High current PCB design

Other Hardware General Electronics
1

Hi,

I need to build a PCB with tracks that can carry around 6 amps at 12 volts.

Due to space and size limitation, I've used 5mm wide tracks on a 70um board, this gives a cross sectional area of 0.37 mm2. Applying a basic rule-of-thumb (multiplying by 4) suggests that this is okay for a current of 1.5 amps.

To overcome this limitation, I've soldered a length of copper wire onto each of the current-carrying tracks, this should work ok, but has a couple of basic limitations 1) it looks really messy, 2) it's very time consuming and 3) it can be very fiddly on some of the more convoluted tracks.

Is there a better solution? is it possible to get PCBs with copper thicker than 70um? (none of my usual UK suppliers do this), or is there a way of somehow thickening the copper?

Thanks

2

Is it possible to re route the track to allow a link to be soldered on the component side of the board?
That would allow a heavy wire link to be used rather than a PCB track.

Weedpharma

3

That would allow a heavy wire link to be used

That's kinda what I've already done by soldering a wire parallel to each track. Is this the only/best solution?

4

What I have in mind replaces the track. If you can fit a straight piece of wire on top, you only solder each end on the bottom. It may require a different layout of components but is a lot quicker and neater than soldering a bendy piece of wire onto the track.

Weedpharma

5

What I have in mind replaces the track.

Ok, I get you. That could work, but the design is moderately complex, so using lots of jumper-wires would still leave the board quite messy. - It might be a good idea for some of the connections, but probably not all of them

thanks

6

The professional approach is custom-formed busbars that are made up for
the board layout - not economic in small quantities, and simple busbar strips
seem really hard to source these days - the sort of thing is a tinned copper strip,
possibly insulated, with pins every few tenths of an inch. Mounted vertically the
strips can be bend around the board to suit.

Years ago you could get "verostrip" I think it was called, which was this with
pins every 0.1 inch, so you had to chop off the pins you didn't need, but a spacing
of 0.5 inch would be ideal I think, since thats the natural spacing of TO220 packages
for instance.

[ this is the sort of thing: http://www.espbus.com/pcb_bus_bars.html ]

7

simple busbar strips seem really hard to source these days - the sort of thing is a tinned copper strip, possibly insulated, with pins every few tenths of an inch. Mounted vertically the strips can be bend around the board to suit.

This sounds ideal, but I can't find anyone who stocks them RS/farnell/Maplin don't seem to, and there don't seem to be any uk retail suppliers.

Looks like I'll just have to continue soldering extra wires onto the board.

Thanks

8

if you want to try something.

first apply the etch resist in reverse. cover everything BUT the tracks you want to be thick.

then using your etch tank as a electroplate tank, use a copper electrode.

there is a huge amount of information about plating through holes. but you can do the same thing on the exposed 'traces'

as I understand, most board houses buy thin copper and then make their own thicker boards with all the copper they etch off.

9

According to this calculator
http://circuitcalculator.com/wordpress/2006/01/31/pcb-trace-width-calculator/
5mm is plenty for 6A with 1oz copper on an external layer.

10

According to this calculator

thanks - that's a completely different formula to the others I had found

11

The problem with a trace calculator is that you make no allowance for peak, inrush or fault
currents - you don't want your trace to act as the fuse in the system (remember above
a certain current you get thermal-runaway as the resistance rises with temperature).

Power dissipation is proportional to the square of current, so even a modest 3 times overload
you are talking 10 times the power (much more once thermal runaway starts), leading to
burnt boards and delamination / fused traces.

One rule of thumb is to ensure the cross-sectional area of the trace is significantly more
than that of the wire in the fuse protecting the system.