Power supply across multiple PCBs

My project is split across 9 PCBs. 5 out of 9 have ICs on them. Each IC has a 0.1uF cap connected between +5V and GND. 8 out of 9 PCBs have +5V supply and GND planes. Main board has Arduino Nano connected to it and I'm connecting all other boards to the main one. Now, is there any special consideration to be taken when dealing with setups like those? I mean, do I need to place caps or some other components on each board to avoid noise or other issues (I don't know if I should even expect issues)?

You're probably fine but more caps is never bad. Apart from the small caps on the ICs you could put a bigger cap on each PCB where the power cables join the board. I'd use polyester type.

Rule one: Always add a tank capacitor at the point the power enters the board. This should be >= 10µF. Bigger is better - 100µF is good.

Rule two: Always add a tank capacitor for each group of 3-4 chips. This should be >= 10µF.

Rule three: You should strive connect your board's power inputs to the lowest source impedance point of the circuit - direct to the power supply / battery is best.

Rule four: Be wary of too much power-up inrush current when running from protected sources like USB, and be prepared to revise down your total power capacitance to keep inrush currents below the maximum the supply can safely provide (too much capacitance could cause your computer to disable the USB port, for instance).

majenko:
Rule one: Always add a tank capacitor at the point the power enters the board. This should be >= 10µF. Bigger is better - 100µF is good.

Never heard of tank capacitor before, some googling reveals its use in AC circuits. However, I’m dealing with DC only… So I’m not too sure about what it does.

majenko:
Rule two: Always add a tank capacitor for each group of 3-4 chips. This should be >= 10µF.

Where exactly to place it?

majenko:
Rule three: You should strive connect your board’s power inputs to the lowest source impedance point of the circuit - direct to the power supply / battery is best.

My power connections are scattered on main board, one in each corner, due to practical reasons.

majenko:
Rule four: Be wary of too much power-up inrush current when running from protected sources like USB, and be prepared to revise down your total power capacitance to keep inrush currents below the maximum the supply can safely provide (too much capacitance could cause your computer to disable the USB port, for instance).

Not sure if I understood this right - your point is not to put too much caps?

kustom:

majenko:
Rule one: Always add a tank capacitor at the point the power enters the board. This should be >= 10µF. Bigger is better - 100µF is good.

Never heard of tank capacitor before, some googling reveals its use in AC circuits. However, I'm dealing with DC only... So I'm not too sure about what it does.

A tank capacitor is basically a capacitor which provides a reserve of power to respond to rapid transients faster than the relatively high impedance of the incoming wires can. Not to be confused with the capacitor in an LC tank circuit.

kustom:

majenko:
Rule two: Always add a tank capacitor for each group of 3-4 chips. This should be >= 10µF.

Where exactly to place it?

Somewhere convenient and close to the chips. On boards with lots of chips (specially old-style DIP chips), you may see this (| = 100nF decoupling, ##### = chip, O = electrolytic):

O | ##### | ##### | ##### O | ##### | ##### ...

... with the power and ground running along beneath all the chips.

kustom:

majenko:
Rule three: You should strive connect your board's power inputs to the lowest source impedance point of the circuit - direct to the power supply / battery is best.

My power connections are scattered on main board, one in each corner, due to practical reasons.

Make sure those connection points connect as directly as possible to the main power point then, without too much spurring off them.

kustom:

majenko:
Rule four: Be wary of too much power-up inrush current when running from protected sources like USB, and be prepared to revise down your total power capacitance to keep inrush currents below the maximum the supply can safely provide (too much capacitance could cause your computer to disable the USB port, for instance).

Not sure if I understood this right - your point is not to put too much caps?

When you first apply power to a board the capacitors will be empty. They immediately start to charge up. This causes a large amount of current draw from the power supply, which reduces as the capacitors charge up. Take this example:

Yellow is voltage, blue is current. You switch on and the voltage across the capacitors rises with a nice curve. But equally there is a big spike of current which tails off rapidly - it's basically the inverse of the voltage.

