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Rpi4B Power Supply

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I broke what looks to be either a diode, capacitor or resistor on the board near the power connector of the pi 4B and I want to try to replace the surface mounted component. I know the chances are pretty slim, but I cannot try without knowing what I need to get.

From other questions, I have included a picture showing the board around the missing part:

Busted part

 New contributor

-3

Question

What is the missing thing?

Answer

psu

My first wild guess of the big black rectangular thing is a choke. There is a similar one on the right side of the micro USB connector.

I read the schematic and found 4 chokes. Perhaps it is one of the bigger ones.

You might use a multi-meter to check if they are connected to the big flat black guy.

Perhaps no choke can still work, only that connected output voltage not that stable. So one quick and dirty solution is to short circuit the broken choke.

PS – Just wild guessing. No guarantee that nothing won’t explode! 🙂

rpi4 psu

I called the black guy a choke and not an inductor, because not too long ago I guessed something inside a wall wart an inductor for making a switching power supply’s oscillator, but then found it called a choke. I guess it is like the choke used in home white neon lights in those were the days. The purpose is to choke down the current so no excess flowing. But then I remember that R-L-C things can be used as low pass filters, to stablize output voltage of a switching power supply, of order of 100kHz. In other words, to make the output clean.

I usually go to Electronics Tutorials to refresh my memory and to verify. And below is something perhaps useful. I am too lazy to watch the youTubes. Perhaps the OP can watch the movies, and let me know if my suggestion is too risky and indeed something might explode.

Of course to play safe, I would suggest the following: (1) Use a multi-meter to check the point is question is a DC voltage. (2) Use a 10k to replace the choke, and see if Rpi is working again, (3) Invest money or time to find a similar valued choke (no need SMD, just a through hole one can do, to show off to friends that he knows how to “improve” the Rpi! :), and try his luck again.

choke

Now let us take a close look of the big guy MXL7704.

mxl7704

I suspect MXL7704 has all LDOs inside. In other words, there is very little chance of using the external inductors for it internal switching PSUs. A recent discussion by another OP on CM3 PSU design (Ref 7) gives more details.

I think perhaps I can study MXL7704 inside to see if my guesses makes any sense.

/ to continue, …

References

(1) Power Supplies for Beginners, Part 4 Buck Regulator : Time: 0:25s Electronics Tutorials

(2) How to calculate inductance 1

(3) How to calculate inductance 2

(4) How to calculate inductance 3

(5) Energy Stored in Inductor

(6) 33uH Inductor used in LM2596 Switching Regulator

(7) Rpi CM3 PSU Discussion

(8) Rpi CM3 IO Board Schematic

  • You may be correct that this is an inductor – this would be an integral part of the regulator. Suggesting that the OP just short-circuit it is irresponsible and guaranteed to cause damage. – Milliways 6 hours ago
  • 1
    This is the very model of a bad answer. – Michael Harvey

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