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2 layer design

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runman_up

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is it do-able to put a trace rated for 120VAC 30A or so on a two layer board that also has 5V dc controlling a microcontroller - what is the best way to do this for a two-layer?
 

alec_t

Well-Known Member
Most Helpful Member
It's possible.
Is one layer a ground plane? If so, does it cover most of the board area?
Is the 5V DC isolated from the AC?
 

kubeek

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It is possible, you simple need to keep the trace at least 5mm away from anything else.
But the bigger problem is that 30A is very large current and you will need some pretty wide traces, for 35um copper and 40K temp. rise this calculator says about 500mil width.http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
Also multiple pins will be needed for the connections, one soldered pin just can´t handle 30A.
 
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eTech

Well-Known Member
is it do-able to put a trace rated for 120VAC 30A or so on a two layer board that also has 5V dc controlling a microcontroller - what is the best way to do this for a two-layer?
How about using two separate PCBs?
One for the 120VAC circuits and one for the Microcontroller circuits?
 

panic mode

Well-Known Member
when making traces for large current problem is cross section of traces.
the larger the cross section, the more current can flow through it.
usually very wide traces are not practical so possible workarounds for example are:

1. use of multiple layers (note that inner layers are worse for this because of limited cooling). layers need to have equal current carrying capacity and have plenty of vias for better contact near component terminals. if the traces or tracks are not even, there should be more vias, possibly along entire track.

2. use thicker copper cladding (standard may be 1oz of Cu per square foot but you can get 2x, 3x or 4x that if you specify it)

3. leaving slots in soldermask along PCB tracks and applying some solder (like "mountain range") to increase cross section.

4. optionally don't place high current conductors on pcb, use other methods for those circuits (wires, bars, ...)

5. also check needed clearances between tracks due higher voltage. if the board has proper coating, clearances between tracks can be reduced.

examples:

before - https://engineering.purdue.edu/ece477/Webs/S08-Grp12/pics/MotorControllerPCB02.jpg
after - https://engineering.purdue.edu/ece477/Webs/S08-Grp12/pics/MotorControllerPCB05.jpg

note that even light coat of solder dramatically changes thickness of the pcb traces.
 
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panic mode

Well-Known Member
(deleted duplicate)
 
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eTech

Well-Known Member
Hi panic,

Agreed with the items you listed except #1.

High current traces should never be on internal layers (heats and expands, cools and contracts) and Via's should be avoided at all costs because the copper area of the via hole needs to be eqivalent to the copper carrying capacity of the trace. So the hole is going to be fairly large. If you use small vias, its equivalent to jumping wide traces with narrow traces...
 

panic mode

Well-Known Member
that is what i said in#1; do NOT use inner layers for high current, outer layers are preferred.
as for vias, they need to be same current carrying capacity as trace IF one via has to carry entire current.
if there is bunch of them sharing current, this does not need to apply.
many manufacturers offer design guidelines for various applications including high current like for SMPS etc.
there are some interesting things on vias etc:

http://www.electro-tech-online.com/custompdfs/2012/10/270_pcb_layout_for_switchers.pdf
http://www.electro-tech-online.com/custompdfs/2012/10/slua366.pdf
 
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