Narrow Traces on PCB

[Shachar85]

Hi
I'm new in PCB designing. I've read a lot and didn't find the answer I seek.

Long story short - my question is what happens when driving too much current through too narrow trace?
Will it limit the current? or will it break and open the cirucit?


The full details:
I'm trying to design a heating bed for a 3d printer.
I'm going to use 24v, copper clad pcb from ebay (0.5 oz/ft^2). 400X200mm.
I need to get to at least 400w.

Tried many calculations, but can't get the numbers right.
The best I got was W=7.8mm, L = 8600mm, distance between traces is 1.5mm.
It gives me 1.08ohm at 25deg(c) and 1.43ohm at 110deg(c).
Both are fine by me, but it gives me 16.8A to 22.2A. Which are both too much for 7.8mm wide trace, according to the online calculators.




And that's why I wonder what happens if I drive too much current - will it break the circuit or lower the current?

Thanks for any help

 

[jiripolivka]

A PCB line is a conductor like any other. If you send a current through it, it will heat up in proportion to its resistance. The thinner or narrower your conductor, the higher its resistance. And heat will rise with current.

If not properly cooled, it will break like any fuse or a thin wire. You can find current rating in manufacturers' data, or run a test with your real PCB line. Reduce the current 5-10 times to increase reliability of your PCB.

Poor designs make sure a PCB line breaks. Try to avoid high currents in your PCBs, send such flows by thick wires.

 

[Shachar85]

Thanks for the answer. If I see it right - there's no way to reach 400w with 0.5oz/ft^2.

Just to make sure - is the "Max Current" is when the copper break? or just the recommended max current?

 

[d123]

Hi,

Probably maximum means "maximum", not "recommended", as much as I often wish otherwise

Some-one in the link to another thread in post #5 suggested this tool, Saturn PCB Toolkit, which is up-to-date and from looking at the download page looks like it has a lot of parameters to make use of, here's the thread:

https://www.edaboard.com/threads/346469/

 

[jiripolivka]

Hi,

Probably maximum means "maximum", not "recommended", as much as I often wish otherwise

Some-one in the link to another thread in post #5 suggested this tool, Saturn PCB Toolkit, which is up-to-date and from looking at the download page looks like it has a lot of parameters to make use of, here's the thread:

https://www.edaboard.com/threads/346469/

As I wrote, reduce your calculated maximum current 5-10 times, otherwise you PCB will be not reliable. PCB lines decompose due to thermal rundown plus chemical corrosion of copper line glued to substrate.

 

[Shachar85]

Ok then....I'll have to say goodbye to the PCB idea.
Even if I had 1oz copper clad, I wouldn't get near 5 times maximum current.

I'll go with another type of heating surface.
Too bad. Could be fun making my own


Thanks for the advice