Struggle to fix a 36W dual channel LED driver

In summary, the conversation is about a malfunctioning LED lamp that uses a smart driver with an LCD display and timer. The user has tested the LED bulbs and found one faulty, replaced it, and checked the diodes, finding one faulty. They replaced it with a 1N5408 diode from a boost DC-DC converter for LED application. However, the LED light is still weak and the user is seeking help to troubleshoot the issue. They are also waiting for a desoldering hot air station and have purchased spare MOSFETs. The user is unsure about the role of the inductor and two zener diodes in the circuit and is seeking suggestions to further diagnose the problem.
  • #1
Good morning and my compliment for their infinite knowledge contained in this forum.

I'm struggling to keep being my girlfriend's hero.

I have here a nail led 36w UV led lamp that failed last week.

Bulbs are tested all ok and connected in two channels each channel in series of 9 LEDs of 2w

Driver is a smart one, it includes a LCD display and a timer that can be setted for auto shutdown after lit up 5sec 20 sec or 30 sec (each time have a separate button to activate)

To trigger the light there is a photocell like sensor which leads i measured during power on are around 5v.

At triac i find 12v.

The main power supply of the board is DC 12v.

All powers up fine but when i blind the photocell it try to lit up LEDs with no luck (i just see a short message on LCD ). If i try second time neither the message come up i need to unplug and plug again power supply to replicate the behaviour.

Change the two capacitors didn't help . Did today.

I inspected the board again and i found a 1200 smd resistor slightly browned. Unsoldered it and measured firm on 120ohm (r14)
The tracks beneath it looks well warmed up but i checked continuity and are ok , and no shorts are found.
Picture attached
I'm lost...


  • 20170712_001756.jpg
    57.2 KB · Views: 497
  • 20170711_233654.jpg
    67.9 KB · Views: 483
Last edited by a moderator:
Engineering news on
  • #2
Welcome to the PF. :smile:
Marcolongt said:
Bulbs are tested all ok and connected in two channels each channel in series of 9 LEDs of 2w
How did you test the LEDs? Did you use the diode check setting on your DVM to check the forward voltage under small current bias, and found around 2V? What color are these LEDs? Do you have access to them without any built-in resistors?
Marcolongt said:
At triac i find 12v.
What triac? Triacs usually are part of AC Mains circuits, not low-voltage 12V LED driver circuits.
Marcolongt said:
All powers up fine but when i blind the photocell it try to lit up LEDs with no luck (i just see a short message on LCD ).
Did you use your DVM to measure the overall voltage driven to the LED strings when this happens?
Marcolongt said:
Change the two capacitors didn't help
What 2 capacitors? What motivated you to change them?

Can you post the model number of the driver and LED lights, and links to any other technical information you have found? Thanks. :smile:
  • #3
LEDs have been tested with another led driver which differs from the original one. One faulty led lamp have been found faulty in the serial chain of one bank and have been replaced.

LEDs are 2w RMS? And in UV wavelength.

When i said triac i should have referred to single n channel MOSFET. Attached data sheet.

This one i didn't test yet...I don't know how to test one of these...


I checked the 3 diodes and found one (d6) that shows 0v on DVM on both directions. I tested it in circuit so far. This diode links positive pads on the capacitor area to the driver output to led banks.
I don't know how to replace this diode will any diode work the same? Attached data sheet

The other 2 diodes are zener and if tested in circuit with DVM diode mode the show regular diode values. Should i test them differently?


  • 20170712_075609.jpg
    50.5 KB · Views: 451
  • 20170712_075529.jpg
    53.9 KB · Views: 466
  • Screenshot_20170712-075628.png
    16.8 KB · Views: 477
  • Screenshot_20170712-084436.png
    23 KB · Views: 478
Last edited by a moderator:
  • #4

I replaced the D6 with a generic 3 amp diode (voltages specs are far more higher compared to the original one data sheet but i don't think it's a problem) code is 1na5408 see attached datasheet. I'm not quite sure does the same job of the smd one ...

Replaced the smd 120ohm smd with a generic one (not smd) however the original one is not faulty as measure correctly.

