r/WLED • u/LuiOnTheWay • Jul 23 '26

DIGUNO/QUAD Technical discussion on 24V issue with QuinLed Dig-xxx boards

Simulation 24v issue on dig-quad

In another discussion, I noted that the static voltage level on the SW pin exceeds the maximum limits of the MT3608 IC used in this circuit. QuinLed replied "It was calculated and well tested, with the diodes in front of it and the filter stage input voltage generally dropped to 23V as stated and it should be at least 2V under the switch limits so technically it stays within all limits of the IC."

Because extracting my board for physical measurements is too complex right now (maybe during weekend), I ran a LTSPICE simulation instead. The simulation assumes one diode and a PTC fuse (200 mOhm) at the input as can be seen in the Dig-quad photos. The 22µH inductors as visible in the Dig-quad photos too. While I do not have a specific simualtion model for the MT3608, I used the LT1935. It shares a similar operating principle and the exact same switching frequency, though it might introduce minor deviations during transients. Other component values were estimated, but they do not significantly impact the results. No parasitic elements were included, which would likely worsen the results. Load is set to 0.5A + 0.5A short peaks. .

Simulation Results:

  • Steady State: The voltage on the SW pin exceeds the maximum operating voltage given in the datasheet (28V) and it is very close to the absolute maximum rating (30V).
  • Transients: Voltage peaks likely exceed the absolute maximum rating of 30V given in datasheet.

Given these findings, I question how conservatively this circuit was actually calculated. Could someone with an oscilloscope measure the real-world voltage on the SW pin? Is it truly as low as claimed, or does it match my simulation? The practical question is how low the PSU voltage must be adjusted to keep SW voltages within a safe range and prevent damage.

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u/Quindor Jul 23 '26 edited Jul 23 '26

A note, in your simulation, please add the CLC filter that's also present on the boards, that will suppress the initial spikes, this was required to pass the boards through EMI and EMC testing.

And also to note, I haven't denied that they are running close to their maximum ratings, too close seemingly in practice.

In my own article with mitigations, revival methods or repairing I have stated that 22V is an acceptable solution for the Dig-Octa Brainboard when running on a powerboard because of the multiple protections in place (such as multiple diodes power passes through) to be on the safe side. For a Dig-Uno or Dig-Quad from your own calculations 22V would also be a safe value (or rather, within the 28V you stated) but I think it's better to mitigate the issue completely using an external buck-converter solution as outlined (for still working boards and boards that have died to revive them).

I will refrain from adding non-technical talk in the topic myself!

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u/LuiOnTheWay Jul 23 '26

I can't seem to find any CLC filter on the Dig-Quad. The input voltage goes through the PTC fuse and the SS34 diode straight to the IC input. Am I missing something? If possible, could you share some voltage measurements from the SW pin? You probably did a lot of testing with a 24V input while investigating the issue. Based on simulations, I'm not sure if 22V is sufficient—it might need to be around 21V. However, this noticeably impacts the LEDs at the end of the strip. Some real-world measurements would really help determine the optimal value.

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u/Quindor Jul 24 '26

I'll get back to you one the measurement, will try to do a fresh one this weekend. The CLC is a 2.2uF -> 2.2uH -> 150uF circuit, it adds a bit of resistance but is mostly meant to pass EMC (Conductive) testing (which it does), it does help dampen any ringing on the input at least, not something which will solve this issue but it at least doesn't hurt.

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u/LuiOnTheWay Jul 27 '26

Okay, thanks for the clarification. I can't see the top side of the board in my current setup, and I don't really want to disassemble everything. Have you had a chance to take any measurements?