With too much capacitance that initial spike (which is called the inrush current) can get quite large (in this case 9.44A), but the period of it is quite short (drops rapidly down to just a few hundred mA). Most basic power supplies will just not provide enough current to honour that spike, so the capacitor charge will be slowed down (Q=IT). However, if the power source is sensitive to the spike of current, say it contains an over-current cut-off switch, which is sensitive to short pulses, or, like USB ports, will be shut off if the current draw exceeds a certain level without proper negotiation, then that initial pulse could cause the power supply to cut off the power. In that situation you need to keep that initial spike below your current limit, and thus will need to reduce your total capacitance accordingly, or implement some kind of "soft start" power system in your circuit.

majenko:
A tank capacitor is basically a capacitor which provides a reserve of power to respond to rapid transients faster than the relatively high impedance of the incoming wires can. Not to be confused with the capacitor in an LC tank circuit.

Okay, so I need electrolytic cap which will act as tank cap, 100nf?

majenko:
Make sure those connection points connect as directly as possible to the main power point then, without too much spurring off them.

If you are reffering to vias and bending, there isn't much of that, I've tried to minimize it as much as possible.

Regarding the power of my circuit, it's standard USB. I've also attached the circuit.

Also, you are very helpful, thank you! I've still got much to learn.

circuit.png

kustom:

majenko:
A tank capacitor is basically a capacitor which provides a reserve of power to respond to rapid transients faster than the relatively high impedance of the incoming wires can. Not to be confused with the capacitor in an LC tank circuit.

Okay, so I need electrolytic cap which will act as tank cap, 100nf?

I think you meant µF not nF there, and as it's USB I'd be inclined to use 10µF per board.

majenko:
Make sure those connection points connect as directly as possible to the main power point then, without too much spurring off them.

If you are reffering to vias and bending, there isn't much of that, I've tried to minimize it as much as possible.

Regarding the power of my circuit, it's standard USB. I've also attached the circuit.

Also, you are very helpful, thank you! I've still got much to learn.

Vias increase the impedance. Bends don't do that much really. So yes, keep the vias to a minimum, and try not to have any other connections coming off from the power traces to power other things. They should go straight back to the main power connection if possible.

Don't forget that this all depends on what's on those PCBs, how much current they draw, what they're switching on/off, etc.

All of that is still a secret so we might be talking rubbish.

majenko:

kustom:

majenko:
A tank capacitor is basically a capacitor which provides a reserve of power to respond to rapid transients faster than the relatively high impedance of the incoming wires can. Not to be confused with the capacitor in an LC tank circuit.

Okay, so I need electrolytic cap which will act as tank cap, 100nf?

I think you meant µF not nF there, and as it's USB I'd be inclined to use 10µF per board.

Yeah, typo sorry. So, 10µF per board + additional 10µF on main board since I have 4 chips there?

majenko:
Vias increase the impedance. Bends don't do that much really. So yes, keep the vias to a minimum, and try not to have any other connections coming off from the power traces to power other things. They should go straight back to the main power connection if possible.

I'll try to come up with some ways of improving connections then.

Kustom,
Via's from power plane, and ground plane, to signal plane are O.K. Keep them as close to the IC's as possible. Signal line vias are O.K. too, just keep them to a minimum.

impedances between grounds cause voltage drops. So, each board should have it's own ground wire and power wire to the supply (DO NOT daisy-chain power and return lines).

The 'Tank' mentioned is actually a Filter (ripple filter). It charges up during power on and supplies enough power to overcome ripples (small power drops) caused switching and stuff.

Another way to handle in-rush may be to calculate a resistive value based upon the current capability of your USB hub, at 5 volts, and insert that value in your supply hub. NOT a great solution, but it is something.

Thank you as well for information. And by the way, PCBs are not secret or anything. I’ve attached them.

PCBs.zip (57.6 KB)

Just a helpful suggestion-- When discussing PCB design concerns, having an image of the board (not an Eagle file, but a .PNG screencap or image export from your PCB fabrication software) posted directly into the post helps. You should avoid using .ZIP files, as it takes a few extra steps to download, uncompress, and view the files. If someone’s browsing by smartphone or tablet, they probably won’t or can’t open those. If they’re browsing by PC, they probably still won’t bother. You’ll get more eyes by making it as convenient as possible for your readers to see it.