Replace back the caps with the stock one as they were higher in temperature toleration.


Timer works! It activate when photocells is triggered and count down.
Led lit up but shows a very weak light flow .

Can this be due to n channel MOSFET issue?
Can it be an issue at the 2 zener?

I'm still struggling but somehow seeing the light!

Also a desoldering hot air station is on the way and i bought 2 MOSFETs spares.


  • Screenshot_20170712-214454.png
    25.5 KB · Views: 443
Last edited by a moderator:
  • #5
Guys i still need your help.

Im sure d6 is ok.

I detached the n channel MOSFET and tested it off circuit. It behaves fine the the gate is triggered by diode tester i read high value from source to drain and when i ground the gate there is no value so expected behaviour.

Now my issue is still weak power at LEDs.
They power up but i barely see the light .

LEDs are ok as they lit up properly with a different driver.

What is the role of the inductor and of the 2 zener diodes in the circuit?

Inductor seem ok not sure how to test it properly.
Zener I'd need to remove them from circuit so not sure if they can be a culprit i will go on and remove them.

Let me know if you have suggestions

Thank you
  • #6
At this point we are all flying blind; although D6 is a suspect. The 1N5408 replacement is intended for power supply applications at power line frequency, it would not work well at high switching frequencies.
If you have access to an Oscilloscope, see what frequency, voltage, and waveform the diode is asked to handle. And also what frequency, voltage, and waveform is at the LED strings. Post 'scope photos if you can, noting both Vertical and Horizontal settings.

Beyond that, it's time you drew a schematic of the circuit, everything that allows the chip to control the LED strings. Show all reference designators, part Nos. and pin Nos. and whatever values you can. Voltages at key points would also be a big help.
  • #7
Well, I've replaced the 1n5408 with another diode from a boost dc dc converter for led application to be sure it can deal with the specs.

I'm just trying to find the root cause of the circuit output being fine (light turn on when photocell is triggered) but light emission is weak.

Unfortunately I don't have the background to fill the root cause analysis :(

what I can barely imagine is that it could be due to the switching capabilities being bad?

Can the MOSFET despite the test i did having issue to reach is duty cycle?
can the oscillator in charge of this duty cycle (i believe is the qartz on che circuit) be also involved in this issue?
  • #8
Guys i may have found other faulty components.
The two resistance on the ground of led out leads have weird values.

See attached picture for values in and off circuit The rest of the resistances in that area measure ok.

Not sure about c5


  • Screenshot_20170717-222828.png
    78.2 KB · Views: 473
  • #9
Guys !

Replacing the two resistor that were going far above their stock values solved the issue.

Lamp is now working.

Thank you for your support

Related to Struggle to fix a 36W dual channel LED driver

1. What is the purpose of a 36W dual channel LED driver?

A 36W dual channel LED driver is used to regulate the power supply for LED lights, specifically those with two channels. It converts the incoming AC power to DC power and delivers a constant and stable current to the LED lights.

2. How do I know if my 36W dual channel LED driver is malfunctioning?

A malfunctioning LED driver can result in flickering or dimming lights, inconsistent color temperature, or complete failure of the lights. You can also test the output voltage of the driver with a multimeter to check for any irregularities.

3. Can I replace my 36W dual channel LED driver with a different wattage?

No, it is important to use the correct wattage LED driver for your lights. Using a higher wattage driver can cause damage to your LED lights, while using a lower wattage driver may result in insufficient power and dimming of the lights.

4. How can I troubleshoot and fix a malfunctioning 36W dual channel LED driver?

First, check the input voltage and make sure it matches the recommended range for your driver. You can also check for loose connections and make sure the driver is not overheating. If these steps do not resolve the issue, it may be necessary to replace the driver.

5. Can I repair a faulty 36W dual channel LED driver on my own?

It is not recommended to attempt repairing a LED driver on your own, as it requires technical knowledge and proper equipment. It is best to consult a professional or replace the driver if it is malfunctioning.

Similar threads

  • Electrical Engineering
  • Electrical Engineering
  • DIY Projects
  • Electrical Engineering
  • Electrical Engineering
  • Electrical Engineering
  • Electrical Engineering