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PCB Design

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Broz

New Member
I've created PCB's before with all through hole components and it was fairly easy to do. I used PCB Express software and services. I ran all of my power and ground traces on the bottom surface and all of my signal traces on the top surface. It was pretty straightforward.

Now I'm trying to build a very small device and I'm laying out my PCB. I went to a combination of surface mount and some through hole components to save space. The problem I'm having now is that I can't get away from daisy chaining power and ground traces. With the combination of components I have to use and their necessary positions on the board, I'm having to use several vias and there is hardly a straight trace anywhere on the board.

I've read that I'm not suppose to daisy chain power and ground traces but I just can't seem to avoid it with this project. What are the dangers of doing so and what do I have to really watch out for at a minimum.

Thanks.
 

Sceadwian

Banned
Each device draws different currents which due to the non zero resistance of the traces will cause the voltage on VCC to sag and the voltage on GND to rise above zero when any device is drawing current, this VCC sag and ground raising will change what VCC and GND chips further down the line see, and it will fluctuate with load so you'll get crosstalk between devices. Decoupling caps will help reduce noise but it won't change the fact that your ground will actually be above ground relative to other parts of the circuit.
 

Broz

New Member
Alright, that makes sense. Let me ask this to see what might be able to get away with. I have 2 output devices (LED's) and one input device (IR detector) that are controlled by a PIC. None of these devices are in operation at the same time as the other. Is it possible that I could get away with controlling these devices reliably with daisy chained power and ground traces given that the PIC is controlling them at separate times?
 

Sceadwian

Banned
If they're not being operated at the same time then the problem doesn't exist.
 

Mr RB

Well-Known Member
Some of those IR detector devices (ie 3pin ones with internal chips) can be fussy with noise on the +5v line, even noise made by your micro.

Most of the commercial designs TV's VCR etc use a decouple cap right on the pwr pins of the IR sensor and a resistor (47 ohm?) to the +5v line.
 

Boncuk

New Member
To avoid mentioned crosstalk you could make the entire empty space on the board both, supply voltage and ground layer. Addionally use decoupling capacitors spaced not more than 2 inches distributed over the board, mainly close to IC supply pins.

Here is an example of a 24V/35A PCB with relay outputs.

Picture1 shows the entire trace routing without using the empty space.
Picture2 shows the bottom layer used for ground entirely (except for signal traces)
Picture3 shows the top layer used for the positive output voltage.

Use the polygon and draw a rectangle around the PCB. Name the rectangle accordingly (ground for the bottom layer) and perform "ratsnest". All excessive board space will then be connected to ground.

Use the same method for the top layer.

It is also good for environmental considerations since less etchant solution will be necessary and less copper will dissolved in it.

Boncuk
 

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