Another tip: Watch the image size. The schematic you posted above is large enough to be projected on a movie theater screen without seeing pixels. That’s excessively large. Try to keep your image resolution down below 1000 pixels (wide OR high), although you’ll need to balance resolution against readability. If there’s so much detail that it can’t be read at less than 2000 pixels, then so be it. Then again, that much detail would be a lot of information for casual viewers to take in, and you might be asking too much of free help. :wink:

Make sure when you resize your images that you use software that does a good job of scaling. The old Windows Paint programs would butcher the details, but most photo editing stuff (Paint.NET is a good free option) will use better algorithms that preserve detail by dithering and anti-aliasing.

Thank you for your long but helpful post! :smiley:

Allright, I'll try to explain what I'm doing. My project is MIDI controller consisting of 42 buttons, 27 LEDs, 31 pots and 2 rotary encoders. Due to the controller layout, splitting project on several PCBs was the only reasonable solution to avoid wire hell. The PCBs are being soldered below the controller plate, right on LEDs and buttons. Buttons are LEDs are connected into matrix with common columns (ULN2803+74HC238+4021).

The layout looks like this:

  1. Red part is "Main Board" (Arduino+4021+74HC238+ULN2803+LM324+buttons+LEDs+10 diodes)

  1. White part is PCB with 4 potentiometers ("Mod pots PCB")

  1. Purple parts are 2 boards (6 Pots UD PCB) for potentiometers (4051 mux on each)

Upper board connects to "Mod pots PCB" via 6-pin output (+5V, GND, S0, S1 and S2 control pins for mux and analogue output from 4051).
Lower board connects directly to Main board.

  1. Green part is top board (buttons+LEDs)

Board connects to main board via 5-pin output (LED rows 2, 3, 4 and columns 5 and 4). It also has 2 7-pin outputs which connect into 4 Buttons L/R PCB (6 button rows+column output).

  1. Yellow parts are 2 PCBs with tactile switches

Its 7-pin output connects to 4 Buttons L/R PCBs. (6 button rows + column output).

  1. Blue parts are 2 PCBs also containing tactile switches (4 on each), 4051 mux and 16 diodes each

Board has several outputs, 6-pin output for button rows to main board and 6-pin output containing +5V, GND, SO, S1, S2 and analogue output from 4051. 3-pin outputs go into main board as well (column outputs).

Hope the connections are clear now.

Make your traces as wide as is reasonable.
Ground planes are preferable.

LarryD:
Make your traces as wide as is reasonable.
Ground planes are preferable.

Every board except for the top one has a ground fill on both sides. Traces are all 10mils.

My preference is 25mil for signal traces and 50 - 75mil for power.
Just a note, 10mil can be easily damaged by over heating when soldering or removing a component later.

LarryD:
My preference is 25mil for signal traces and 50 - 75mil for power.
Just a note, 10mil can be easily damaged by over heating when soldering or removing a component later.

Isn’t that a bit of overkill (50mil)? In the past I was using both 10mils and 24 mils max and everything worked just fine.

Depends. 10 mils is fine for digital signalling. Unless it isn't, for various reasons like trace length and inductance. You may choose to use larger traces for a more robust physical path.

For power traces, there is no de facto. It depends on how much current is required. Larger never hurts, as it will reduce impedance. For ground, you just can't get big enough. If you can make the entire surface a ground plane, you should do so. (With respect to separation of returns to a central point and all that, of course..)

SirNickity:
Depends. 10 mils is fine for digital signalling. Unless it isn't, for various reasons like trace length and inductance. You may choose to use larger traces for a more robust physical path.

For power traces, there is no de facto. It depends on how much current is required. Larger never hurts, as it will reduce impedance. For ground, you just can't get big enough. If you can make the entire surface a ground plane, you should do so. (With respect to separation of returns to a central point and all that, of course..)

Actually I've spent my whole day converting all those boards to 24mils, so that part should be solved. :slight_smile: I've also placed those electrolytic caps on boards with power supply. Is there anything else I should pay attention to?