Introduction: Fixing and Improving a Night Light
Today on the healing bench we have this small night lamp that belongs to my daughter. It no longer works so we will try to fix it and also make it better as it has a terrible flicker.
This repair deals with mains voltage. If mishandled, mains electricity can hurt you or even possibly kill you. Make sure to always be careful and have the device unplugged at all times while working on it.
This lamp is really cheap, but because of the cute application on the front, my daughter loves it. However, it has this awful flicker that drives me crazy and I wanted to fix that for quite some time. Now that it no longer works at all, I have to fix it.
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Step 1: Open the Case
The case is made from translucent plastic that is molded in two parts. The cover has the application printed and it is held in place by three screws that are accessible from the back. To remove them I’ve used a Philips head screwdriver that can fit into the recessed holes.
Step 2: Diagnose the Issue
Once the three of them are removed, I separated the two covers to expose the PCB inside and by just looking at it I was able to pinpoint the issue. On one of the pins that go into the outlet, the wire came loose and it no longer makes electrical contact.
This can happen from a bad solder joint that had released either by thermal expansion or by some mechanical force if the lamp was tossed to the ground. To fix it, I’ve turned on my soldering iron and removed the single screw holding the PCB in place to gain better access to the loose wire.
Step 3: Repair the Light
I first heated the pin with the soldering iron and applied a bit of solder to it. I’ve also added some fresh solder to the exposed wire and by some fiddling around I pressed both together to join the solder and establish a good solder joint. This wasn’t easy, because of the small space and its weird position inside but I managed to do it.
Now, with the wire fixed, I moved the wires back in place behind the PCB and secured it with its screw. To check if the repair worked, I’ve plugged it to the outlet.
Step 4: Fix the Light Flicker
As you can see, the repair worked but it now exposed that horrible flicker that this lamp always had. It looks even worse on camera but it is definitely visible for the eyes as well.
The cause of this flicker is the absence of any smoothing of the voltage after the bridge rectifier. Its output is connected directly to the LEDs thus turning them off on every half cycle of the mains sine wave.
The circuit of the lamp is quite simple where a dropper capacitor with a discharge resistor is connected on one side of the bridge rectifier and on the other it is directly connected to the other side of the mains voltage. The output of the rectifier has a resistor in series and the three LEDs are also connected in series with that resistor.
To smooth the voltage, I’ll add a capacitor to the output of the rectifier and the only rule that we need to really follow here is that it is rated for the voltage that is produced. So I measured the output voltage of the rectifier and it was close to 12V so I got one 25V, 100 microfarads capacitor to add to the circuit. The capacitance here is not really important but the rule of thumb is that the higher it is, the smoother the voltage will be.
To solder it in place, I first removed the light from the mains socket and added a bit of fresh solder to the output pads on the rectifier as well as the legs of the capacitor that I’ve trimmed to length. It is important to note the capacitor and the rectifier polarity so the negative side of the capacitor is soldered to the negative output of the rectifier. If these two are mixed, the capacitor can explode so be really careful.
With both of the legs soldered directly to the rectifier output, it was time to test the light again. As expected, the light output is much nicer now without any visible flicker even for the camera.
Step 5: Enjoy a Much Better Night Light
Being happy with the repair, I returned the cover to the lamp and secured it with the three screws from the back officially declaring the lamp as fixed.
If I turn off the fixed exposure on the camera, it can still detect a slight shimmer in the output of the light but this is not detectable by our eyes and it looks a lot more pleasant now.
With that, I hope that this Instructable was educational for you and that you managed to learn something. If that is true than hit the like button, make sure to subscribe to my YouTube channel and I'll see you in the next one.
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