PCB Trace widths

[Mark245]

Hey everyone,

I am building a smaller amp with maybe 20 watts per channel. I am routing all of the power traces for 50 mils and the traces I do not expect much power on for 25 mils. Is this a good assumption? I was thinking about going with 75 mils for power. Is this overkill?

Also, I have decided to go with a double sided pcb since I have like 80 parts to route. Basically, the traces I could not fit all on one side I put on the other. Is it ok to do this? Will it cause noise? I have heard some people say to put all power and ground on one side and everything else on the other. Any rule of thumb for this?

Thanks,

Mark

[rtarbell]

Generally speaking, yes - the power and ground traces are usually on the bottom of a two-sided PCB - all of your signal traces are on the top. This makes troubleshooting much easier (if you want to probe a certain signal, you don't have to flip the board upside down).

Just another suggestion for you - depending upon what kinds of components you are using (surface mount, lead-less packages, etc.), I would suggest that you do NOT get a solder mask on your board. This way, if your traces are too thin, then you can solder some bus wire right over the top of them to enhance their current carrying capability. If you have a solder mask, you would have to scrape the solder mask away from your traces before doing this.

Do a google search for

pcb trace width calculator

This should get you to several pages that can give you numbers as to how thick a PCB trace should be for a given current.

[syn08]

One inch of 50mils 1 oz thick copper has a DC resistance of about 11 milliohm. A 20W amp into 8ohm will have a peak current of 2.2A, hence a voltage drop on critical traces of max 24mV peak per inch. That's way to much to my taste, I would double the high current traces width. Even so, you would still require a good layout to avoid ground loops.

[Mr Evil]

Just make the tracks as wide as will physically fit.

If you're using a double-sided board, note that running a track on one side in parallel with one on the other side will capacitively couple them. This is good in some cases, and bad in others.

[Steve Dunlap]

Originally posted by syn08
One inch of 50mils 1 oz thick copper has a DC resistance of about 11 milliohm. A 20W amp into 8ohm will have a peak current of 2.2A, hence a voltage drop on critical traces of max 24mV peak per inch. That's way to much to my taste, I would double the high current traces width. Even so, you would still require a good layout to avoid ground loops.

I agree with this. Another option is to go with 2 oz copper (or heavier) also. This will make very little difference in price for small runs of just a few pieces.

If you have room, you can go with both our suggestions.

[Eva]

Use traces for small signal nodes and polygons for speaker current paths...

Also, double sided PCB calls for a ground plane

[sakis]

also need to be carefull in case your circuit is layout sensitive ....


some amps are some arent that much .... generaly all amps are

also ground plane might be an issue

some manged with it others didnt

regards sakis

[nigelwright7557]

If your PCB CAD software allows powerplanes I would use them as much as possible to keep the ground plane as low impedance as possible.

Otherwise fill out tracks the best you can.
I would use a minimum of 75 thou and thicken up where room allows.

I have a 900w amp and use upto 200 thou tracks on high power lines.

[jackinnj]

See if you can use 100 mils for most of the power and output nodes. Where this isn't workable you can narrow the trace -- I'm lucky to be able to "right-click" to narrow or widen the trace, but ymmv.

When you discover a mistake with a solder mask, just grab the Dremel with a cutting blade.

Audioman45 suggested (in an Application Note) that groundplanes weren't helpful in audio amp circuitry -- so I laid out another board using the LM4702 sans ground plane, using a central locus to bring the signal ground together. The difference was measurable in terms of the THD% being a smidge lower, and by a smidge I mean a tiny but consistently measurable amount.

[tinitus]

I dont know
This one has supply caps all the way from bridge to output
Maybe thats slightly different
Oh, connections from power transistors to speaker out will be solid core wire
Well, the whole thing might be hard wired, or just part of it
I should say this too, its a Dx amp