the flange black with a marker, partly to prevent rust, partly to integrate the flange (which was now somewhat exposed) visually with the color of the rest of the unit.  It wasn't really necessary, but why not?

Finally, I re-assembled the unit with everything in the correct order, meaning the flange was now inside.  I re-united the the LIGHTING module flange with the face plate using a nut and bolt - that type of mechanical connection will not fail any time soon!

Final Testing

I tested the unit once again for voltage leaks.  There were none.   It seemed safe.  I also took the disco light to my balcony and shined it on a large tree about 30 feet away. To my delight and satisfaction, the disco light seemed to work just fine: 

Party On!

Based on my testing, the FX LAB disco light appeared to be both safe and 100% fully functional. 

It was ready to be put back into service once again.



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