Intro: Troubleshooting a Cheap Imported LED Fixture
As an engineer that works mainly in LED lighting, I'm always getting samples of new products at a bunch of different price points and with wildly varying quality. Against my better judgement I took a chance on some sub-$40 18-watt RGB LED fixtures from Amazon a few weeks ago to try and find a low cost fixture that still had DMX input and had a regular 120VAC plug.
From the moment the Amazon box showed up labelled "Super wonderful Par LEDlighting,party essential", I knew that it was going to be *far* from wonderful and I was going to be in for some debugging. Alas, I plug it in for the first time and... nothing. No blinks, beeps, anything.
Here is a short step-by-step on how to teardown and troubleshoot cheap imported goods. Obviously this can't be a comprehensive guide, but highlights a few important things you should look for when it just won't power up.
Step 1: Tear It Apart, But Carefully
This fixture came apart in just a few pieces. Usually there will be some sort of beauty ring or facia followed by beefier screws that actual hold together whatever you are taking apart. Glue is often plentifully used in these imported plastic goods so you may have to do a little prying.
Make sure you keep all the different kinds of screws separate; if it looks like there are going to be a bunch of them it doesn't hurt to label them in steps as you take your item apart.
Step 2: Start Checking Things Out With Visual Observation
CAUTION: Things that plug in to the wall likely have exposed areas sitting at 120VAC which you definitely don't want to touch, so use caution when poking screwdrivers, probes, etc into a live fixture.
OK! Now we have a good view of the guts of this light. Very standard for an LED PAR can, there is:
- DC Power Supply board (top one, non-soldermasked PCB)
- Logic Board with constant current LED supplies (green PCB in middle)
- Button/display satellite board (on right)
- LED carrier board (under the lens assembly on left; just arrays of SMD LEDs in series)
You'll want to spend a few minutes evaluating what you're dealing with anytime you take a piece of electronics apart. Quickly identify things that could be or indicate a problem. Common things to look for are leaking or blown electrolytic caps, burn marks under resistors, burned wires or burned up PCB traces, blown fuses, condensation, critters living inside, etc.
Given the topology of this light, a logical start would be to make sure that there is appropriate power in the right places on the board.
Step 3: Check the DC Power Supply to See If It Is Borked
As this light has an AC/DC power supply on a separate board, I'm going to make sure that is working first. There isn't too much to this one- a transformer, switching IC, a FET, and some rectifier diodes, so let's just start by checking for an output voltage.
Finding the DC input pads usually just means tracing thin wires from the power supply board to the logic board. Most LED par cans are built like this, as they'll use the power supply PCB in many different models. Often the wires will be color colded RED/BLACK, but in this case they're just the black wires coming off of the left hand side.
The silkscreen on a logic board can often give you hints about what voltages live where. In this case, there is a silkscreen outline for a connector that was no-loaded so the wires could just be soldered on, and the pads are easy to hit with the multimeter. Carefully positioning the multimeter probes, we see 0VDC on the pads where there should be around 24V, given the way they wired the LEDs on this light.
Step 4: Snap, No DC. Well, Is There Even AC Hitting This Board?
So it is possible that the DC converter is fried, but this one is pretty darn simple so that's unlikely. Maybe it just isn't getting power.
Usually these power supply PCBs will have a fuse (occasionally a PPTC resettable fuse). Look for that near where the AC power hits the board and make sure it isn't blown (visual inspection or check continuity with multimeter). In this case, the fuse is fine.
Next I plugged the unit back in to sanity check that we are actually getting 120VAC at the input. And guess what- nothing. (well 1.7V according to the meter, but that can be ignored)
Step 5: Could It Possibly Be the Plug? Yup.
Now this was a long shot, but checking continuity on the neutral and hot legs from the plug to the board showed a break in the hot line. Now that could be a break in the wire somewhere in that 6ft or so, or a bad spot weld to the actual blade of the plug.
In this case, I ripped the plastic molding apart to reveal that the wires are stupidly thin and have no insulation where they meet the blades. Something must have happened in shooting the over molding onto the plug which broke the connection.
Step 6: Ok, So We'll Just Put a New Plug on It.
I cut off the old defective plug and installed a new plastic/vinyl grounded plug from my local hardware store. Note that the wiring the manufacturer chose for this light is *insanely* thin and difficult to get landed under the screws. Must have been 24 gauge or so.
Especially in this case where the wires are less-than-beefy, you'll want to make sure you use the strain relief clamps that are part of the plug assembly. These will keep your wires from getting yanked out if someone tries to remove the plug from a socket by pulling on the wire instead of the plug housing.
Step 7: Test It, and Put It Back Together!
You'll definitely want to test whatever fixes you make before putting the whole thing back together. Just don't go poking around in there while it's powered up. It works!
Notice that I always seem to have one screw leftover. This is the sign of a true master, and demonstrates how I could have helped cost reduce this thing even further.