Power supply design always wrong

All my projects are usually breadboard prototyped.
I find that 9 times out of 10 however, something changes when it ends up in the final project.

Take this Aliens machine gun I made a while back. Fully prototyped it before, yet I had SO many issues once it was assembled. Works now however (I have so much crap in my garage!)


Well all seemed well, but now, randomly, the Mega reboots for no apparent reason.
It's not under any load (no RGB leds are running), and when it's running off the bench power supply, it is pulling less than an amp.

But... randomly (sometimes 20+ minutes apart). it will just reboot. No idea what that would be :roll_eyes:

No idea how to track that down.

This Mega has been very problematic. I think the time has come to bin it and re-install the entire rig with a Teensy 3.2 I have spare here.

Is anything getting hot?

What do you mean? To me 'less than an amp' suggests maybe 750mA or something like that. How much is it actually drawing? I'd expect a lot less than an amp for a Mega.

So let's be clear - you are definitely not attempting to power the Mega 2560 via "Vin" or the "barrel jack"?

Can you PLEASE post a schematic?

Thanks.. Tom... :smiley: :coffee: :coffee: :coffee: :coffee: :australia:

Yes... the 12v barrel is powering the Mega. I don't have any room left for yet 'another' 5v PSU (already fitted 5).
But... that 12v barrel is only powering the Mega and a JQ8400 MP3 module.... nothing else.
There is no load on any of the pins (LED wise). Why is the jack so bad? Made MANY projects over the years using the 12v jack. If it's that useless, why fit it?

I drew a schematic, but then radically changed all the wiring... so sitting here now re-doing it.

It's weird. For some reason, plugging in the USB lead is not recognised (therefore I can't use the debug serial) unless I unplug the power to the project first, find the USB connection, and then re-power the device. What would cause that kind of interferrence?

Update... for reasons I don't yet understand, the regulator on the Mega is hot to the touch (took some internal parts removal to reach it).

So... Buck converter number 6 being installed :roll_eyes:


So your project is not vert serviceable?
You need to address that too, in the field how will you fix problems quickly.
Have you got places where you can probe to check voltages and signals?
(A design and repair Electronics Engineer here, ya go to make it serviceable.
Otherwise I'll charge like a wounded bull to fix it. :smiley:)

Possible gnd fault, open circuit somewhere.

Tom... :smiley: :+1: :coffee: :australia:


Floating GNDs between USB and external power supply.

So the Mega is supplying power to something else.

The power supplies are likely not your problem; it's the apparent chaos in your wiring/project approach. Sorry to put it so bluntly, but there it is.

It's not that bad; problem is that people start powering all kinds of things from the 5V while using the jack. Trying to drive motors, use it for the coil of relays and so on.

Right first... the wiring is not Chaos. It is as neat as can be achieved in the space I have.

Why is 5 Buck regulators weird? I had them, so I have divided the load of all the LEDS across multiple regulators.

This is a prop, not an item I am selling. By their nature, they are a little chaotic in their final assembly. They are one off builds.
Yes, in hindsight, I would do things differently, but it changes as the build progresses.

This is kind of the layout now .... broadly speaking.

I unplugged everything from the Mega's output pins and the regulator still ran hot.
I just unsoldered the SMD reg from the Mega and replaced it. The new one runs cool.
But, it has it's own 5v external supply now anyway.

A Mega can deliver a maximum average of about 150mA with 12volt on V-in or the DC socket.

The datasheet of that Mp3 module states that even USB power is not enough,
because it needs at least 1Amp.

It seems that most of the devices need 5volt. Can't you run everything on a 5volt supply?
What are those amplifier modules. A bridge-mode class-D amplifier running on 5volt can be equally powerful than a class-AB amp running on 12volt.

About time!

Did I not explain that back in #4?

And it is not at all a "12v jack". If you must, a "7.5V jack".

It's a "legacy" part. Once provided, it is provided in successive generations "just in case" - for "compatibility". More recent designs actually do have proper regulators.

What gives me the shivers about your last diagram is the implication that the data lines to the LED strips (and the 7-segment displays) are actually separate from the grounds. :astonished:

Grounds run with the data lines where they can (which is most of the cable run). They obvisouly split at the Nano and carry on back to the common ground point, but they are all twisted looms to each RGB strip and each LED display.

Why the 47R in series with the PA power supply?

I assume all those DC-DC converters have the gnd inputs connected to gnd output.

Have you got the DC-DC connected to the 12V supply in star configuration?
Do you have a terminal strip for the 12V connections?

What supply noise do you hear, hum or hash?
If mains hum, you probably have a ground loop.

Don't forget the metal tab on the LM7805 is connected to gnd pin.

Tom... :smiley: :+1: :coffee: :australia:

7805 is mounted to the metal front plate... which is also Gnd.

All the DC-DC modules connect to a 12v 'hub' in star formation (2x 5 way Wagos for easy disconnection).
I will solder those when finished.

The 47R removes noise on my audio system. Read it somewhere and it seems to work.
Without it, there is a slight background hum.

That means that the sound amps are low power.
You should have used a 5volt supply for everything (no buck converters).
And maybe... a 5 to 12volt boost converter for the unspecified amps.
But 5volt class-D should be better there, so a 5volt-only setup.
A 12volt supply is just plain silly.
Was the built-in class-D amplifier on the Mp3 module not powerful enough?

The 12v supply isn't silly.... stupid thing to say :roll_eyes:

I have 12v LEDS, 12v relays (that cannot be changed to 5v), a 12v strobe etc

Anyway, it's working now

None of them are shown in your diagram.
The advice was based on that diagram where all devices were 5volt, except for the low power sound.